Abstract. Wind forcing plays a pivotal role in driving upper-ocean physical and biogeochemical processes, yet direct wind observations remain sparse in many regions of the global ocean. While passive acoustics have been used to estimate wind speed from moored and mobile platforms, their application to profiling floats has been demonstrated only in limited cases. Here we report the first deployment of a biogeochemical profiling float equipped with a passive acoustic sensor explicitly designed for wind retrieval, aimed at detecting wind-driven surface signals from depth. The float was deployed in the northwestern Mediterranean Sea near the DYFAMED (DYnamique des Flux Atmosphériques en MEDiterranée) meteorological buoy from February to April 2025 and operated at parking depths of 500–1000 m. We demonstrate that wind speed can be successfully retrieved from subsurface ambient noise using established acoustic algorithms, with float-derived estimates showing good agreement with collocated surface observations. To evaluate scalability to remote regions, we simulate a remote deployment scenario by refitting the acoustic model of Nystuen et al. (2015) using ERA5 reanalysis as a reference for surface wind. The ERA5-based calibration performs well under moderate winds but exhibits systematic high-wind bias (≥ 10 m s−1). Finally, we apply a residual learning framework to correct these estimates using a limited subset of DYFAMED wind data, simulating conditions where only brief surface observations are available. The corrected wind time series achieved a 38.6 % reduction in RMSE, demonstrating the effectiveness of combining reanalysis with sparse in-situ calibration. This framework improves agreement with in-situ wind observations relative to reanalysis alone, supporting a scalable strategy for float-based wind monitoring in data-sparse ocean regions. Such capability has direct implications for improving estimates of air–sea exchanges, interpreting biogeochemical fluxes, and advancing climate-relevant ocean observing.

Abstract Understanding factors controlling the biological carbon pump (BCP) at the regional scale is of major interest for better characterizing carbon sequestration into the deep ocean and, therefore, the ocean's role in climate regulation. This study focuses on high‐latitude marine regions, which are responsible for the majority of marine CO2 absorption. Using data from Biogeochemical‐Argo floats, a bioregionalization method was performed on 335 annual time series of chlorophyll a concentration and particulate backscattering coefficient, variables from which particulate organic carbon (POC) could be estimated. This analysis highlighted six regimes characterized by distinct seasonality in productivity, export, and transfer of small POC (<100 μm). Both hemispheres exhibited regimes with strong summer blooms and others with deep chlorophyll maxima. Across these regimes, variations in phytoplankton phenology and particle assemblages drove three distinct systems of BCP strength and efficiency for small particles. Despite these differences, processes such as gravitational sinking, the mixed layer pump, or particle fragmentation facilitated the export of small particles down to ∼1,000 m across all regions. This resulted in an average annual contribution of ∼10% of small particles to total organic carbon fluxes at depth, highlighting the role of small particles in long‐term carbon sequestration. These findings emphasize the need for future investigations into processes driving small‐particle carbon export and transfer in the mesopelagic zone at annual and seasonal scales.

The recent roadmap IndOOS-2 has stressed the need to expand the biogeochemical-Argo observing system in the Indian Ocean. The Monaco Explorations Indian Ocean expedition offered a unique opportunity to meet this goal in the southwestern sector which was, in this regard and at that time, one of the least covered oceanic regions. We designed a deployment strategy for the biogeochemical float array grounded on past experiences, existing knowledge, and the analysis of historical datasets to cover the contrasting biophysical regimes from the Seychelles Chagos Thermocline Ridge to the subtropical gyre. Aligning with IndOOS-2 recommendations, a denser float distribution was set in the tropical band to enhance biogeochemical observations in upwelling zones. Following this strategy, a fleet of seventeen biogeochemical floats was successfully deployed during the expe- dition in October–November 2022. After two years of operations, the spatio-temporal distribution covered by the fleet confirmed that the goals of the deployment strategy have been reached, revealing seasonal modulations of the meridional trophic gradient with respect to phytoplankton biomass from tropical mesotrophy to subtropical oligotrophy

Abstract Photosynthetic available radiation (PAR) is the light usable by photosynthetic organisms. Photosynthetic available radiation measurements at depth are required to quantify the light availability for primary production. Direct PAR measurements may be measured with full‐spectrum quantum sensors for the range 400 to 700 nm. When spectrally resolved light is measured, as for the downwelling irradiance spectrum , PAR may be computed by numerically integrating within those limits. As radiation varies across a spectral continuum, needs to be resolved at a sufficiently large number of bands, to provide an unbiased PAR estimate. When is available at a small number of spectral bands, as for multispectral sensors, it is still possible to numerically integrate , but the estimation will contain errors. Here, we propose a method that delivers unbiased PAR estimates, based on two‐layer neural networks, formulable in a small number of matrix equations, and thus exportable to any software platform. The method was calibrated with a dataset of hyperspectral acquired by new types of BioGeoChemical (BGC)‐Argo floats deployed in a variety of open ocean locations, representative of a wide range of bio‐optical properties. This procedure was repeated for several band configurations, including those existing on multispectral radiometers presently the standard for the BGC‐Argo fleet. Validation results against independent data were highly satisfactory, displaying minimal uncertainties across a wide PAR range, with the performance varying as a function of each sensor configuration, overall supporting the operational implementation in the Argo program. Model codes are findable at https://github.com/jaipipor/PAR_BGC_Argo .

The ocean plays an essential role in regulating Earth’s climate, influencing weather conditions, providing sustenance for large populations, moderating anthropogenic climate change, encompassing massive biodiversity, and sustaining the global economy. Human activities are changing the oceans, stressing ocean health, threatening the critical services the ocean provides to society, with significant consequences for human well-being and safety, and economic prosperity. Effective and sustainable monitoring of the physical, biogeochemical state and ecosystem structure of the ocean, to enable climate adaptation, carbon management and sustainable marine resource management is urgently needed. The Argo program, a cornerstone of the Global Ocean Observing System (GOOS), has revolutionized ocean observation by providing real-time, freely accessible global temperature and salinity data of the upper 2,000m of the ocean (Core Argo) using cost-effective simple robotics. For the past 25 years, Argo data have underpinned many ocean, climate and weather forecasting services, playing a fundamental role in safeguarding goods and lives. Argo data have enabled clearer assessments of ocean warming, sea level change and underlying driving processes, as well as scientific breakthroughs while supporting public awareness and education. Building on Argo’s success, OneArgo aims to greatly expand Argo’s capabilities by 2030, expanding to full-ocean depth, collecting biogeochemical parameters, and observing the rapidly changing polar regions. Providing a synergistic subsurface and global extension to several key space-based Earth Observation missions and GOOS components, OneArgo will enable biogeochemical and ecosystem forecasting and new long-term climate predictions for which the deep ocean is a key component. Driving forward a revolution in our understanding of marine ecosystems and the poorly-measured polar and deep oceans, OneArgo will be instrumental to assess sea level change, ocean carbon fluxes, acidification and deoxygenation. Emerging OneArgo applications include new views of ocean mixing, ocean bathymetry and sediment transport, and ecosystem resilience assessment. Implementing OneArgo requires about $100 million annually, a significant increase compared to present Argo funding. OneArgo is a strategic and cost-effective investment which will provide decision-makers, in both government and industry, with the critical knowledge needed to navigate the present and future environmental challenges, and safeguard both the ocean and human wellbeing for generations to come.

Phytoplankton community composition significantly influences important biogeochemical processes, particularly the biological carbon pump. Assessing the global distribution and dynamics of main phytoplankton groups is therefore of the utmost importance. Taking advantage of the synoptic view of satellite ocean color and altimetry observations combined with vertically-resolved proles of chlorophyll fluorescence collected by the global BioGeoChemical-Argo (BGC-Argo) fleet, we previously developed a neural network-based approach to infer a global tridimensional (3D) gridded product of chlorophyll a (Chla), i.e. the SOCA-Chla method. Expanding upon SOCA-Chla, we introduce SOCA-PFT, a novel method for deriving a global 3D product of phytoplankton functional types (PFT). SOCA-PFT follows the same principle as SOCA-Chla but requires an initial step to enrich the training BGC-Argo database with the PFT information that would not otherwise be available. This step involves developing a neural network trained on a large-scale database of concurrent shipborne measurements of vertical proles of pigments determined by High Performance Liquid Chromatography (HPLC), fluorescence and temperature/salinity (T/S). Applied to the BGC-Argo database, this intermediate method yields a PFT-enriched BGC-Argo database, which is further matched up with satellite observations to train the SOCA-PFT method. The resulting global PFT product provides depth-resolved Chla associated with pico-, nano-, and microphytoplankton as well as concentrations of pigment biomarkers representing major phytoplankton groups. This new product is expected to be useful for various applications, from understanding the response of phytoplankton communities to environmental conditions, to improving the quantification of biogeochemical budgets or validating biogeochemical models that explicitly incorporate multiple phytoplankton groups.

The Arabian Sea is a highly productive tropical ecosystem of the Indian Ocean that supports high fluxes of particulate organic carbon to the mesopelagic zone from two distinct periods of elevated biological productivity associated with the semiannual reversals of the monsoonal wind system. There are now strong indications that the Arabian Sea's monsoonal wind patterns and hydrographic conditions are being impacted by long-term temperature increases, but the consequences of these changes on primary production and carbon export to the mesopelagic zone are unknown. This is especially true for the summer monsoon period when cloud cover obscures much of the Arabian Sea basin and therefore precludes remotely sensed ocean color measurements for estimating phytoplankton biomass and productivity. Here we overcome this limitation by using a database of bio-optical profiles from Biogeochemical Argo floats collected over the last decade to evaluate the impact of interannual temperature increases on Arabian Sea primary production and carbon export. We classify individual years of float observations based on the spatial extent of the Arabian Sea Mini Warm Pool that appears in the southeast Arabian Sea before the onset of the summer monsoon. This Mini Warm Pool, which begins to build in winter and collapses with the onset of the summer monsoon in late spring, has gained considerable interest on account of its influence on the timing of the onset of the summer monsoon. We observed a 35 percent decrease in primary production during the summer monsoon phytoplankton bloom in strong warm pool years, and a 13 percent decrease in particle stocks in the upper mesopelagic zone following the peak of the bloom. Decreases in production and export were additionally accompanied by a decrease in average particle size, indicating a shift from larger cells like diatoms that appear from fertilization of the oligotrophic waters to smaller phytoplankton size classes in response to a deepening of the thermocline and increased stratification of the water column. These results suggest changes in phytoplankton community structure and further decreases in primary production and carbon export in the Arabian Sea in response to future warming.

Human activities are causing a sustained increase in the concentration of carbon dioxide (CO2) and other greenhouse gases in the atmosphere. The resulting harmful effects on Earth’s climate require decarbonizing the economy and, given the slow pace and inherent limitations of decarbonization of some industries such as aviation, also the active removal and safe sequestration of CO2 away from the atmosphere (i.e., carbon dioxide removal or CDR; NASEM, 2022). Limiting global warming to 1.5°C—a target that may already have been exceeded—would require CDR on the order of 100–1000 Gt CO2 over the twenty-first century (IPCC, 2018).

The ocean’s meso- and submeso-scales (1-100 km, days to weeks) host features like filaments and eddies that have a key structuring effect on phytoplankton distribution, but that due to their ephemeral nature, are challenging to observe. This problem is exacerbated in regions with heavy cloud coverage and/or difficult access like the Southern Ocean, where observations of phytoplankton distribution by satellite are sparse, manned campaigns costly, and automated devices limited by power consumption. Here, we address this issue by considering high-resolution in-situ data from 18 bio-logging devices deployed on southern elephant seals ( Mirounga leonina) in the Kerguelen Islands between 2018 and 2020. These devices have submesoscale-resolving capabilities of light profiles due to the high spatio-temporal frequency of the animals’ dives (on average 1.1 +-0.6 km between consecutive dives, up to 60 dives per day), but observations of fluorescence are much coarser due to power constraints. Furthermore, the chlorophyll a concentrations derived from the (uncalibrated) bio-logging devices’ fluorescence sensors lack a common benchmark to properly qualify the data and allow comparisons of observations. By proposing a method based on functional data analysis, we show that a reliable predictor of chlorophyll a concentration can be constructed from light profiles (14 686 in our study). The combined use of light profiles and matchups with satellite ocean-color data enable effective (1) homogenization then calibration of the bio-logging devices’ fluorescence data and (2) filling of the spatial gaps in coarse-grained fluorescence sampling. The developed method improves the spatial resolution of the chlorophyll a field description from ~30 km to ~12 km. These results open the way to empirical study of the coupling between physical forcing and biological response at submesoscale in the Southern Ocean, especially useful in the context of upcoming high-resolution ocean-circulation satellite missions.

<div> <p><span data-contrast="none">Producing comprehensive information about the ocean has become a top priority to monitor and predict the ocean and climate change.</span><span data-contrast="none"> Complementary to ocean state estimate provided by modelling/assimilation systems, a multi observations-based approach is developed thought the Copernicus Marine Service MultiOBservation Thematic Assembly (</span><span data-contrast="auto">MOB TAC). Recent advances in data fusion techniques and use of machine-learning approach open the possibility of producing estimators of ocean physic and biogeochemistry (BGC) operationally, using input data from diverse sensors, satellites and in-situ programs.</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:160,&quot;335559740&quot;:259}"> </span></p> </div> <div> <p><span data-contrast="auto">MOB TAC provides the following multi observations products at global scale: </span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:60,&quot;335559740&quot;:259}"> </span></p> </div> <div> <p><span data-contrast="auto">Blue ocean</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:60,&quot;335559740&quot;:259}"> </span></p> </div> <div> <div> <ul> <li data-leveltext="" data-font="Wingdings" data-listid="1" data-list-defn-props="{&quot;335552541&quot;:1,&quot;335559684&quot;:-2,&quot;335559685&quot;:720,&quot;335559991&quot;:360,&quot;469769226&quot;:&quot;Wingdings&quot;,&quot;469769242&quot;:[9642],&quot;469777803&quot;:&quot;left&quot;,&quot;469777804&quot;:&quot;&quot;,&quot;469777815&quot;:&quot;hybridMultilevel&quot;}" aria-setsize="-1" data-aria-posinset="1" data-aria-level="1"><span data-contrast="auto">3D temperature, salinity, geopotential height and geostrophic current fields, both in near-real-time (NRT) and as long time series (REP=Reprocessing) in delayed-mode;</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></li> <li data-leveltext="" data-font="Wingdings" data-listid="1" data-list-defn-props="{&quot;335552541&quot;:1,&quot;335559684&quot;:-2,&quot;335559685&quot;:720,&quot;335559991&quot;:360,&quot;469769226&quot;:&quot;Wingdings&quot;,&quot;469769242&quot;:[9642],&quot;469777803&quot;:&quot;left&quot;,&quot;469777804&quot;:&quot;&quot;,&quot;469777815&quot;:&quot;hybridMultilevel&quot;}" aria-setsize="-1" data-aria-posinset="2" data-aria-level="1"><span data-contrast="auto">2D sea surface salinity and sea surface density fields, both in NRT and as REP;</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></li> <li data-leveltext="" data-font="Wingdings" data-listid="1" data-list-defn-props="{&quot;335552541&quot;:1,&quot;335559684&quot;:-2,&quot;335559685&quot;:720,&quot;335559991&quot;:360,&quot;469769226&quot;:&quot;Wingdings&quot;,&quot;469769242&quot;:[9642],&quot;469777803&quot;:&quot;left&quot;,&quot;469777804&quot;:&quot;&quot;,&quot;469777815&quot;:&quot;hybridMultilevel&quot;}" aria-setsize="-1" data-aria-posinset="3" data-aria-level="1"><span data-contrast="auto">2D total surface and near-surface currents, both in NRT and as REP;</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></li> <li data-leveltext="" data-font="Wingdings" data-listid="1" data-list-defn-props="{&quot;335552541&quot;:1,&quot;335559684&quot;:-2,&quot;335559685&quot;:720,&quot;335559991&quot;:360,&quot;469769226&quot;:&quot;Wingdings&quot;,&quot;469769242&quot;:[9642],&quot;469777803&quot;:&quot;left&quot;,&quot;469777804&quot;:&quot;&quot;,&quot;469777815&quot;:&quot;hybridMultilevel&quot;}" aria-setsize="-1" data-aria-posinset="4" data-aria-level="1"><span data-contrast="auto">3D Vertical velocity fields as REP;</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></li> <li data-leveltext="" data-font="Wingdings" data-listid="1" data-list-defn-props="{&quot;335552541&quot;:1,&quot;335559684&quot;:-2,&quot;335559685&quot;:720,&quot;335559991&quot;:360,&quot;469769226&quot;:&quot;Wingdings&quot;,&quot;469769242&quot;:[9642],&quot;469777803&quot;:&quot;left&quot;,&quot;469777804&quot;:&quot;&quot;,&quot;469777815&quot;:&quot;hybridMultilevel&quot;}" aria-setsize="-1" data-aria-posinset="5" data-aria-level="1"><span data-contrast="auto">L2Q and L4 sea surface salinity from SMOS in REP and NRT (only L2Q)</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></li> </ul> </div> </div> <div> <div> <p><span data-contrast="auto">Green ocean</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559685&quot;:0,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></p> </div> <div> <ul> <li data-leveltext="" data-font="Wingdings" data-listid="1" data-list-defn-props="{&quot;335552541&quot;:1,&quot;335559684&quot;:-2,&quot;335559685&quot;:720,&quot;335559991&quot;:360,&quot;469769226&quot;:&quot;Wingdings&quot;,&quot;469769242&quot;:[9642],&quot;469777803&quot;:&quot;left&quot;,&quot;469777804&quot;:&quot;&quot;,&quot;469777815&quot;:&quot;hybridMultilevel&quot;}" aria-setsize="-1" data-aria-posinset="1" data-aria-level="1"><span data-contrast="auto">2D surface carbon data sets of FCO2, pCO2, DIC, Alkalinity, saturation states of surface waters with respect to calcite and aragonite as REP;</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></li> <li data-leveltext="" data-font="Wingdings" data-listid="1" data-list-defn-props="{&quot;335552541&quot;:1,&quot;335559684&quot;:-2,&quot;335559685&quot;:720,&quot;335559991&quot;:360,&quot;469769226&quot;:&quot;Wingdings&quot;,&quot;469769242&quot;:[9642],&quot;469777803&quot;:&quot;left&quot;,&quot;469777804&quot;:&quot;&quot;,&quot;469777815&quot;:&quot;hybridMultilevel&quot;}" aria-setsize="-1" data-aria-posinset="2" data-aria-level="1"><span data-contrast="auto">Nutrient and Carbon vertical distribution (including Nitrates, Phosphates, Silicates, pH, pCO2, Alkalinity, DIC) profiles as REP and NRT;</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></li> <li data-leveltext="" data-font="Wingdings" data-listid="1" data-list-defn-props="{&quot;335552541&quot;:1,&quot;335559684&quot;:-2,&quot;335559685&quot;:720,&quot;335559991&quot;:360,&quot;469769226&quot;:&quot;Wingdings&quot;,&quot;469769242&quot;:[9642],&quot;469777803&quot;:&quot;left&quot;,&quot;469777804&quot;:&quot;&quot;,&quot;469777815&quot;:&quot;hybridMultilevel&quot;}" aria-setsize="-1" data-aria-posinset="3" data-aria-level="1"><span data-contrast="auto">3D Particulate Organic Carbon (POC), particulate backscattering coefficient (bbp) and Chlorophyll a (Chl-a) fields as REP.</span><span data-ccp-props="{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:200,&quot;335559740&quot;:276}"> </span></li> </ul> </div> <div> <p><span data-contrast="auto">Parallel to its portfolio, MOB TAC has and will further develop specific expertise about the integration of multiple satellites and in-situ based observations coming from the other CMEMS TACs and projects. </span><span data-contrast="none">Furthermore, MOB TAC provides specific Ocean Monitoring Indicators (OMIs), based on the above products, to monitor and the global ocean carbon sink. </span></p> </div> </div>

Biogeochemical- (BGC-) Argo aims to deploy and maintain a global array of autonomous profiling floats to monitor ocean biogeochemistry. With over 250,000 profiles collected so far, the BGC-Argo network is rapidly expanding toward the target of a sustained fleet of 1,000 floats. These floats prioritize the measurement of six key properties: oxygen, nitrate, pH, chlorophyll-a, suspended particles, and downwelling light. To assess the current biogeochemical state of the ocean, its variability, and trends with confidence, it is crucial to quality control these measurements. Accordingly, BGC-Argo maintains a quality control system using manual inspection and parameter-specific algorithms for flagging and adjusting data. In this study, we provide a census of the quantity and quality of measurements from BGC-Argo based on their quality flagging system. The purpose of this census is to assess the current status of the array in terms of data quality, how data quality has changed over time, and to provide a better understanding of the quality-controlled data to current and future users. Alongside increasing profile numbers and spatial coverage, we report that for most parameters between 80 and 95% of the profiles collected so far contain high-quality BGC data, with an exception for pH. The quality of pH profiles has seen a large improvement in the last five years and is on track to match the data quality of other BGC parameters. We highlight how BGC-Argo is improving and discuss strategies to increase the quality and quantity of BGC profiles available to users. This census shows that tracking percentages of high-quality data through time is useful for monitoring float sensor technology and helpful for ensuring the long-term success of BGC-Argo.

Abstract The gravitational sinking of particles in the mesopelagic layer (∼200–1,000 m) transfers to the deep ocean a part of atmospheric carbon fixed by phytoplankton. This process, called the gravitational pump, exerts an important control on atmospheric CO 2 levels but remains poorly characterized given the limited spatio‐temporal coverage of ship‐based flux measurements. Here, we examined the gravitational pump with BioGeoChemical‐Argo floats in the Southern Ocean, a critically under‐sampled area. Using time‐series of bio‐optical measurements, we characterized the concentration of particles in the productive zone, their export and transfer efficiency in the underlying mesopelagic zone, and the magnitude of sinking flux at 1,000 m. We separated float observations into six environments delineated by latitudinal fronts, sea‐ice coverage, and natural iron fertilization. Results show a significant increase in the sinking‐particle flux at 1,000 m with increasing latitude, despite comparable particle concentrations in the productive layer. The variability in deep flux was driven by changes in the transfer efficiency of the flux, related to the composition of the phytoplanktonic community and the size of particles, with intense flux associated with the predominance of micro‐phytoplankton and large particles at the surface. We quantified the relationships between the nature of surface particles and the flux at 1,000 m and used these results to upscale our flux survey across the whole Southern Ocean using surface observations by floats and satellites. We then estimated the basin‐wide Spring‐Summer flux of sinking particles at 1,000 m over the Southern Ocean (0.054 ± 0.021 Pg C).

The Underwater Vision Profiler (UVP) has been developed to study the number, size and shape of particles (size \textgreater 80µm) and plankton (size \textgreater 700µm) in situ. Over the last decade, thousands of profiles have been collected in the world's oceans by the UVP5 to better understand and quantify processes affecting community compositions of large plankton and the biological carbon pump. These data, used together with modeling approaches helped estimate plankton global carbon biomass and particle vertical flux. The most recent UVP (UVP6) sensors have been developed to be mounted on autonomous platforms, mooring and CTD rosettes down to 6000 m depth. Fully inter-calibrated, they record particles and identify plankton and marine snow after recovery or during deployment using an embedded recognition algorithm. A complete software ecosystem is used to pilot the instrument, record the data, and make them available to fulfill the global need of easy data access expressed by scientists, policy makers and the public. Because of the cost reduction of the UVP6, its capability to be mounted on many platforms including autonomous ones, the Ocean is being quickly populated by this sensor (125 sensors have been in operation in the last 2 years). Recent plankton community composition, particle mass, and flux data from three different basins in the Atlantic will be presented. In the next decade, the massive global monitoring of these key biological Essential Oceanographic Variables will significantly advance our understanding of key aquatic processes including the biological carbon pump.

The vertical distribution of light and its spectral composition are critical factors influencing numerous physical, chemical, and biological processes within the oceanic water column. In this study, we present vertically resolved models of downwelling irradiance (ED) at three different wavelengths and photosynthetically available radiation (PAR) on a global scale. These models rely on the SOCA (Satellite Ocean Color merged with Argo data to infer bio-optical properties to depth) methodology, which is based on an artificial neural network (ANN). The new light models are trained with light profiles (ED/PAR) acquired from BioGeoChemical-Argo (BGC-Argo) floats. The model inputs consist of surface ocean color radiometry data (i.e., Rrs, PAR, and kd(490)) derived by satellite and extracted from the GlobColour database, temperature and salinity profiles originating from BGC-Argo, as well as temporal components (day of the year and local time in cyclic transformation). The model outputs correspond to ED profiles at the three wavelengths of the BGC-Argo measurements (i.e., 380, 412, and 490 nm) and PAR profiles. We assessed the retrieval of light profiles by these light models using three different datasets: BGC-Argo profiles that were not used for the training (i.e., 20% of the initial database); data from four independent BGC-Argo floats that were used neither for the training nor for the 20% validation dataset; and the SeaBASS database (in situ data collected from various oceanic cruises). The light models show satisfactory predictions when thus compared with real measurements. From the 20% validation database, the light models retrieve light variables with high accuracies (root mean squared error (RMSE)) of 76.42 μmol quanta m−2 s−1 for PAR and 0.04, 0.08, and 0.09 W m−2 nm−1 for ED380, ED412, and ED490, respectively. This corresponds to a median absolute percent error (MAPE) that ranges from 37% for ED490 and PAR to 39% for ED380 and ED412. The estimated accuracy metrics across these three validation datasets are consistent and demonstrate the robustness and suitability of these light models for diverse global ocean applications.

Abstract. Numerical models of ocean biogeochemistry are becoming the major tools used to detect and predict the impact of climate change on marine resources and to monitor ocean health. However, with the continuous improvement of model structure and spatial resolution, incorporation of these additional degrees of freedom into fidelity assessment has become increasingly challenging. Here, we propose a new method to provide information on the model predictive skill in a concise way. The method is based on the conjoint use of a k-means clustering technique, assessment metrics, and Biogeochemical-Argo (BGC-Argo) observations. The k-means algorithm and the assessment metrics reduce the number of model data points to be evaluated. The metrics evaluate either the model state accuracy or the skill of the model with respect to capturing emergent properties, such as the deep chlorophyll maximums and oxygen minimum zones. The use of BGC-Argo observations as the sole evaluation data set ensures the accuracy of the data, as it is a homogenous data set with strict sampling methodologies and data quality control procedures. The method is applied to the Global Ocean Biogeochemistry Analysis and Forecast system of the Copernicus Marine Service. The model performance is evaluated using the model efficiency statistical score, which compares the model–observation misfit with the variability in the observations and, thus, objectively quantifies whether the model outperforms the BGC-Argo climatology. We show that, overall, the model surpasses the BGC-Argo climatology in predicting pH, dissolved inorganic carbon, alkalinity, oxygen, nitrate, and phosphate in the mesopelagic and the mixed layers as well as silicate in the mesopelagic layer. However, there are still areas for improvement with respect to reducing the model–data misfit for certain variables such as silicate, pH, and the partial pressure of CO2 in the mixed layer as well as chlorophyll-a-related, oxygen-minimum-zone-related, and particulate-organic-carbon-related metrics. The method proposed here can also aid in refining the design of the BGC-Argo network, in particular regarding the regions in which BGC-Argo observations should be enhanced to improve the model accuracy via the assimilation of BGC-Argo data or process-oriented assessment studies. We strongly recommend increasing the number of observations in the Arctic region while maintaining the existing high-density of observations in the Southern Oceans. The model error in these regions is only slightly less than the variability observed in BGC-Argo measurements. Our study illustrates how the synergic use of modeling and BGC-Argo data can both provide information about the performance of models and improve the design of observing systems.

Phytoplankton diversity affects marine ecosystems and biogeochemical functioning ▪ Phytoplankton community composition is a major driver of carbon fluxes (e.g.

<p>To better understand the global ocean biogeochemical processes, it is crucial to strengthen the spatial coverage of high-quality biogeochemical variables. In this context, we provide high-quality nutrients (nitrate, phosphate and silicate) and carbonate system variables (total alkalinity, dissolved inorganic carbon, pH and partial pressure of carbon dioxide) profiles for BGC-Argo floats equipped with oxygen sensors and data qualified in delayed mode. These variables are derived using neural network models called CANYON-B and CONTENT for nutrients and carbonate system variables, respectively. For the Mediterranean Sea, we deliver these variables from a regional dedicated model called CANYON-MED. These variables are distributed from September 2002 to August 2022 as part of CMEMS MOBTAC service. The last update of the product will be available in CMEMS portal from March 2023.</p> <p>At the global scale, nitrate, phosphate and silicate are retrieved with an accuracy (from the root mean squared difference) of 0.68, 0.051, 2.3 µmol kg^-1_, respectivelyand the carbonate system variables, i.e., total alkalinity, dissolved inorganic carbon, pH and partial pressure of carbon dioxide are retrieved with an accuracy of 6.2 µmol kg^-1, 6.9 µmol kg^-1, 0.013 (unitless), and 15 µatm, respectively. The global models (CANYON-B and CONTENT) have also been validated with independent data collected from recent various oceanic cruises not used for the development of the methods (from GLODAPv2.2021 database) and the Hawaii Ocean Time series (HOT). These independent validations demonstrate the validity of these models for global ocean applications. The nitrate and pH products were again validated against measured nitrate and pH from BGC-Argo floats equipped with oxygen sensors. Overall validation results were quite satisfactory at global and regional spatial scales.</p> <p>Currently, the profiles are available for BGC-Argo floats with concurrent profiles of temperature, salinity and oxygen qualified in delayed mode. This product will be also available from near real-time observations from 2024.</p>

Abstract OneArgo is a major expansion of the Argo program, which has provided two decades of transformative physical data for the upper 2 km of the global ocean. The present Argo array will be expanded in three ways: (1) Global Core: the existing upper ocean measurements will be extended to high latitudes and marginal seas and with enhanced coverage in the tropics and western boundaries of the major ocean basins; (2) Deep: deep ocean measurements will be obtained for the 50% of the global oceans that are below 2,000-m depth; and (3) Biogeochemical: dissolved oxygen, pH, nitrate, chlorophyll, optical backscatter, and irradiance data will be collected to investigate biogeochemical variability of the upper ocean and the processes by which these cycles respond to a changing climate. The technology and infrastructure necessary for this expansion is now being developed through large-scale regional pilots to further refine the floats and sensors and to demonstrate the utility of these measurements. Further innovation is expected to improve the performance of the floats and sensors and to develop the analyses necessary to provide research-quality data. A fully global OneArgo should be operational within 5‐10 years.

Background: Biogeochemical-Argo floats are collecting an unprecedented number of profiles of optical backscattering measurements in the global ocean. Backscattering (BBP) data are crucial to understanding ocean particle dynamics and the biological carbon pump. Yet, so far, no procedures have been agreed upon to quality control BBP data in real time. Methods: Here, we present a new suite of real-time quality-control tests and apply them to the current global BBP Argo dataset. The tests were developed by expert BBP users and Argo data managers and have been implemented on a snapshot of the entire Argo dataset. Results: The new tests are able to automatically flag most of the “bad” BBP profiles from the raw dataset. Conclusions: The proposed tests have been approved by the Biogeochemical-Argo Data Management Team and will be implemented by the Argo Data Assembly Centres to deliver real-time quality-controlled profiles of optical backscattering. Provided they reach a pressure of about 1000 dbar, these tests could also be applied to BBP profiles collected by other platforms.

Influence of the phytoplankton community composition on the in situ fluorescence signal: Implication for an improved estimation of the chlorophyll-a concentration from BiogeoChemical-Argo profiling floats.

Abstract. The Green Edge project was designed to investigate the onset, life and fate of a phytoplankton spring bloom (PSB) in the Arctic Ocean. The lengthening of the ice-free period and the warming of seawater, amongst other factors, have induced major changes in arctic ocean biology over the last decades. Because the PSB is at the base of the Arctic Ocean food chain, it is crucial to understand how changes in the arctic environment will affect it. Green Edge was a large multidisciplinary collaborative project bringing researchers and technicians from 28 different institutions in seven countries, together aiming at understanding these changes and their impacts into the future. The fieldwork for the Green Edge project took place over two years (2015 and 2016) and was carried out from both an ice-camp and a research vessel in the Baffin Bay, canadian arctic. This paper describes the sampling strategy and the data set obtained from the research cruise, which took place aboard the Canadian Coast Guard Ship (CCGS) Amundsen in spring 2016. The dataset is available at https://doi.org/10.17882/59892 (Massicotte et al., 2019a).

Biogeographical classifications of the global ocean generalize spatiotemporal trends in species or biomass distributions across discrete ocean biomes or provinces. These classifications are generally based on a combination of remote-sensed proxies of phytoplankton biomass and global climatologies of biogeochemical or physical parameters. However, these approaches are limited in their capacity to account for subsurface variability in these parameters. The deployment of autonomous profiling floats in the Biogeochemical Argo network over the last decade has greatly increased global coverage of subsurface measurements of bio-optical proxies for phytoplankton biomass and physiology. In this study, we used empirical orthogonal function analysis to identify the main components of variability in a global data set of 422 annual time series of Chlorophyll a fluorescence and optical backscatter profiles. Applying cluster analysis to these results, we identified six biomes within the global ocean: two high-latitude biomes capturing summer bloom dynamics in the North Atlantic and Southern Ocean and four mid- and low-latitude biomes characterized by variability in the depth and frequency of deep chlorophyll maximum formation. We report the distribution of these biomes along with associated trends in biogeochemical and physicochemical environmental parameters. Our results demonstrate light and nutrients to explain most variability in phytoplankton distributions for all biomes, while highlighting a global inverse relationship between particle stocks in the euphotic zone and transfer efficiency into the mesopelagic zone. In addition to partitioning seasonal variability in vertical phytoplankton distributions at the global scale, our results provide a potentially novel biogeographical classification of the global ocean.

Fluorescence is a practical method implemented in the BioGeoChemical-Argo (BGC-Argo) network for estimating the chlorophyll a concentration (Chla), a widely used proxy of phytoplankton biomass. Despite a strong correlation between the Chla and fluorescence signal on restricted spatial and temporal scales, large regional variations with a clear latitudinal gradient in the Chla-to-fluorescence ratio, referred to as “slope factor”, have been observed in the global ocean. This indicates the potential influence of phytoplankton community composition, resulting from the combined effects of phytoplankton absorption and quantum yield of fluorescence. As phytoplankton communities play a key role in global biogeochemical cycles, it is critical to understand their variability to accurately determine Chla. In order to examine the role of phytoplankton community composition on the fluorescence signal, we used a global concurrent dataset of Chla estimated from BGC-Argo float fluorescence measurements and High-Performance Liquid Chromatography determinations, as well as phytoplankton absorption measurements. The community composition of phytoplankton shows a strong influence on absorption, with smaller cells characterized by reduced package effect and high absorption by accessory pigments in the blue spectral region. The quantum yield of fluorescence presents a clear trend with lower values in oligotrophic areas than in high latitude regions. In oligotrophic regions, picophytoplankton exhibit low values of the fluorescence quantum yield, which we attribute to the non-photosynthetic pigment zeaxanthin. The present work, showing that the slope factor is significantly correlated to the size structure of phytoplankton communities, is a first step towards a better estimation of Chla from BGC-Argo floats. Different methods have been proposed for assessing community size structure from BGC-Argo floats and thus could be used to better constrain the calibration of fluorescence in Chla.

A global in situ data set for validation of ocean colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI) is presented. This version of the compilation, starting in 1997, now extends to 2021, which is important for the validation of the most recent satellite optical sensors such as Sentinel 3B OLCI and NOAA-20 VIIRS. The data set comprises in situ observations of the following variables: spectral remote-sensing reflectance, concentration of chlorophyll-a, spectral inherent optical properties, spectral diffuse attenuation coefficient, and total suspended matter. Data were obtained from multi-project archives acquired via open internet services or from individual projects acquired directly from data providers. Methodologies were implemented for homogenization, quality control, and merging of all data. Minimal changes were made on the original data, other than conversion to a standard format, elimination of some points, after quality control and averaging of observations that were close in time and space. The result is a merged table available in text format. Overall, the size of the data set grew with 148 432 rows, with each row representing a unique station in space and time (cf. 136 250 rows in previous version; Valente et al., 2019). Observations of remote-sensing reflectance increased to 68 641 (cf. 59 781 in previous version; Valente et al., 2019). There was also a near tenfold increase in chlorophyll data since 2016. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) are included in the final table. By making the metadata available, provenance is better documented and it is also possible to analyse each set of data separately. The compiled data are available at https://doi.org/10.1594/PANGAEA.941318 (Valente et al., 2022).

This study assesses marine community production based on the diel variability of bio-optical properties monitored by two BioGeoChemical-Argo (BGC-Argo) floats. Experiments were conducted in two distinct Mediterranean systems, the northwestern Ligurian Sea and the central Ionian Sea, during summer months. We derived particulate organic carbon (POC) stock and gross community production integrated within the surface, euphotic and subsurface chlorophyll maximum (SCM) layers, using an existing approach applied to diel cycle measurements of the particulate beam attenuation (c_p) and backscattering (b_bp) coefficients. The diel cycle of c_p provided a robust proxy for quantifying biological production in both systems; that of b_bp was comparatively less robust. Derived primary production estimates vary by a factor of 2 depending upon the choice of the bio-optical relationship that converts the measured optical coefficient to POC, which is thus a critical step to constrain. Our results indicate a substantial contribution to the water column production of the SCM layer (16 %-42 %), which varies largely with the considered system. In the Ligurian Sea, the SCM is a seasonal feature that behaves as a subsurface biomass maximum (SBM) with the ability to respond to episodic abiotic forcing by increasing production. In contrast, in the Ionian Sea, the SCM is permanent, primarily induced by phytoplankton photoacclimation, and contributes moderately to water column production. These results clearly demonstrate the strong potential for transmissometers deployed on BGC-Argo profiling floats to quantify non-intrusively in situ biological production of organic carbon in the water column of stratified oligotrophic systems with recurring or permanent SCMs, which are widespread features in the global ocean.

Oceanic particulate organic carbon (POC) is a small but dynamic component of the global carbon cycle. Biogeochemical models historically focused on reproducing the sinking flux of POC driven by large fast-sinking particles (LPOC). However, suspended and slow-sinking particles (SPOC, here < 100 µm) dominate the total POC (TPOC) stock, support a large fraction of microbial respiration, and can make sizable contributions to vertical fluxes. <P />Recent developments in the parameterization of POC reactivity in PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies model; PISCESv2_RC) have improved its ability to capture POC dynamics. Here we evaluated this model by matching a global 3D simulation and 1D simulations at 50 different locations with observations made from biogeochemical (BGC-) Argo floats and satellites. Our evaluation covers globally representative biomes between 0 and 1000 m depth and relies on (1) a refined scheme for converting particulate backscattering at 700 nm (b<SUB>bp700</SUB>) to POC, based on biome-dependent POC / b<SUB>bp700</SUB> ratios in the surface layer that decrease to an asymptotic value at depth; (2) a novel approach for matching annual time series of BGC-Argo vertical profiles to PISCES 1D simulations forced by pre-computed vertical mixing fields; and (3) a critical evaluation of the correspondence between in situ measurements of POC fractions, PISCES model tracers, and SPOC and LPOC estimated from high vertical resolution b<SUB>bp700</SUB> profiles through a separation of the baseline and spike signals. <P />We show that PISCES captures the major features of SPOC and LPOC across a range of spatiotemporal scales, from highly resolved profile time series to biome-aggregated climatological profiles. Model-observation agreement is usually better in the epipelagic (0-200 m) than in the mesopelagic (200-1000 m), with SPOC showing overall higher spatiotemporal correlation and smaller deviation (typically within a factor of 1.5). Still, annual mean LPOC stocks estimated from PISCES and BGC-Argo are highly correlated across biomes, especially in the epipelagic (r=0.78; n=50). Estimates of the SPOC / TPOC fraction converge around a median of 85 % (range 66 %-92 %) globally. Distinct patterns of model-observations misfits are found in subpolar and subtropical gyres, pointing to the need to better resolve the interplay between sinking, remineralization, and SPOC-LPOC interconversion in PISCES. Our analysis also indicates that a widely used satellite algorithm overestimates POC severalfold at high latitudes during the winter. The approaches proposed here can help constrain the stocks, and ultimately budgets, of oceanic POC.

Summertime wildfire activity is increasing in boreal forest and tundra ecosystems in the Northern Hemisphere. However, the impact of long range transport and deposition of wildfire aerosols on biogeochemical cycles in the Arctic Ocean is unknown. Here, we use satellite-based ocean color data, atmospheric modeling and back trajectory analysis to investigate the transport and fate of aerosols emitted from Siberian wildfires in summer 2014 and their potential impact on phytoplankton dynamics in the Arctic Ocean. We detect large phytoplankton blooms near the North Pole (up to 82°N in the eastern Eurasian Basin). Our analysis indicates that these blooms were induced by the northward plume transport and deposition of nutrient-bearing wildfire aerosols. We estimate that these highly stratified surface waters received large amounts of wildfire-derived nitrogen, which alleviated nutrient stress in the phytoplankton community and triggered an unusually large bloom event. Our findings suggest that changes in wildfire activity may strongly influence summertime productivity in the Arctic Ocean.

Autonomous and cabled platforms are revolutionizing our understanding of ocean systems by providing 4D monitoring of the water column, thus going beyond the reach of ship-based surveys and increasing the depth of remotely sensed observations. However, very few commercially available sensors for such platforms are capable of monitoring large particulate matter (100-2000 μm) and plankton despite their important roles in the biological carbon pump and as trophic links from phytoplankton to fish. Here, we provide details of a new, commercially available scientific camera-based particle counter, specifically designed to be deployed on autonomous and cabled platforms: the Underwater Vision Profiler 6 (UVP6). Indeed, the UVP6 camera-and-lighting and processing system, while small in size and requiring low power, provides data of quality comparable to that of previous much larger UVPs deployed from ships. We detail the UVP6 camera settings, its performance when acquiring data on aquatic particles and plankton, their quality control, analysis of its recordings, and streaming from in situ acquisition to users. In addition, we explain how the UVP6 has already been integrated into platforms such as BGC-Argo floats, gliders and long-term mooring systems (autonomous platforms). Finally, we use results from actual deployments to illustrate how UVP6 data can contribute to addressing longstanding questions in marine science, and also suggest new avenues that can be explored using UVP6-equipped autonomous platforms.

The Green Edge project was designed to investigate the onset, life, and fate of a phytoplankton spring bloom (PSB) in the Arctic Ocean. The lengthening of the ice-free period and the warming of seawater, amongst other factors, have induced major changes in Arctic Ocean biology over the last decades. Because the PSB is at the base of the Arctic Ocean food chain, it is crucial to understand how changes in the Arctic environment will affect it. Green Edge was a large multidisciplinary, collaborative project bringing researchers and technicians from 28 different institutions in seven countries together, aiming at understanding these changes and their impacts on the future. The fieldwork for the Green Edge project took place over two years (2015 and 2016) and was carried out from both an ice camp and a research vessel in Baffin Bay, in the Canadian Arctic. This paper describes the sampling strategy and the dataset obtained from the research cruise, which took place aboard the Canadian Coast Guard ship (CCGS) Amundsen in late spring and early summer 2016. The sampling strategy was designed around the repetitive, perpendicular crossing of the marginal ice zone (MIZ), using not only ship-based station discrete sampling but also high-resolution measurements from autonomous platforms (Gliders, BGC-Argo floats …) and under-way monitoring systems. The dataset is available at https://doi.org/10.17882/86417 (Bruyant et al., 2022).

As part of Copernicus Marine Environmental Monitoring Service (CMEMS), the multi-observations thematic assembly center aims to provide products based on observations and data fusion techniques (Guinehut et al., 2021). Sauzede et al., (2016) have demonstrated the potential of using hydrological measurements and ocean color satellite observations to infer the vertical distribution of backscattering coeffi cient, a proxy for the stock of particulate organic carbon (POC). The 'Satellite Ocean-Color merged with Argo data to infer bio-optical properties to depth' (SOCA) method is a neural-network-based method trained using the Biogeochemical-Argo database. SOCA has been upgraded to improve the POC retrieval and additionally retrieve the chlorophyll-a concentration (Chl). Using this method with CMEMS hydrological and satellite products, weekly 3-dimensional fi elds of POC and associated uncertainty were retrieved for the 1998-2018 period and made available from the CMEMS online portal since July 2020. The 3-dimensional products of SOCA-retrieved Chl will be made available by the end of 2021. Both of these products will be updated yearly as new input data become available. These new CMEMS products represent a most valuable source of data useful not only for supporting the quality control of Biogeochemical-Argo fl oat observations but also for data assimilation and initialization/validation of biogeochemical models.

Abstract. Upper suboxic water masses confine a majority of the microbial communities that can produce up to 90 % of oceanic N2. This effective N2-yielding section encloses a suspended small-particle layer, inferred from particle backscattering (bbp) measurements. It is thus hypothesized that this layer (hereafter, the bbp-layer) is linked to N2-yielding microbial communities such as anammox and denitrifying bacteria – a hypothesis yet to be evaluated. Here, data collected by three BGC-Argo floats deployed in the Black Sea are used to investigate the origin of this bbp-layer. To this end, we evaluate how key drivers of anammox-denitrifying bacteria dynamics impact on the vertical distribution of bbp and the thickness of the bbp-layer. In conjunction with published data on N2 excess, our results suggest that the bbp-layer is at least partially composed of anammox-denitrifying bacteria for three main reasons: (1) strong correlations are recorded between bbp and nitrate; (2) the top location of the bbp-layer is driven by the ventilation of oxygen-rich subsurface waters, while its thickness is modulated by the amount of nitrate available to produce N2; (3) the maxima of both bbp and N2 excess coincide at the same isopycnals where denitrifying-anammox bacteria coexist. We thus advance that bbp and O2 can be exploited as a combined proxy to delineate the N2-yielding section of the Black Sea. This proxy can potentially contribute to refining delineation of the effective N2-yielding section of oxygen-deficient zones via data from the growing BGC-Argo float network.

The MALINA oceanographic campaign was conducted during summer 2009 to investigate the carbon stocks and the processes controlling the carbon fluxes in the Mackenzie River estuary and the Beaufort Sea. Dur- ing the campaign, an extensive suite of physical, chemical and biological variables was measured across seven shelf–basin transects (south-north) to capture the meridional gradient between the estuary and the open ocean.Key variables such as temperature, absolute salinity, radiance, irradiance, nutrient concentrations, chlorophyll-a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, and carbon stocks and fluxes were routinely measured onboard the Canadian research icebreaker CCGS Amundsen and from a barge in shallow coastal areas or for sampling within broken ice fields. Here, we present the results of a joint effort to tidy and standardize the collected data sets that will facilitate their reuse in further studies of the changing Arctic Ocean.

Stratified oceanic systems are characterized by the presence of a so-called Deep Chlorophyll a Maximum (DCM) not detectable by ocean color satellites. A DCM can either be a phytoplankton (carbon) biomass maximum (Deep Biomass Maximum, DBM), or the consequence of photoacclimation processes (Deep photoAcclimation Maximum, DAM) resulting in the increase of chlorophyll a per phytoplankton carbon. Even though these DCM (further qualified as either DBMs or DAMs) have long been studied, no global-scale assessment has yet been undertaken and large knowledge gaps still remain in relation to the environmental drivers responsible for their formation and maintenance. In order to investigate their spatial and temporal variability in the open ocean, we use a global data set acquired by more than 500 Biogeochemical-Argo floats given that DCMs can be detected from the comparative vertical distribution of chlorophyll a concentrations and particulate backscattering coefficients. Our findings show that the seasonal dynamics of the DCMs are clearly region-dependent. High-latitude environments are characterized by a low occurrence of intense DBMs, restricted to summer. Meanwhile, oligotrophic regions host permanent DAMs, occasionally replaced by DBMs in summer, while subequatorial waters are characterized by permanent DBMs benefiting from favorable conditions in terms of both light and nutrients. Overall, the appearance and depth of DCMs are primarily driven by light attenuation in the upper layer. Our present assessment of DCM occurrence and of environmental conditions prevailing in their development lay the basis for a better understanding and quantification of their role in carbon budgets (primary production and export).

Abstract. Numerical models of ocean biogeochemistry are becoming a major tool to detect and predict the impact of climate change on marine resources and ocean health. Classically, validation of such models relies on comparison with surface quantities from satellite (such as chlorophyll-a concentrations), climatologies, or sparse in situ data (such as cruises observations, and permanent fixed oceanic stations). However, these datasets are not fully suitable to assess how models represent many climate-relevant biogeochemical processes. These limitations now begin to be overcome with the availability of a large number of vertical profiles of light, pH, oxygen, nitrate, chlorophyll-a concentrations and particulate backscattering acquired by the Biogeochemical-Argo (BGC-Argo) floats network. Additionally, other key biogeochemical variables such as dissolved inorganic carbon and alkalinity, not measured by floats, can be predicted by machine learning-based methods applied to float oxygen concentrations. Here, we demonstrate the use of the global array of BGC-Argo floats for the validation of biogeochemical models at the global scale. We first present 18 key metrics of ocean health and biogeochemical functioning to quantify the success of BGC model simulations. These metrics are associated with the air-sea CO2 flux, the biological carbon pump, oceanic pH, oxygen levels and Oxygen Minimum Zones (OMZs). The metrics are either a depth-averaged quantity or correspond to the depth of a particular feature. We also suggest four diagnostic plots for displaying such metrics.

Measuring the underwater light field is a key mission of the international Biogeochemical-Argo program. Since 2012, 0–250 dbar profiles of downwelling irradiance at 380, 412 and 490 nm besides photosynthetically available radiation (PAR) have been acquired across the globe every 1 to 10 days. The resulting unprecedented amount of radiometric data has been previously quality-controlled for real-time distribution and ocean optics applications, yet some issues affecting the accuracy of measurements at depth have been identified such as changes in sensor dark responsiveness to ambient temperature, with time and according to the material used to build the instrument components. Here, we propose a quality-control procedure to solve these sensor issues to make Argo radiometry data available for delayed-mode distribution, with associated error estimation. The presented protocol requires the acquisition of ancillary radiometric measurements at the 1000 dbar parking depth and night-time profiles. A test on >10,000 profiles from across the world revealed a quality-control success rate >90% for each band. The procedure shows similar performance in re-qualifying low radiometry values across diverse oceanic regions. We finally recommend, for future deployments, acquiring daily 1000 dbar measurements and one night profile per year, preferably during moonless nights and when the temperature range between the surface and 1000 dbar is the largest.

A recent paradigm explains that the downward pumping of biogenic carbon in the ocean is performed by the combined action of six different biological carbon pumps (BCPs): the biological gravitational pump, the physically driven pumps (Mixed Layer Pump, Eddy Subduction Pump and Large-scale Subduction Pump), and the animal-driven pumps (diurnal and seasonal vertical migrations of zooplankton and larger animals). Here, we propose a research community approach to implement the new paradigm through the integrated study of these BCPs in the World Ocean. The framework to investigate the BCPs combines measurements from different observational platforms, i.e. , oceanographic ships, satellites, moorings, and robots (gliders, floats, and robotic surface vehicles such as wavegliders and saildrones). We describe the following aspects of the proposed research framework: variables and processes to be measured in both the euphotic and twilight zones for the different BCPs; spatial and temporal scales of occurrence of the various BCPs; selection of key regions for integrated studies of the BCPs; multi-platform observational strategies; and upscaling of results from regional observations to the global ocean using deterministic models combined with data assimilation and machine learning to make the most of the wealth of unique measurements. The proposed approach has the potential not only to bring together a large multidisciplinary community of researchers, but also to usher the community toward a new era of discoveries in ocean sciences.

Complementary to ocean state estimates provided by modelling/assimilation systems, a multi observations-based approach is available through the MULTI OBSERVATIONS (MULTIOBS) Thematic Assembly Center (TAC) of the European Copernicus Marine Environment Monitoring Service (CMEMS). CMEMS MULTIOBS TAC provides multi observation-based ocean products at global scale derived from the combination of two or more different sensors from satellite and in situ, and using state-of-the-art data fusion techniques. These products cover the blue ocean for physics and the green ocean for the carbonate system and biogeochemical variables. MULTIOBS products are available in Near-Real-Time (NRT) or as Multi-Year Products (MYP) for the past 25 to 35 years with regular temporal extensions. MULTIOBS TAC provides also associated Ocean Monitoring Indicators (OMIs). It uses mostly inputs from other CMEMS TACs.

We deployed sensors for physical and biogeochemical measurements on one Eulerian mooring and two Lagrangian biogeochemical Argo-floats on the Kerguelen Plateau. High temporal and vertical resolution measurements revealed an abrupt shoaling of both the mixed-layer depth and mixing-layer depth. The sudden stratification was concomitant with the start of significant biological activity detected by chlorophyll-a accumulation, oxygen oversaturation and dissolved inorganic carbon drawdown. The net community production computed in the mixing-layer during the onset period of 9 days was 119±7 mmol m-2 d-1. While it is generally admitted that bloom initiation is mostly driven by the onset of positive heat fluxes, our results suggest this is not a sufficient condition. Here we report that the decrease in the depth over which wind mixes the upper layer drives the initiation of the bloom. These results suggest that future atmospheric changes in Southern Ocean could impact the phenology of the blooms.

The Green Edge initiative was developed to investigate the processes controlling the primary productivity and the fate of organic matter produced during the Arctic phytoplankton spring bloom (PSB) and to determine its role in the ecosystem. Two field campaigns were conducted in 2015 and 2016 at an ice camp located on landfast sea ice southeast of Qikiqtarjuaq Island in Baffin Bay (67.4797N, 63.7895W). During both expeditions, a large suite of physical, chemical and biological variables was measured beneath a consolidated sea ice cover from the surface to the bottom at 360 m depth to better understand the factors driving the PSB. Key variables such as temperature, salinity, radiance, irradiance, nutrient concentrations, chlorophyll-a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, carbon stocks and fluxes were routinely measured at the ice camp. Here, we present the results of a joint effort to tidy and standardize the collected data sets that will facilitate their reuse in other Arctic studies. The dataset is available at http://www.seanoe.org/data/00487/59892/ (Massicotte et al., 2019a).

The shallower oxygen-poor water masses of the ocean confine a majority of the microbial communities that can produce up to 90 % of oceanic N 2. This effective N 2yielding section encloses a suspended small-particle layer, inferred from particle backscattering (b bp) measurements. It is thus hypothesized that this layer (hereafter, the b bp-layer) is linked to microbial communities involved in N 2 yielding such as nitrate-reducing SAR11 as well as sulfur-oxidizing, anammox, and denitrifying bacteria-a hypothesis yet to be evaluated. Here, data collected by three BGC-Argo floats deployed in the Black Sea are used to investigate the origin of this b bp-layer. To this end, we evaluate how the key drivers of N 2-yielding bacteria dynamics impact the vertical distribution of b bp and the thickness of the b bp-layer. In conjunction with published data on N 2 excess, our results suggest that the b bp-layer is at least partially composed of the bacteria driving N 2 yielding for three main reasons: (1) strong correlations are recorded between b bp and nitrate; (2) the top location of the b bp-layer is driven by the ventilation of oxygen-rich subsurface waters, while its thickness is modulated by the amount of nitrate available to produce N 2 ; and (3) the maxima of both b bp and N 2 excess coincide at the same isopycnals where bacteria involved in N 2 yielding coexist. We thus advance that b bp and O 2 can be exploited as a combined proxy to delineate the N 2-yielding section of the Black Sea. This proxy can potentially contribute to refining delineation of the effective N 2-yielding section of oxygendeficient zones via data from the growing BGC-Argo float network.

The international array of profiling floats known as Argo is a major component of the global ocean-and climate-observing system. In 2010, the NAOS (Novel Argo Observing System) project was selected as part of France's Equipex "Investissement d'Avenir" program. The objectives of NAOS were to consolidate the French contribution to the Argo core mission (global temperature and salinity measurements down to 2,000 m) as well as to develop the future generation of French Argo profiling floats and prepare the next phase of the Argo program with an extension to the deep ocean (Deep-Argo), biogeochemistry (BGC-Argo) and polar seas. This paper summarizes the main technological advances and at-sea validations carried out as part of NAOS: development of a deep (4,000 m) float, a new BGC float for Research & Development (R&D) applications, and a BGC float for deployments in Arctic areas, assessment of a new density and Absolute Salinity optical sensor, improvement of the reliability of the standard Argo float, and upgraded satellite-transmission performance. French profiling floats developed in this way are now operational and among the most deployed worldwide, and the density sensor is the most promising of its kind for profiling floats applications.

During the North Atlantic Aerosols and Marine Ecosystems Study in the western North Atlantic, float‐based profiles of fluorescent dissolved organic matter and backscattering exhibited distinct spike layers at ∼ 300 m. The locations of the spikes were at depths similar or shallower to where a ship‐based scientific echo sounder identified layers of acoustic backscatter, an Underwater Vision Profiler detected elevated concentration of zooplankton, and mesopelagic fish were sampled by a mesopelagic net tow. The collocation of spike layers in bio‐optical properties with mesopelagic organisms suggests that some can be detected with float‐based bio‐optical sensors. This opens the door to the investigation of such aggregations/layers in observations collected by the global biogeochemical‐Argo array allowing the detection of mesopelagic organisms in remote locations of the open ocean under‐sampled by traditional methods.

A regional neural network-based method, "CANYON-MED" is developed to estimate nutrients and carbonate system variables specifically in the Mediterranean Sea over the water column from pressure, temperature, salinity, and oxygen together with geolocation and date of sampling. Six neural network ensembles were developed, one for each variable (i.e., three macronutrients: nitrates (NO − 3), phosphates (PO 3− 4) and silicates (SiOH 4), and three carbonate system variables: pH on the total scale (pH T), total alkalinity (A T), and dissolved inorganic carbon or total carbon (C T), trained using a specific quality-controlled dataset of reference "bottle" data in the Mediterranean Sea. This dataset is representative of the peculiar conditions of this semi-enclosed sea, as opposed to the global ocean. For each variable, the neural networks were trained on 80% of the data chosen randomly and validated using the remaining 20%. CANYON-MED retrieved the variables with good accuracies (Root Mean Squared Error): 0.73 µmol.kg −1 for NO − 3 , 0.045 µmol.kg −1 for PO 3− 4 and 0.70 µmol.kg −1 for Si(OH) 4 , 0.016 units for pH T , 11 µmol.kg −1 for A T and 10 µmol.kg −1 for C T. A second validation on the ANTARES independent time series confirmed the method's applicability in the Mediterranean Sea. After comparison to other existing methods to estimate nutrients and carbonate system variables, CANYON-MED stood out as the most robust, using the aforementioned inputs. The application of CANYON-MED on the Mediterranean Sea data from autonomous observing systems (integrated network of Biogeochemical-Argo floats, Eulerian moorings and ocean gliders measuring hydrological properties together with oxygen concentration) could have a wide range of applications. These include data quality control or filling gaps in time series, as well as biogeochemical data assimilation and/or the initialization and validation of regional biogeochemical models still lacking crucial reference data. Matlab and R code are available at https:// github.com/MarineFou/CANYON-MED/.

Argo, the international array of profiling floats, is a major component of the global ocean and climate observing system. In 2010, the NAOS (Novel Argo Observing System) project was selected as part of the French "Investissements d'Avenir" Equipex program. The objectives of NAOS were to consolidate the French contribution to Argo's core mission (global temperature and salinity measurements down to 2000 m), and also to develop the future generation of French Argo profiling floats and prepare the next phase of the Argo program with an extension to the deep ocean (Deep Argo), biogeochemistry (BGC-Argo) and polar seas. This paper summarizes how NAOS has met its objectives. The project significantly boosted France's contribution to Argo's core mission by deploying more than 100 NAOS standard Argo profiling floats. In addition, NAOS deployed new-generation floats as part of three scientific experiments: biogeochemical floats in the Mediterranean Sea, biogeochemical floats in the Arctic Ocean, and deep floats with oxygen sensors in the North Atlantic. The experiment in the Mediterranean Sea, launched in 2012, implemented and maintained a network of BGC-Argo floats at basin scale for the first time. The 32 BGC-Argo floats deployed and about 4000 BGC profiles collected have vastly improved characterization of the biogeochemical and ecosystem dynamics of the Mediterranean. Meanwhile, experiments in the Arctic and in the North Atlantic, starting in 2015 and deploying 20 Arctic BGC floats and 23 deep floats, have provided unique observations on biogeochemical cycles in the Arctic and deep-water masses, as well as ocean circulation variability in the North Atlantic. NAOS has therefore paved the way to the new operational phase of the Argo program in France that includes BGC and Deep Argo extensions. The objectives and characteristics of this new phase of Argo-France are discussed in the conclusion.

The South Pacific Subtropical Gyre (SPSG) is a vast and remote oceanic system where the variability in phytoplankton biomass and production is still largely uncertain due to the lack of in situ biogeochemical observations. The SPSG is an oligotrophic environment where the ecosystem is controlled predominantly by nutrient depletion in surface waters. However, this dynamic is altered in the vicinity of islands where increased biological activity occurs (i.e. the island mass effect, IME). This study mainly focuses on in situ observations which show evidence of an IME leeward of Tahiti (17.7°S - 149.5°W), French Polynesia. Physical and biogeochemical observations collected with two Biogeochemical-Argo profiling floats are used to investigate the dynamics of phytoplankton biomass. Data from the first float, drifting from April 2015 to November 2016 over >1000 km westward of Tahiti, describe the open ocean conditions. The second float, deployed leeward of Tahiti in October 2015, stayed within 45 km off Tahiti for three months before it stopped communicating. In the oligotrophic central SPSG, our observations show that the deepening of the deep chlorophyll maximum (DCM) from winter to summer is light-driven and that the wintertime increase in chlorophyll a concentration in the upper layer is likely to be due to the process of photoacclimation, consistent with previous observations in oligotrophic environments. In contrast, leeward of Tahiti, the DCM widens toward the surface during late spring in association with a biological enhancement in the upper layer. Using Biogeochemical-Argo data, meteorological data from Tahiti, Hybrid Coordinate Ocean Model outputs and satellite-derived products (i.e., horizontal currents and associated fronts), the physical mechanisms involved in producing this biological enhancement leeward of Tahiti have been investigated. The IME occurs during a period of strong precipitation and in a zone of weak currents downstream of the island. We conjecture that the land drainage induces a significant supply of nitrate in the ocean upper layer (down to ~100 m) while a zone of weak currents in the southwestern zone behind Tahiti allows an accumulation zone to form, hence increasing phytoplankton growth up to 20 km away from the coastlines. A bio-optical-based community index suggests that the composition of the phytoplankton community differs leeward of Tahiti from that in the open ocean area, with more microphytoplankton within the IME, which is associated with an increase in the carbon export to the deeper ocean.

Due to its particular characteristics, the Mediterranean Sea is often viewed as a microcosm of the World Ocean. Its proportionally-reduced dimensions and peculiar hydrological circulation render it susceptible to environmental and climatic constraints, which are rapidly evolving. The Mediterranean is therefore an ideal site to examine, in order to better understand a number of key oceanographic phenomena. This is especially true of the Ligurian Sea where, due to its geology, oceanic conditions are found close to the coast. As such, 30 years ago, an offshore time-series site provided a fresh impetus to a long history of marine biology research, which has generated a very important body of data and knowledge. This is the second volume, in a two-volume series, that summarizes this research. Across these two books, the reader will find 13 chapters that examine the geology, physics, chemistry and biology of the Ligurian Sea ? always with the goal of providing key elements of oceanography in a changing world.

The decline of sea-ice thickness, area, and volume due to the transition from multi-year to first-year sea ice has improved the under-ice light environment for pelagic Arctic ecosystems. One unexpected and direct consequence of this transition, the proliferation of under-ice phytoplankton blooms (UIBs), challenges the paradigm that waters beneath the ice pack harbor little planktonic life. Little is known about the diversity and spatial distribution of UIBs in the Arctic Ocean, or the environmental drivers behind their timing, magnitude, and taxonomic composition. Here, we compiled a unique and comprehensive dataset from seven major research projects in the Arctic Ocean (11 expeditions, covering the spring sea-ice-covered period to summer ice-free conditions) to identify the environmental drivers responsible for initiating and shaping the magnitude and assemblage structure of UIBs. The temporal dynamics behind UIB formation are related to the ways that snow and sea-ice conditions impact the under-ice light field. In particular, the onset of snowmelt significantly increased under-ice light availability (>0.1–0.2 mol photons m–2 d–1), marking the concomitant termination of the sea-ice algal bloom and initiation of UIBs. At the pan-Arctic scale, bloom magnitude (expressed as maximum chlorophyll a concentration) was predicted best by winter water Si(OH)4 and PO43– concentrations, as well as Si(OH)4:NO3– and PO43–:NO3– drawdown ratios, but not NO3– concentration. Two main phytoplankton assemblages dominated UIBs (diatoms or Phaeocystis), driven primarily by the winter nitrate:silicate (NO3–:Si(OH)4) ratio and the under-ice light climate. Phaeocystis co-dominated in low Si(OH)4 (i.e., NO3:Si(OH)4 molar ratios >1) waters, while diatoms contributed the bulk of UIB biomass when Si(OH)4 was high (i.e., NO3:Si(OH)4 molar ratios <1). The implications of such differences in UIB composition could have important ramifications for Arctic biogeochemical cycles, and ultimately impact carbon flow to higher trophic levels and the deep ocean.

The necessity of wide, global-scale observing systems for marine biogeochemistry emerged dramatically in the last decade. A global network based on Biogeochemical (BGC) Argo floats is considered to be one of the most promising approaches for reaching this goal. As a first step, pilot studies were encouraged to test the feasibility of a global BGC-Argo array, to consolidate the methods and practices under development, and to set up the array's characteristics. A pilot study in The Mediterranean Sea-deemed a suitable candidate for a test case because it combines a relatively large diversity of oceanic BGC conditions in a reduced open-ocean basin-was consequently approved as a part of the "Novel Argo ocean Observing System" (NAOS) project, a French national initiative to promote, consolidate, and develop the Argo network. We present here a first assessment of the NAOS Mediterranean array, in view of scientific choices on observing-system strategy, on implementation and statistics on network performances, and on data-quality control.

The export and fate of organic carbon in the mesopelagic zone are still poorly understood and quantified due to lack of observations. We exploited data from a biogeochemical-Argo float that was deployed in the Red Sea to study how a warm and hypoxic environment can affect the fate of the organic carbon in the ocean's interior. We observed that only 10% of the particulate organic carbon (POC) exported survived at depth due to remineralization processes in the upper mesopelagic zone. We also found that POC exported was rapidly degraded in a first stage and slowly in a second one, which may be dependent on the palatability of the organic matter. We observed that apparent oxygen utilization (AOU)-based loss rates (a proxy of the remineralization of total organic matter) were significantly higher than the POC-based loss rates, likely because changes in AOU are mainly attributed to changes in dissolved organic carbon. Finally, we showed that POC-and AOU-based loss rates could be expressed as a function of temperature and oxygen concentration. These findings advance our understanding of the biological carbon pump and mesopelagic ecosystem.

A critical driver of the ocean carbon cycle is the downward flux of sinking organic particles, which acts to lower the atmospheric CO2 concentration. This downward flux is reduced by over 70% in the mesopelagic zone (100-1000 m), but this loss cannot be fully accounted for by current measurements. For decades, it has been hypothesized that the missing loss could be explained by the fragmentation of large aggregates into small particles, although data to test this

Coccolithophores (calcifying phytoplankton) form extensive blooms in temperate and subpolar oceans as evidenced from ocean-color satellites. This study examines the potential to detect coccolithophore blooms with BioGeoChemical-Argo (BGC-Argo) floats, autonomous ocean profilers equipped with bio-optical and physicochemical sensors. We first matched float data to ocean-color satellite data of calcite concentration to select floats that sampled coccolithophore blooms. We identified two floats in the Southern Ocean, which measured the particulate beam attenuation coefficient (c p) in addition to two core BGC-Argo variables, Chlorophyll-a concentration ([Chl-a]) and the particle backscattering coefficient (b bp). We show that coccolithophore blooms can be identified from floats by distinctively high values of (1) the b bp /c p ratio, a proxy for the refractive index of suspended particles, and (2) the b bp /[Chl-a] ratio, measurable by any BGC-Argo float. The latter thus paves the way to global investigations of environmental control of coccolithophore blooms and their role in carbon export. Plain Language Summary Coccolithophores are a group of phytoplankton that form an armor of calcite plates. Coccolithophores may form intense blooms which can be identified from space by so-called ocean-color satellites, providing global images of the color of the surface ocean. BioGeoChemical-Argo (BGC-Argo) floats, robots profiling down to 2,000 m with a variety of physicochemical and bio-optical sensors, present an increasingly attractive and cost-effective platform to study phytoplankton blooms and their impact on oceanic biogeochemical cycles. We show that coccolithophore blooms can be detected by BGC-Argo floats with high confidence, hence providing a new way to study them at the global scale as well as their role in sinking carbon.

A global compilation of in situ data is useful to evaluate the quality of ocean-colour satellite data records. Here we describe the data compiled for the validation of the ocean-colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI). The data were acquired from several sources (including, inter alia, MOBY, BOUSSOLE, AERONET-OC, SeaBASS, NOMAD, MERMAID, AMT, ICES, HOT and GeP&CO) and span the period from 1997 to 2018. Observations of the following variables were compiled: spectral remote-sensing reflectances, concentrations of chlorophyll a, spectral inherent optical properties, spectral diffuse attenuation coefficients and total suspended matter. The data were from multi-project archives acquired via open internet services or from individual projects, acquired directly from data providers. Methodologies were implemented for homogenization, quality control and merging of all data. No changes were made to the original data, other than averaging of observations that were close in time and space, elimination of some points after quality control and conversion to a standard format. The final result is a merged table designed for validation of satellite-derived ocean-colour products and available in text format. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) was propagated throughout the work and made available in the final table. By making the metadata available, provenance is better documented, and it is also possible to analyse each set of data separately. This paper also describes the changes that were made to the compilation in relation to the previous version (Valente et al., 2016). The compiled data are available at https://doi.org/10.1594/PANGAEA.898188 (Valente et al., 2019).

As commonly observed in oligotrophic stratified waters, a subsurface (or deep) chlorophyll maximum (SCM) frequently characterizes the vertical distribution of phyto-plankton chlorophyll in the Mediterranean Sea. Occurring far from the surface layer "seen" by ocean colour satellites , SCMs are difficult to observe with adequate spatio-temporal resolution and their biogeochemical impact remains unknown. Biogeochemical-Argo (BGC-Argo) profiling floats represent appropriate tools for studying the dynamics of SCMs. Based on data collected from 36 BGC-Argo floats deployed in the Mediterranean Sea, our study aims to address two main questions. (1) What are the different types of SCMs in the Mediterranean Sea? (2) Which environmental factors control their occurrence and dynamics? First, we analysed the seasonal and regional variations in the chlorophyll concentration (Chl a), particulate backscattering coefficient (b bp), a proxy of the particulate organic carbon (POC) and environmental parameters (photosynthetically active radiation and nitrates) within the SCM layer over the Mediter-ranean Basin. The vertical profiles of Chl a and b bp were then statistically classified and the seasonal occurrence of each of the different types of SCMs quantified. Finally, a case study was performed on two contrasted regions and the environmental conditions at depth were further investigated to understand the main controls on the SCMs. In the eastern basin, SCMs result, at a first order, from a photoacclima-tion process. Conversely, SCMs in the western basin reflect a biomass increase at depth benefiting from both light and nitrate resources. Our results also suggest that a variety of intermediate types of SCMs are encountered between these two endmember situations.

Understanding spatial and temporal dynamics of non-algal particles (NAP) in open ocean is of the utmost importance to improve estimations of carbon export and sequestration. These particles covary with phytoplankton abundance but also accumulate independently of algal dynamics. The latter likely represents an important fraction of organic carbon but it is largely overlooked. A possible way to study these particles is via their optical backscattering properties (b bp) and relationship with chlorophyll-a (Chl). To this aim, we estimate the fraction of b bp associated with the NAP portion () that does not covary with Chl by using a global Biogeochemical-Argo dataset. We quantify the spatial, temporal and vertical variability of. In the northern productive areas, is a small fraction of b bp and shows a clear seasonal cycle. In the Southern Ocean, b k bp is a major fraction of total b bp. In oligotrophic areas, has a smooth annual cycle.

Hydrothermal activity is significant in regulating the dynamics of trace elements in the ocean. Biogeochemical models suggest that hydrothermal iron might play an important role in the iron-depleted Southern Ocean by enhancing the biological pump. However, the ability of this mechanism to affect large-scale biogeochemistry and the pathways by which hydrothermal iron reach the surface layer have not been observationally constrained. Here we present the first observational evidence of upwelled hydrothermally influenced deep waters stimulating massive phytoplankton blooms in the Southern Ocean. Captured by profiling floats, two blooms were observed in the vicinity of the Antarctic Circumpolar Current, downstream of active hydrothermal vents along the Southwest Indian Ridge. These hotspots of biological activity are supported by mixing of hydrothermally sourced iron stimulated by flow-topography interactions. Such findings reveal the important role of hydrothermal vents on surface biogeochemistry, potentially fueling local hotspot sinks for atmospheric CO₂ by enhancing the biological pump.

The Argo Program has been implemented and sustained for almost two decades, as a global array of about 4000 profiling floats. Argo provides continuous observations of ocean temperature and salinity versus pressure, from the sea surface to 2000 dbar. The successful installation of the Argo array and its innovative data management system arose opportunistically from the combination of great scientific need and technological innovation. Through the data system, Argo provides fundamental physical observations with broad societally-valuable applications, built on the cost-efficient and robust technologies of autonomous profiling floats. Following recent advances in platform and sensor technologies, even greater opportunity exists now than 20 years ago to (i) improve Argo’s global coverage and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems, and (iv) consider experimental sensors that might be included in the future, for example to document the spatial and temporal patterns of ocean mixing. For Core Argo and each of these enhancements, the past, present, and future progression along a path from experimental deployments to regional pilot arrays to global implementation is described. The objective is to create a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System (Legler et al., 2015). The integrated system will deliver operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities.

The timing of phytoplankton growth (phenology) in tropical oceans is a crucial factor influencing the survival rates of higher trophic levels, food web structure and the functioning of coral reef ecosystems. Phytoplankton phenology is thus categorised as an ‘ecosystem indicator’, which can be utilised to assess ecosystem health in response to environmental and climatic perturbations. Ocean-colour remote sensing is currently the only technique providing global, long-term, synoptic estimates of phenology. However, due to limited available in situ datasets, studies dedicated to the validation of satellite-derived phenology metrics are sparse. The recent development of autonomous oceanographic observation platforms provides an opportunity to bridge this gap. Here, we use satellite-derived surface chlorophyll-a (Chl-a) observations, in conjunction with a Biogeochemical-Argo dataset, to assess the capability of remote sensing to estimate phytoplankton phenology metrics in the northern Red Sea – a typical tropical marine ecosystem. We find that phenology metrics derived from both contemporary platforms match with a high degree of precision (within the same 5-day period). The remotely-sensed surface signatures reflect the overall water column dynamics and successfully capture Chl-a variability related to convective mixing. Our findings offer important insights into the capability of remote sensing for monitoring food availability in tropical marine ecosystems, and support the use of satellite-derived phenology as an ecosystem indicator for marine management strategies in regions with limited data availability.

Long-term perspectives for the development of CMEMS are described and implications for the evolution of the in situ and satellite observing systems are outlined. Results from Observing System Evaluations (OSEs) and Observing System Simulation Experiments (OSSEs) illustrate the high dependencies of CMEMS systems on observations. Finally future CMEMS requirements for both satellite and in situ observations are detailed.

The North Atlantic subtropical gyre (NASTG) is a model of the future ocean under climate change. Ocean warming signals are hidden within the blue color of these clear waters and can be tracked by understanding the dynamics among phytoplankton chlorophyll ([Chl]) and colored dissolved organic matter (CDOM). In NASTG, [Chl] and CDOM are strongly correlated. Yet, this unusual correlation for open oceans remains unexplained. Here, we test main hypotheses by analyzing high spatiotemporal resolution data collected by Biogeochemical-Argo floats between 2012 and 2018. The direct production of CDOM via phytoplankton metabolism is the main occurring mechanism. More importantly, CDOM dynamics strongly depend on the abundance of picophytoplankton. Our findings thus highlight the critical role of these small organisms under the ocean warming scenario. Picophytoplankton will enhance the production of colored dissolved compounds and, ultimately, impact on the ocean carbon cycle. Plain Language Summary Colored dissolved organic matter (CDOM) is ubiquitous in aquatic ecosystems. CDOM absorbs sunlight and, ultimately, colors the oceans. In the blue and clear subtropical gyre of the North Atlantic Ocean, the temporal dynamics of CDOM are strongly correlated with the concentration of chlorophyll contained into tiny plants called phytoplankton. The reasons of such a correlation are unexplained. Here, we use field data collected by autonomous robotic platforms and show that CDOM is a fresh product of phytoplankton metabolism in the sampled area. More importantly, we observe that this production is driven by the presence of the smallest phytoplankton on the planet. The role of picophytoplankton (i.e., all phytoplankton with size smaller than 2 μm) as a producer of CDOM will thus become critical for the ocean carbon cycle in the future ocean, as climate change allows subtropical gyres expanding.

The ocean’s ability to sequester carbon away from the atmosphere exerts an important control on global climate. The biological pump drives carbon storage in the deep ocean and is thought to function via gravitational settling of organic particles from surface waters. However, the settling flux alone is often insufficient to balance mesopelagic carbon budgets or to meet the demands of subsurface biota. Here we review additional biological and physical mechanisms that inject suspended and sinking particles to depth. We propose that these ‘particle injection pumps’ probably sequester as much carbon as the gravitational pump, helping to close the carbon budget and motivating further investigation into their environmental control.

The detrainment of organic matter from the mixed layer, a process known as the mixed layer pump (ML pump), has long been overlooked in carbon export budgets. Recently, the ML pump has been investigated at seasonal scale and appeared to contribute significantly to particulate organic carbon export to the mesopelagic zone, especially at high latitudes where seasonal variations of the mixed layer depth are large. However, the dynamics of the ML pump at intraseasonal scales remains poorly known, mainly because the lack of observational tools suited to studying such dynamics. In the present study, using a dense network of autonomous profiling floats equipped with bio‐optical sensors, we captured widespread episodic ML pump‐driven export events, during the winter and early spring period, in a large part of the subpolar North Atlantic Ocean. The intraseasonal dynamics of the ML pump exports fresh organic material to depth (basin‐scale average up to 55 mg C·m−2·day−1), providing a significant source of energy to the mesopelagic food web before the spring bloom period. This mechanism may sustain the seasonal development of overwintering organisms such as copepods with potential impact on the characteristics of the forthcoming spring phytoplankton bloom through predator‐prey interactions.

The North Atlantic bloom corresponds to a strong seasonal increase in phytoplankton that produces organic carbon through photosynthesis. It is still debated what physical and biological conditions trigger the bloom, because comprehensive time series of the vertical distribution of phytoplankton biomass are lacking. Vertical profiles from nine floats that sampled the waters of the North Atlantic every few days for a couple of years reveal that phyto-plankton populations start growing in early winter at very weak rates. A proper bloom with rapidly accelerating population growth rates instead starts only in spring when atmospheric cooling subsides and the mixed layer rapidly shoals. While the weak accumulation of phy-toplankton in winter is crucial to maintaining a viable population, the spring bloom dominates the overall seasonal production of organic carbon.

A new Scientific Committee for Ocean Research (SCOR, http://www.scor-int.org /) working group has been formed, entitled SCOR WG-154 “Integration of Plankton-Observing Sensor Systems to Existing Global Sampling Programs (P-OBS, http://www.scor-int.org/SCOR_WGs_WG154.htm.).” The working group (P-OBS WG) is reviewing biological sensing technologies and measurements that are ready for integration into existing regional and global ocean observing programs. Multidisciplinary sets of measurements, whose choice is guided by research and societal benefit go als, will transform our understanding of ocean biology and its impacts on Earth systems.

Characterizing phytoplankton distribution and dynamics in the world's open oceans requires in situ observations over a broad range of space and time scales. In addition to temperature/salinity measurements , Biogeochemical-Argo (BGC-Argo) profiling floats are capable of autonomously observing at high-frequency bio-optical properties such as the chlorophyll fluorescence, a proxy of the chlorophyll a concentration (Chla), the particulate backscattering coefficient (b bp), a proxy of the stock of particulate organic carbon , and the light available for photosynthesis. We analyzed an unprecedented BGC-Argo database of more than 8,500 multivariable profiles collected in various oceanic conditions, from subpolar waters to subtropical gyres. Our objective is to refine previously established Chla versus b bp relationships and gain insights into the sources of vertical, seasonal, and regional variability in this relationship. Despite some regional, seasonal and vertical variations, a general covariation occurs at a global scale. We distinguish two main contrasted situations: (1) concomitant changes in Chla and b bp that correspond to actual variations in phytoplankton biomass, e.g., in subpolar regimes; (2) a decoupling between the two variables attributed to photoacclima-tion or changes in the relative abundance of nonalgal particles, e.g., in subtropical regimes. The variability in the b bp :Chla ratio in the surface layer appears to be essentially influenced by the type of particles and by photoacclimation processes. The large BGC-Argo database helps identifying the spatial and temporal scales at which this ratio is predominantly driven by one or the other of these two factors.

The Black Sea, the largest semienclosed anoxic basin on Earth, can be considered as an excellent natural laboratory for oxic and anoxic biogeochemical processes. The suboxic zone, a thin interface between oxic and anoxic waters, still remains poorly understood because it has been undersampled. This has led to alternative concepts regarding the underlying processes that create it. Existing hypotheses suggest that the interface originates either by isopycnal intrusions that introduce oxygen or the dynamics of manganese redox cycling that are associated with the sinking of particles or chemosynthetic bacteria. Here we reexamine these concepts using high-resolution oxygen, sulfide, nitrate, and particle concentration profiles obtained with sensors deployed on profiling floats. Our results show an extremely stable structure in density space over the entire basin with the exception of areas near the Bosporus plume and in the southern areas dominated by coastal anticyclones. The absence of large-scale horizontal intrusive signatures in the open-sea supports a hypothesis prioritizing the role of biogeochemical processes.

An efficient system to produce in situ high quality radiometric measurements is compulsory to rigorously perform the vicarious calibration of satellite sensors dedicated to Ocean Color Radiometry (OCR) and to validate their derived products. This requirement is especially needed during the early stages of an OCR satellite activity or for remote areas poorly covered by oceanographic cruises with possible bio-optical anomalies. Taking advantage of Argo's profiling float technology, we present a new autonomous profiling float dedicated to in situ radiometric measurements. The float is based on the Provor CTS5 (manufacturer NKE) with an added novel two protruding arm design allowing for sensor redundancies, shading mitigation and near-surface data. Equipped with two identical radiometers on each arm that measure downward irradiance and upwelling radiance at seven wavelengths, the ProVal float generates both redundant radiometric profiles as well as an estimate of Remote Sensing Reflectance. Results from 449 profiles obtained in the NW Mediterranean Sea and in the Indian sector of the Southern Ocean are presented to illustrate the ProVal float technical maturity. Analysis of the behavior of the profiling float, including tilting and ascent speeds is presented. The vertical stability of the ProVal exhibits 85% of surface data of the Mediterranean Sea with a tilt smaller than 10 degrees. This percentage is 40% in the Southern Ocean due to rougher seas. Redundant sensors provide a characterization of the relative drift between sensors over the deployment which is found to be <0.15% per month over a year. Post-cruise calibration of a recovered float revealed no significant drift. As an example of the utility of ProVal floats, a match-up of Remote Sensing Reflectance measured with the European Space Agency Ocean and Land Color Imager (OLCI onboard Sentinel-3A) is shown. It follows that profiling floats, such as ProVal, could provide a significant contribution to an upcoming global System Vicarious Calibration of space-based radiometers.

In-situ high quality measurements of radiometric quantities are mandatory to enable a "system vicarious calibration" (SVC) of satellite sensors dedicated to Ocean Color Radiometry (OCR) as well as to validate their derived products. High density of acquisition is particularly critical during the early stages of an OCR satellite activity. The ProVal float measures downward irradiance and upwelling radiance at seven wavelengths on two arms that allow radiometer redundancy and shading mitigation. We analyzed more than 500 profiles sampled in the Southern Ocean and Mediterranean Sea to date. We find that 45% and 85% of data in the surface layer exhibit tilts lower than 10°in the Southern Ocean and Mediterranean Sea respectively. Floats deployed in the Mediterranean Sea were recovered allowing post-deployment calibrations of radiometers that confirmed the low sensor drift. In addition, platform shading, estimated from the difference between the two radiometers, shows good agreement with Monte-Carlo simulations. Finally, comparisons of Remote Sensing Reflectance with the OLCI sensor (Sentinel-3A) show results in agreement with other sources of in-situ data but with extended coverage capabilities.

Toward deeper development of Biogeochemical-Argo floats XING Xiao-Gang a , CLAUSTRE Hervé b , BOSS Emmanuel c and CHAI Fei a,c a second institute of oceanography, state oceanic Administration, hangzhou, china;

This article presents data regarding the Si bio-geochemical cycle during two oceanographic cruises conducted in the tropical South Pacific (BIOSOPE and OUT-PACE cruises) in 2005 and 2015. It involves the first Si stock measurements in this understudied region, encompassing various oceanic systems from New Caledonia to the Chilean upwelling between 8 and 34 • S. Some of the lowest levels of biogenic silica standing stocks ever measured were found in this area, notably in the southern Pacific gyre, where Chlorophyll a concentrations are the most depleted worldwide. Integrated biogenic silica stocks are as low as 1.08 ± 0.95 mmol m −2 and are the lowest stocks measured in the South Pacific. Size-fractionated biogenic sil-ica concentrations revealed a non-negligible contribution of the pico-sized fraction (<2-3 µm) to biogenic silica standing stocks, representing 26% ± 12% of total biogenic sil-ica during the OUTPACE cruise and 11% ± 9% during the BIOSOPE cruise. These results indicate significant accumulation in this size class, which was undocumented for 2005, but has since then been related to Si uptake by Synechococcus cells. Si uptake measurements carried out during BIOSOPE confirmed biological Si uptake by this size fraction. We further present diatoms community structure associated with the stock measurements for a global overview of the Si cycle in the tropical South Pacific.

In the original version of this Article, the data accession 10.17882/42182 was omitted from the Data Availability statement.In the first paragraph of the Methods subsection entitled `Float data processing', the WET Labs ECO-triplet fluorometer was incorrectly referred to as `WETLabs ECO PUK'. In the final paragraph of this subsection, the WET Labs ECO-series fluorometer was incorrectly referred to as `WETLabs 413 ECO-series'.In the Methods subsection `Float estimates of phytoplankton carbon biomass', the average particulate organic carbon-bbp ratio of 37,537 mgC m^-2 was incorrectly given as 37,357 mgC m^-2.In the second paragraph of the Methods subsection `Float estimates of population division rates', the symbol for Celsius (C) was omitted from the phrase `a 10°C increase in temperature'.These errors have now been corrected in the PDF and HTML versions of the Article.

which shows a significant, high latitude-intensified increase between +0.1 and +0.4 units per decade. This shows the utility that such transfer functions with realistic uncertainty estimates provide to ocean biogeochemistry and global climate change research. In addition, CANYON-B provides robust and accurate estimates of nitrate, phosphate, and silicate. Matlab and R code are available at https://github.com/HCBScienceProducts/.

In situ chlorophyll fluorometers have been used to quantify the distribution of chlorophyll concentration in natural waters for decades. However, chlorophyll fluorescence is depressed during daylight hours due to non-photochemical quenching (NPQ). Corrections attempted to date have provided improvement but still remain unsatisfactory, often overestimating the expected value. In this study, we examine the relationship between NPQ and instantaneous Photosynthetically Active Radiation (iPAR) using field data from BGC-Argo floats equipped with Chlorophyll-a fluorometers and radiometers. This analysis leads to an improved NPQ correction that incorporates both iPAR and mixed layer depth (MLD) and is validated against data collected at sunrise or sunset. The optimal NPQ light threshold is found to be iPAR = 15 μmol quanta m −2 s −1 , and the proposed methods based on such a light threshold correct the NPQ effect more accurately than others, except in "shallow-mixing" waters (NPQ light threshold depth deeper than MLD). For these waters, an empirical-relationship-based method is proposed for improvement of NPQ correction using an iPAR profile. It is therefore recommended that, for optimal NPQ corrections, profiling floats measuring chlorophyll fluorescence in daytime be equipped with iPAR radiometers.

Since 2012, an array of 105 Biogeochemical-Argo (BGC-Argo) floats has been deployed across the world's oceans to assist in filling observational gaps that are required for characterizing open-ocean environments. Profiles of biogeochemical (chlorophyll and dissolved organic matter) and optical (single-wavelength particulate optical backscattering, downward irradiance at three wavelengths, and photosynthetically available radiation) variables are collected in the upper 1000m every 1 to 10 days. The database of 9837 vertical profiles collected up to January 2016 is presented and its spatial and temporal coverage is discussed. Each variable is quality controlled with specifically developed procedures and its time series is quality-assessed to identify issues related to biofouling and/or instrument drift. A second database of 5748 profile-derived products within the first optical depth (i.e., the layer of interest for satellite remote sensing) is also presented and its spatiotemporal distribution discussed. This database, devoted to field and remote ocean color applications, includes diffuse attenuation coefficients for downward irradiance at three narrow wavebands and one broad waveband (photosynthetically available radiation), calibrated chlorophyll and fluorescent dissolved organic matter concentrations, and single-wavelength particulate optical backscattering. To demonstrate the applicability of these databases, data within the first optical depth are compared with previously established bio-optical models and used to validate remotely derived bio-optical products. The quality-controlled databases are publicly available from the SEANOE (SEA scieNtific Open data Edition) publisher at https://doi.org/10.17882/49388 and https://doi.org/10.17882/47142 for vertical profiles and products within the first optical depth, respectively.

Satellite ocean color observations revealed that unusually deep convection events in 2005, 2006, 2010, and 2013 led to an increased phytoplankton biomass during the spring bloom over a large area of the northwestern Mediterranean Sea (NWM). Here we investigate the effects of these events on the seasonal phytoplankton community structure, we quantify their influence on primary production, and we discuss the potential biogeochemical impact. For this purpose, we compiled in situ phytoplankton pigment data from five ship surveys performed in the NWM and from monthly cruises at a fixed station in the Ligurian Sea. We derived primary production rates from a light photosynthesis model applied to these in situ data. Our results confirm that the maximum phytoplankton biomass during the spring bloom is larger in years associated with intense deep convection events (151%). During these enhanced spring blooms, the contribution of diatoms to total phytoplankton biomass increased (133%), as well as the primary production rate (1115%). The occurrence of a highly productive bloom is also related to an increase in the phytoplankton bloom area (1155%) and in the relative contribution of diatoms to primary production (163%). Therefore, assuming that deep convection in the NWM could be significantly weakened by future climate changes, substantial decreases in the spring production of organic carbon and of its export to deep waters can be expected.

Chlorophyll fluorometers provide the largest in situ global data set for estimating phytoplankton biomass because of their ease of use, size, power consumption, and relatively low price. While in situ chlorophyll a (Chl) fluorescence is proxy for Chl a concentration, and hence phytoplankton biomass, there exist large natural variations in the relationship between in situ fluorescence and extracted Chl a concentration. Despite this large natural variability, we present here a global validation data set for the WET Labs Environmental Characterization Optics (ECO) series chlorophyll fluorometers that suggests a factor of 2 overestimation in the factory calibrated Chl a estimates for this specific manufacturer and series of sensors. We base these results on paired High Pressure Liquid Chromatography (HPLC) and in situ fluorescence match ups for which non-photochemically quenched fluorescence observations were removed. Additionally, we examined match-ups between the factory-calibrated in situ fluorescence and estimates of chlorophyll concentration determined from in situ radiometry, absorption line height, NASA's standard ocean color algorithm as well as laboratory calibrations with phytoplankton monocultures spanning diverse species that support the factor of 2 bias. We therefore recommend the factor of 2 global bias correction be applied for the WET Labs ECO sensors , at the user level, to improve the global accuracy of chlorophyll concentration estimates and products derived from them. We recommend that other fluorometer makes and models should likewise undergo global analyses to identify potential bias in factory calibration.

The Southern Ocean (SO), an area highly sensitive to climate change, is currently experiencing rapid warming and freshening. Such drastic physical changes might significantly alter the SO's biological pump. For more accurate predictions of the possible evolution of this pump, a better understanding of the environmental factors controlling SO phytoplankton dynamics is needed. Here we present a satellite-based study deciphering the complex environmental control of phytoplankton biomass (PB) and phenology (PH; timing and magnitude of phytoplankton blooms) in the SO. We reveal that PH and PB are mostly organized in the SO at two scales: a large latitudinal scale and a regional scale. Latitudinally, a clear gradient in the timing of bloom occurrence appears tightly linked to the seasonal cycle in irradiance, with some exceptions in specific light-limited regimes (i.e., well-mixed areas). Superimposed on this latitudinal scale, zonal asymmetries, up to 3 orders of magnitude, in regional-scale PB are mainly driven by local advective and iron supply processes. These findings provide a global understanding of PB and PH in the SO, which is of fundamental interest for identifying and explaining ongoing changes as well as predicting future changes in the SO biological pump.

The Southern Ocean (SO) hosts plankton communities that impact the biogeochemical cycles of the global ocean. However, weather conditions in the SO restrict mainly in situ observations of plankton communities to spring and summer, preventing the description of biological successions at an annual scale. Here, we use shipboard observations collected in the Indian sector of the SO to develop a multivariate relationship between physical and bio-optical data, and, the composition and carbon content of the plankton community. Then we apply this multivariate relationship to five biogeochemical Argo (BGC-Argo) floats deployed within the same bio-geographical zone as the ship-board observations to describe spatial and seasonal changes in plankton assemblage. The floats reveal a high contribution of bacteria below the mixed layer, an overall low abundance of picoplankton and a seasonal succession from nano-to microplankton during the spring bloom. Both naturally iron-fertilized waters downstream of the Crozet and Kerguelen Plateaus show elevated phytoplankton biomass in spring and summer but they differ by a nano-or microplankton dominance at Crozet and Kerguelen, respectively. The estimated plankton group successions appear consistent with independent estimations of particle diameter based on the optical signals. Furthermore, the comparison of the plankton community composition in the surface layer with the presence of large mesopelagic particles diagnosed by spikes of optical signals provides insight into the nature and temporal changes of ecological vectors that drive particle export. This study emphasizes the power of BGC-Argo floats for investigating important biogeochemical processes at high temporal and spatial resolution.

The North Western Mediterranean Sea exhibits recurrent and significant autumnal and spring phytoplankton blooms. The existence of these two blooms coincides with typical temperate dynamics. To determine the potential control of physical and biogeochemical factors on these phytoplankton blooms, data from a multiplatform approach (combining ships, Argo and BGC‐Argo floats, and bio‐optical gliders) were analyzed in association with satellite observations in 2012–2013. The satellite framework allowed a simultaneous analysis over the whole annual cycle of in situ observations of mixed layer depth, photosynthetical available radiation, particle backscattering, nutrients (nitrate and silicate), and chlorophyll‐a concentrations. During the year 2012–2013, satellite ocean color observations, confirmed by in situ data, have revealed the existence of two areas (or bioregions) with comparable autumnal blooms but contrasting spring blooms. In both bioregions, the ratio of the euphotic zone (defined as the isolume 0.415 mol photons m−2 d−1, Z0.415) and the MLD identified the initiation of the autumnal bloom, as well as the maximal annual increase in [Chl‐a] in spring. In fact, the autumnal phytoplankton bloom might be initiated by mixing of the summer shallowing deep chlorophyll maximum, while the spring restratification (when Z0.415/MLD ratio became >1) might induce surface phytoplankton production that largely overcomes the losses. Finally, winter deep convection events that took place in one of the bioregions induced higher net accumulation rate of phytoplankton in spring associated with a diatom‐dominated phytoplankton community principally. We suggest that very deep winter MLD lead to an increase in surface silicates availability, which favored the development of diatoms.

We explore a novel and spatially extensive data set obtained from Biogeochemical-Argo (or BGC-Argo) floats, containing 16,796 profiles of the particulate backscattering coefficient at 700 nm (b bp (700)) measured with three different sensors. We focus at the 900-950m depth interval (within the mesopelagic), where we found values to be relatively constant. While we find significant differences between estimates of b bp (700) obtained with different sensors (≈30% disagreement), the median values in most oceanic regions obtained with a single type of sensor are within 50% of each other and are consistent with measurements of suspended mass conducted in the early 1970s. Deviations from the quasi-constant background value likely indicate times and locations associated with higher particulate export to depth. Indeed, we observe that in productive high-latitude regions, a deep seasonal signal is observed, with enhanced values recorded a few months after surface spring/summer maximal concentrations. In addition, the deep b bp (700) is highest in regions exhibiting suboxic-anoxic conditions (e.g., Northern Indian Ocean), which have been associated with local particulate production as well as reduced particle flux attenuation.

An array of Bio-Argo floats equipped with radiometric sensors has been recently deployed in various open ocean areas representative of the diversity of trophic and bio-optical conditions prevailing in the so-called case 1 waters. Around solar noon and almost every day, each float acquires 0-250-m vertical profiles of photosynthetically available radiation and downward irradiance at three wavelengths (380, 412, and 490 nm). Up until now, more than 6500 profiles for each radiometric channel have been acquired. As these radiometric data are collected out of an operator's control and regardless of meteorological conditions, specific and automatic data processing protocols have to be developed. This paper presents a data quality-control procedure aimed at verifying profile shapes and providing near-real-time data distribution. This procedure is specifically developed to 1) identify main issues of measurements (i.e., dark signal, atmospheric clouds, spikes, and wavefocusing occurrences) and 2) validate the final data with a hierarchy of tests to ensure a scientific utilization. The procedure, adapted to each of the four radiometric channels, is designed to flag each profile in a way compliant with the data management procedure used by the Argo program. Main perturbations in the light field are identified by the new protocols with good performances over the whole dataset. This highlights its potential applicability at the global scale. Finally, the comparison with modeled surface irradiances allows for assessing the accuracy of quality-controlled measured irradiance values and identifying any possible evolution over the float lifetime due to biofouling and instrumental drift.

Abstract. A compiled set of in situ data is important to evaluate the quality of ocean-colour satellite-data records. Here we describe the data compiled for the validation of the ocean-colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI). The data were acquired from several sources (MOBY, BOUSSOLE, AERONET-OC, SeaBASS, NOMAD, MERMAID, AMT, ICES, HOT, GeP&CO), span between 1997 and 2012, and have a global distribution. Observations of the following variables were compiled: spectral remote-sensing reflectances, concentrations of chlorophyll a, spectral inherent optical properties and spectral diffuse attenuation coefficients. The data were from multi-project archives acquired via the open internet services or from individual projects, acquired directly from data providers. Methodologies were implemented for homogenisation, quality control and merging of all data. No changes were made to the original data, other than averaging of observations that were close in time and space, elimination of some points after quality control and conversion to a standard format. The final result is a merged table designed for validation of satellite-derived ocean-colour products and available in text format. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) were preserved throughout the work and made available in the final table. Using all the data in a validation exercise increases the number of matchups and enhances the representativeness of different marine regimes. By making available the metadata, it is also possible to analyse each set of data separately. The compiled data are available at doi:10.1594/PANGAEA.854832 (Valente et al., 2015).

The ocean region known as the mesopelagic zone, which is at depths of about 100–1,000 m, harbours one of the largest ecosystems and fish stocks on the planet1, 2. Life in this region is believed to rely on particulate organic carbon supplied by the biological carbon pump3. Yet this supply appears insufficient to meet mesopelagic metabolic demands4, 5, 6. An additional organic carbon source to the mesopelagic zone could be provided by the seasonal entrainment of surface waters in deeper layers, a process known as the mixed-layer pump7, 8, 9, 10, 11. Little is known about the magnitude and spatial distribution of this process globally or its potential to transport carbon to the mesopelagic zone. Here we combine mixed-layer depth data from Argo floats with satellite estimates of particulate organic carbon concentrations to show that the mixed-layer pump supplies an important seasonal flux of organic carbon to the mesopelagic zone. We estimate that this process is responsible for a global flux of 0.1–0.5 Pg C yr−1. In high-latitude regions where the mixed layer is usually deep, this flux amounts on average to 23% of the carbon supplied by fast sinking particles, but it can be greater than 100%. We conclude that the seasonal mixed-layer pump is an important source of organic carbon for the mesopelagic zone.

The present study proposes a novel method that merges satellite ocean color bio-optical products with Argo temperature-salinity profiles to infer the vertical distribution of the particulate backscattering coefficient (bbp). This neural network-based method (SOCA-BBP for Satellite Ocean-Color merged with Argo data to infer the vertical distribution of the Particulate Backscattering coefficient) uses three main input components: (1) satellite-based surface estimates of bbp and chlorophyll a concentration matched up in space and time with (2) depth-resolved physical properties derived from temperature-salinity profiles measured by Argo profiling floats and (3) the day of the year of the considered satellite-Argo matchup. The neural network is trained and validated using a database including 4725 simultaneous profiles of temperature-salinity and bio-optical properties collected by Bio-Argo floats, with concomitant satellite-derived products. The Bio-Argo profiles are representative of the global open-ocean in terms of oceanographic conditions, making the proposed method applicable to most open-ocean environments. SOCA-BBP is validated using 20% of the entire database (global error of 21%). We present additional validation results based on two other independent data sets acquired (1) by four Bio-Argo floats deployed in major oceanic basins, not represented in the database used to train the method; and (2) during an AMT (Atlantic Meridional Transect) field cruise in 2009. These validation tests based on two fully independent data sets indicate the robustness of the predicted vertical distribution of bbp. To illustrate the potential of the method, we merged monthly climatological Argo profiles with ocean color products to produce a depth-resolved climatology of bbp for the global ocean.

D'Ortenzio and Ribera d'Alcalà (2009, DR09 hereafter) divided the Mediterranean Sea into " bioregions " based on the climatological seasonality (phenology) of phy-toplankton. Here we investigate the interannual variability of this bioregionalization. Using 16 years of available ocean color observations (i.e., SeaWiFS and MODIS), we analyzed the spatial distribution of the DR09 trophic regimes on an annual basis. Additionally, we identified new trophic regimes, exhibiting seasonal cycles of phytoplankton biomass different from the DR09 climatological description and named " Anomalous ". Overall, the classification of the Mediter-ranean phytoplankton phenology proposed by DR09 (i.e., " No Bloom " , " Intermittently " , " Bloom " and " Coastal "), is confirmed to be representative of most of the Mediterranean phytoplankton phenologies. The mean spatial distribution of these trophic regimes (i.e., bioregions) over the 16 years studied is also similar to the one proposed by DR09, although some annual variations were observed at regional scale. Discrepancies with the DR09 study were related to interannual variability in the sub-basin forcing: winter deep convection events, frontal instabilities, inflow of Atlantic or Black Sea Waters and river runoff. The large assortment of phytoplank-ton phenologies identified in the Mediterranean Sea is thus verified at the interannual scale, further supporting the " sen-tinel " role of this basin for detecting the impact of climate changes on the pelagic environment.

The distribution of the chlorophyll a concentration ([Chl a]) in the Mediterranean Sea, mainly obtained from satellite surface observations or from scattered in situ experiments, is updated by analyzing a database of fluorescence profiles converted into [Chl a]. The database, which includes 6790 fluorescence profiles from various origins, was processed with a specific quality control procedure. To ensure homogeneity between the different data sources, 65 % of fluorescence profiles have been intercalibrated on the basis of their concomitant satellite [Chl a] estimation. The climatological pattern of [Chl a] vertical profiles in four key sites of the Mediterranean Sea has been analyzed. Climatological results confirm previous findings over the range of existing [Chl a] values and throughout the principal Mediterranean trophic regimes. They also provide new insights into the seasonal variability in the shape of the vertical [Chl a] profile, inaccessible through remote-sensing observations. An analysis based on the recognition of the general shape of the fluorescence profile was also performed. Although the shape of [Chl a] vertical distribution characterized by a deep chlorophyll maximum (DCM) is ubiquitous during summer, different forms are observed during winter, thus suggesting that factors affecting the vertical distribution of the biomass are complex and highly variable. The [Chl a] spatial distribution in the Mediterranean Sea mimics, on smaller scales, what is observed in the global ocean. As already evidenced by analyzing satellite surface observations, midlatitude- and subtropical-like phytoplankton dynamics coexist in the Mediterranean Sea. Moreover, the Mediterranean DCM variability appears to be characterized by patterns already observed on the global scale.

In 2013, as part of the French NAOS (Novel Argo Oceanic observing System) program, five profiling floats equipped with nitrate sensors (SUNA-V2) together with CTD and bio-optical sensors were deployed in the Mediterranean Sea. At present day, more than 500 profiles of physical and biological parameters were acquired, and significantly increased the number of available nitrate data in the Mediterra-nean Sea. Results obtained from floats confirm the general view of the basin, and the well-known west-to-east gradient of oligotrophy. At seasonal scale, the north western Mediterranean displays a clear temperate pattern sustained by both deep winter mixed layer and shallow nitracline. The other sampled areas follow a subtropical regime (nitracline depth and mixed layer depth are generally decoupled). Float data also permit to highlight the major contribution of high-frequency processes in controlling the nitrate supply during winter in the north western Mediterranean Sea and in altering the nitrate stock in subsurface in the eastern basin.

An analysis of seasonal variations in climatological surface chlorophyll points to distinct biogeographical zones in the North Atlantic subpolar gyre. In particular, the Labrador Sea appears well delineated into two regions on either side of the 60°N parallel, with very different climatological phytoplankton biomass cycles. Indeed, north of 60°N, an early and short spring bloom occurs in late April, while south of 60°N, the bloom gradually develops 1 month later and significant biomass persists all summer long. Nevertheless, at climatological scale, the first-order mechanism that controls the bloom is identical for both bioregions. The light-mixing regime can explain the bloom onset in both bioregions. In the Labrador Sea, the blooms seem to rely on a mean community compensation irradiance threshold value of 2.5 mol photon m−2 d−1 over the mixed layer.

Thanks to the new genera.on of profiling floats and in par.cular to iridium telemetry, the acquisi.on frequency can be drama.cally increased. We inves.gate here the possibility to extract informa.on related to sea-state from the analysis of high-resolu.on measurements of the pressure data. We par.cularly focus on the study of the speed anomaly as compared to a nominal speed expected for a calm sea-state. By comparison between speed anomaly of a float in the Med Sea and concurrent seastate measurements by a weather buoy in the same area, we suggest that float behaviour can be an indicator of sea-state. In the context of remOcean and NOAS projects, we set up a high frequency mode (every 2 s) for the sub-surface layer and for more than fourty floats deployed in various open ocean areas, we present a preliminary analysis of the speed anomaly. observa.on of float behavior : Characteris.cs of the NKE CTS4 float in the upper layer:-when the float passes 10 dbars, it does not ac.vate its pump for a dura.on of 600 sec-ader 600 sec, the float starts to pump 360 sec to emerge-under standard (calm) weather condi.ons, it takes 90-100 sec for the float to rise the surface from 10m.-data acquisi.on from 1m to 0 m: mean value-data acquisi.on from 10 to 1m : raw data @ 2 sec (0.5Hz)-data acquisi.on from 350 to 10m : raw data @ 10 sec (0.1Hz) CTS4, NKE 600 s 4.5 m 80 s 0 m Standard behavior sta.s.c & climagraphs Non Standard behavior MOORED AUTOMATIC WEATHER STATIONS valida.on with weather mooring Time-serie of behavior indicators of two floats compared to meteorological buoy data in the NorthWestern Mediterranean Sea. Even if the distance of the floats to the mooring are greater than 100 km, the comparison of both time series confirms that float behavior indicators can be used with a certain confidence to track sea-state. These indicators can thus be extracted from all the CTS4 floats we manage to date. We now envisage to extract similar float behavior indicators from standard Argo profiles and in particular the information related to big event.. big events > 10m re-dive 0.4% what next Characteris.cs of the NKE CTS4 float in Indicators of float behavior in the upper layer In this poster we explore the idea that the float behavior is related to the sea state. Time spend between 10m and 1m (s) proxy of speed anomaly Maximum re−dive between 10m and 1m [m],

Abstract. In vivo chlorophyll a fluorescence is a proxy of chlorophyll a concentration, and is one of the most frequently measured biogeochemical properties in the ocean. Thousands of profiles are available from historical databases and the integration of fluorescence sensors to autonomous platforms has led to a significant increase of chlorophyll fluorescence profile acquisition. To our knowledge, this important source of environmental data has not yet been included in global analyses. A total of 268 127 chlorophyll fluorescence profiles from several databases as well as published and unpublished individual sources were compiled. Following a robust quality control procedure detailed in the present paper, about 49 000 chlorophyll fluorescence profiles were converted into phytoplankton biomass (i.e., chlorophyll a concentration) and size-based community composition (i.e., microphytoplankton, nanophytoplankton and picophytoplankton), using a method specifically developed to harmonize fluorescence profiles from diverse sources. The data span over 5 decades from 1958 to 2015, including observations from all major oceanic basins and all seasons, and depths ranging from the surface to a median maximum sampling depth of around 700 m. Global maps of chlorophyll a concentration and phytoplankton community composition are presented here for the first time. Monthly climatologies were computed for three of Longhurst's ecological provinces in order to exemplify the potential use of the data product. Original data sets (raw fluorescence profiles) as well as calibrated profiles of phytoplankton biomass and community composition are available on open access at PANGAEA, Data Publisher for Earth and Environmental Science.

A neural network-based method is developed to assess the vertical distribution of (1) chlorophyll a concentration ([Chl]) and (2) phytoplankton community size indices (i.e., microphytoplankton, nanophytoplankton, and picophytoplankton) from in situ vertical profiles of chlorophyll fluorescence. This method (FLAVOR for Fluorescence to Algal communities Vertical distribution in the Oceanic Realm) uses as input only the shape of the fluorescence profile associated with its acquisition date and geo-location. The neural network is trained and validated using a large database including 896 concomitant in situ vertical profiles of High-Performance Liquid Chromatography (HPLC) pigments and fluorescence. These profiles were collected during 22 oceanographic cruises representative of the global ocean in terms of trophic and oceanographic conditions, making our method applicable to most oceanic waters. FLAVOR is validated with respect to the retrieval of both [Chl] and phytoplankton size indices using an independent in situ data set and appears to be relatively robust spatially and temporally. To illustrate the potential of the method, we applied it to in situ measurements of the BATS (Bermuda Atlantic Time Series Study) site and produce monthly climatologies of [Chl] and associated phytoplankton size indices. The resulting climatologies appear very promising compared to climatologies based on available in situ HPLC data. With the increasing availability of spatially and temporally well-resolved data sets of chlorophyll fluorescence, one possible global-scale application of FLAVOR could be to develop 3-D and even 4-D climatologies of [Chl] and associated composition of phytoplankton communities. The Matlab and R codes of the proposed algorithm are provided as supporting information.

Based on in situ data sets collected using two Bio-Argo floats deployed in the subpolar North Atlantic from June 2008 to May 2010, the present study focuses on the seasonal variability of three bio-optical properties, i.e., chlorophyll-a concentration ([Chla]), particle backscattering coefficient at 532 nm (b(bp)(532)), and particle beam attenuation coefficient at 660 nm (c(p)(660)). In addition, the interrelationships among these properties are examined. Our results show that: (1) [Chla], b(bp)(532) and c(p)(660) are largely well coupled with each other in the upper layer, all being minimum in mid-winter (January) and maximum in summer; (2) the backscattering coefficient presents an abrupt increase in late summer in the Icelandic Basin, likely due to a large contribution of coccolithophores following the diatom spring bloom; (3) the intercorrelations between the three bio-optical properties are basically consistent with previous studies; (4) seasonal variation in the of [Chla] to c(p)(660) ratio exhibits a clear light-dependence, most likely due to the phytoplankton photoacclimation.

We deployed four Bio-Argo profiling floats in various oligotrophic locations of the Pacific subtropical gyres and Mediterranean Sea to address the seasonal phytoplankton dynamics in the euphotic layer and explore its dependence on light regime dynamics. Results show that there is a similar phytoplankton biomass seasonal pattern in the four observed oceanic regions. In the lower part of the euphotic layer, the seasonal displacement of the deep chlorophyll maximum (DCM) is light driven. During winter, the chlorophyll a concentration ([Chl a]) always increases in the upper euphotic mixed layer. This increase always results from a photoacclimation to the reduced irradiance. Depending on the location, however, the concentration can also be associated with an actual increase in biomass. The winter increase in [Chl a] results in an increase in irradiance attenuation that impacts the position of the isolume (level where the daily integrated photon flux is constant) and DCM, which becomes shallower. In summer when the [Chl a] in the upper layer decreases along with light attenuation, the DCM deepens and becomes closer to (and sometimes reaches) the nitracline, which enhances the phytoplankton biomass at the DCM. The bio-optical mechanisms and their relationship to light regimes that are revealed by the time series appear to be generic and potentially characteristic of all of the areas where a DCM forms, which is 50% of the open ocean.

Two profiling floats, equipped with nitrate concentration sensors were deployed in the northwestern Mediterranean from summer 2012 to summer 2013. Satellite ocean color data were extracted to evaluate surface chlorophyll concentration at float locations. Time series of mixed layer depths and nitrate and chlorophyll concentrations were analyzed to characterize the interplay between the physical-chemical and biological dynamics in the area. Deep convection (mixed layer depth > 1000 m) was observed in January-February, although high-nitrate surface concentrations could be already observed in December. Chlorophyll increase is observed since December, although high values were observed only in March. The early nitrate availability in subsurface layers, which is likely due to the permanent cyclonic circulation of the area, appears to drive the bloom onset. The additional nitrate supply associated to the deep convection events, although strengthening the overall nitrate uptake, seems decoupled of the December increase of chlorophyll.

The present data publication provides permanent links to original and updated versions of validated data files. The data files include properties of seawater, particulate matter and dissolved matter that were measured from discrete water samples collected with Niskin bottles during the 2009-2013 Tara Oceans expedition. Properties include pigment concentrations from HPLC analysis (10 depths per vertical profile, 25 pigments per depth), the carbonate system (Surface and 400m; pH (total scale), CO₂, pCO₂, <em>f</em>CO₂, HCO₃, CO₃, Total alkalinity, Total carbon, OmegaAragonite, OmegaCalcite, and dosage Flags), nutrients (10 depths per vertical profile; NO₂, PO₄, NO₂/NO₃, SI, quality Flags), DOC, CDOM, and dissolved oxygen isotopes. The Service National d'Analyse des Paramètres Océaniques du CO₂, at the Université Pierre et Marie Curie, determined CT and AT potentiometrically (Edmond 1970; DOE 1994) on samples preserved according to Dickson et al. (2007). More than 250 vertical profiles of these properties were made across the world ocean. DOC, CDOM and dissolved oxygen isotopes are available only for the Arctic Ocean and Arctic Seas (2013).

Phytoplankton phenology is primarily affected by physical forcing. However, its quantification is far from being completely understood. Among the physical forcing factors, the mixed layer depth (MLD) is considered to have the strongest impact on phytoplankton dynamics, and consequently, on their phenology. The role of MLD variations in shaping the phytoplankton phenology was explored in the Mediterranean Sea, a basin displaying contrasting phenological regimes. A database of MLD estimations was merged with ocean color chlorophyll concentrations ([Chl]SAT) to generate concomitant annual MLD and [Chl]SAT cycles. Several indices were calculated to quantitatively analyze these cycles. The relevance of indices summarizing the temporal difference between main characteristics of MLD and [Chl]SAT cycles was emphasized. As previously observed, two dominant phenological regimes coexist in the Mediterranean Sea. The first is marked by a typical spring bloom, as in temperate regions. The second displays a low seasonality and an absence of an intense [Chl]SAT peak as in subtropical areas. The MLD is shown to play a key role in determining the dominant phenological regime in a given area. Results also show that regions having low seasonality display concomitant MLD and [Chl]SAT maxima, whereas [Chl]SAT peaks are generally observed 30 days after MLD peaks in regions with strongest seasonality. Over the whole basin, [Chl]SAT increase starts 1 month after the initiation of MLD deepening. Finally, after examining the impact of MLD on light and nutrient availability for phytoplankton, mechanisms were proposed to explain the time lags between MLD and [Chl]SAT increase and MLD and [Chl]SAT maxima.

We analyze an original large data set of concurrent in situ measurements of fluorescence, temperature and salinity provided by sensors mounted on the elephant seals of Kerguelen Island. Our results were mainly gathered in regions of the Southern Ocean where the typical iron limitation is relieved by natural iron fertilization. Thus the role of light as the proximal factor of control of phytoplankton can be examined. We show that self-shading, and consequently stratification, are major factors controlling the integrated biomass during the bloom induced by iron fertilization. When the mixed layer was the shallowest, the maximum ChlMLachievable by the given light-mixing regime was however not reached, most likely due to silicic acid limitation. We also show that a favorable light-mixing regime prevails after the spring equinox and is maintained for roughly seven months (October-April). Citation: Blain, S., S. Renaut, X. Xing, H. Claustre, and C. Guinet (2013), Instrumented elephant seals reveal the seasonality in chlorophyll and light-mixing regime in the ironfertilized Southern Ocean.

A dataset consisting of AC-S measurements of (hyper-) spectral particulate absorption, scattering and attenuation coefficients were obtained from measurements performed on the flow-through system of the R/V Tara during its 2.5-year long expedition.The AC-S instruments were robust, working continuously with weekly maintenance for about 3 months at a time, and provided absorption (attenuation) data for 454 (375) days, or 90% (75%) of total possible days during the expedition.This dataset has been mapped to 1 km×1 km bins to avoid over emphasizing redundant data, and to match the spatial scale of typical ocean color satellite sensors. It consists of nearly 70,000 particulate absorption spectra and about 60,000 particulate scattering and attenuation spectra. These data are found to be consistent with chlorophyll extraction and with the published average shapes of particulate absorption and scattering spectra and bio-optical relationships. This dataset is richer than previous ones in the data from open-ocean (oligotrophic) environments making it more representative of global distributions and of utility for global algorithm development.

Abstract. A global pigment database consisting of 35 634 pigment suites measured by high performance liquid chromatography was assembled in support of the MARine Ecosytem DATa (MAREDAT) initiative. These data originate from 136 field surveys within the global ocean, were solicited from investigators and databases, compiled, and then quality controlled. Nearly one quarter of the data originates from the Laboratoire d'Océanographie de Villefranche (LOV), with an additional 17% and 19% stemming from the US JGOFS and LTER programs, respectively. The MAREDAT pigment database provides high quality measurements of the major taxonomic pigments including chlorophylls a and b, 19'-butanoyloxyfucoxanthin, 19'-hexanoyloxyfucoxanthin, alloxanthin, divinyl chlorophyll a, fucoxanthin, lutein, peridinin, prasinoxanthin, violaxanthin and zeaxanthin, which may be used in varying combinations to estimate phytoplankton community composition. Quality control measures consisted of flagging samples that had a total chlorophyll a concentration of zero, had fewer than four reported accessory pigments, or exceeded two standard deviations of the log-linear regression of total chlorophyll a with total accessory pigment concentrations. We anticipate the MAREDAT pigment database to be of use in the marine ecology, remote sensing and ecological modeling communities, where it will support model validation and advance our global perspective on marine biodiversity. The original dataset together with quality control flags as well as the gridded MAREDAT pigment data may be downloaded from PANGAEA: http://doi.pangaea.de/10.1594/PANGAEA.793246.

In situ observation of the marine environment has traditionally relied on ship-based platforms. The obvious consequence is that physical and biogeochemical properties have been dramatically undersampled, especially in the remote Southern Ocean (SO). The difficulty in obtaining in situ data represents the major limitations to our understanding, and interpretation of the coupling between physical forcing and the biogeochemical response. Southern elephant seals (Mirounga leonina) equipped with a new generation of oceanographic sensors can measure ocean structure in regions and seasons rarely observed with traditional oceanographic platforms. Over the last few years, seals have allowed for a considerable increase in temperature and salinity profiles from the SO, but we were still lacking information on the spatiotemporal variation of phytoplankton concentration. This information is critical to assess how the biological productivity of the SO, with direct consequences on the amount of CO2 "fixed" by the biological pump, will respond to global warming. In this research programme, we use an innovative sampling fluorescence approach to quantify phytoplankton concentration at sea. For the first time, a low energy consumption fluorometer was added to Argos CTD-SRDL tags, and these novel instruments were deployed on 27 southern elephant seals between 25 December 2007 and the 4 February 2011. As many as 3388 fluorescence profiles associated with temperature and salinity measurements were thereby collected from a vast sector of the Southern Indian Ocean. This paper addresses the calibration issue of the fluorometer before being deployed on elephant seals and presents the first results obtained for the Indian sector of the Southern Ocean. This in situ system is implemented in synergy with satellite ocean colour radiometry. Satellite-derived data is limited to the surface layer and is restricted over the SO by extensive cloud cover. However, with the addition of these new tags, we are able to assess the 3-dimension distribution of phytoplankton concentration by foraging southern elephant seals. This approach reveals that for the Indian sector of the SO, the surface chlorophyll a (chl a) concentrations provided by MODIS were underestimated by a factor 2 compared to chl a concentrations estimated from HPLC corrected in situ fluorescence measurements. The scientific outcomes of this programme include an improved understanding of both the present state and variability in ocean biology, and the accompanying biogeochemistry, as well as the delivery of real-time and open-access data to scientists (doi:10.7491/MEMO.1x)

A global dataset of in situ particulate absorption spectra has been decomposed into component functions representing absorption by phytoplankton pigments and non-algal particles. The magnitudes of component Gaussian functions, used to represent absorption by individual or groups of pigments, are well correlated with pigment concentrations determined using High Performance Liquid Chromatography. We are able to predict the presence of chlorophylls a , b, and c, as well as two different groups of summed carotenoid pigments with percent errors between 30% and 57%. Existing methods of analysis of particulate absorption spectra measured in situ provide for only chlorophyll a; the method presented here, using high spectral resolution particulate absorption, shows the ability to obtain the concentrations of additional pigments, allowing for more detailed studies of phytoplankton ecology than currently possible with in-situ spectroscopy.

As the proxy for Chlorophyll a (Chl a) concentration, thousands of fluorescence profiles were measured by instrumented elephant seals in the Kerguelen region (Southern Ocean). For accurate retrieval of Chl a concentrations acquired by in vivo fluorometer, a two-step procedure is applied: 1) A predeployment intercalibration with accurate determination by high performance liquid chromatography (HPLC) analysis, which not only calibrates fluorescence in appropriate Chl a concentration units, but also strongly reduces variability between fluorometers, and 2) a profile-by-profile quenching correction analysis, which effectively eliminates the fluorescence quenching issue at surface around noon, and results in consistent profiles between day and night. The quenching correction is conducted through an extrapolation of the deep fluorescence value toward surface. As proved by a validation procedure in the Western Mediterranean Sea, the correction method is practical and relatively reliable when there is no credible reference, especially for deep mixed waters, as in the Southern Ocean. Even in the shallow mixed waters, the method is also effective in reducing the influence of quenching.

Understanding the ocean carbon cycle requires a precise assessment of phytoplankton biomass in the oceans. In terms of numbers of observations, satellite data represent the largest available data set. However, as they are limited to surface waters, they have to be merged with in situ observations. Amongst the in situ data, fluorescence profiles constitute the greatest data set available, because fluorometers have operated routinely on oceanographic cruises since the 1970s. Nevertheless, fluorescence is only a proxy of the total chlorophyll a concentration and a data calibration is required. Calibration issues are, however, sources of uncertainty, and they have prevented a systematic and wide range exploitation of the fluorescence data set. In particular, very few attempts to standardize the fluorescence databases have been made. Consequently , merged estimations with other data sources (e.g. satellite) are lacking. We propose a merging method to fill this gap. It consists firstly in adjusting the fluorescence profile to impose a zero chlorophyll a concentration at depth. Secondly, each point of the fluorescence profile is then multiplied by a correction coefficient, which forces the chlorophyll a integrated content measured on the fluorescence profile to be consistent with the concomitant ocean colour observation. The method is close to the approach proposed by Boss et al. (2008) to correct fluorescence data of a profiling float, although important differences do exist. To develop and test our approach , in situ data from three open ocean stations (BATS, HOT and DYFAMED) were used. Comparison of the so-called "satellite-corrected" fluorescence profiles with con-comitant bottle-derived estimations of chlorophyll a concentration was performed to evaluate the final error (estimated at 31 %). Comparison with the Boss et al. (2008) method, using a subset of the DYFAMED data set, demonstrated that the methods have similar accuracy. The method was applied to two different data sets to demonstrate its utility. Using fluo-rescence profiles at BATS, we show that the integration of "satellite-corrected" fluorescence profiles in chlorophyll a climatologies could improve both the statistical relevance of chlorophyll a averages and the vertical structure of the chlorophyll a field. We also show that our method could be efficiently used to process, within near-real time, profiles obtained by a fluorometer deployed on autonomous platforms, in our case a bio-optical profiling float. The application of the proposed method should provide a first step towards the generation of a merged satellite/fluorescence chlorophyll a product, as the "satellite-corrected" profiles should then be consistent with satellite observations. Improved climatolo-gies with more consistent satellite and in situ data are likely to enhance the performance of present biogeochemical models .

An approach that combines a recently developed procedure for improved estimation of surface chlorophyll a concentration (Chl surf) from ocean color and a phytoplankton class-specific bio-optical model was used to examine primary production in the Mediterranean Sea. Specifically, this approach was applied to the 10 year time series of satellite Chl surf data from the Sea-viewing Wide Field-of-view Sensor. We estimated the primary production associated with three major phytoplankton classes (micro, nano, and picophytoplankton), which also yielded new estimates of the total primary production (P tot). These estimates of P tot (e.g., 68 g C m À2 yr À1 for the entire Mediterranean basin) are lower by a factor of $2 and show a different seasonal cycle when compared with results from conventional approaches based on standard ocean color chlorophyll algorithm and a non-class-specific primary production model. Nanophytoplankton are found to be dominant contributors to P tot (43-50%) throughout the year and entire basin. Micro and picophytoplankton exhibit variable contributions to P tot depending on the season and ecological regime. In the most oligotrophic regime, these contributions are relatively stable all year long with picophytoplankton ($32%) playing a larger role than microphytoplankton ($22%). In the blooming regime, picophytoplankton dominate over microphytoplankton most of the year, except during the spring bloom when microphytoplankton (27-38%) are considerably more important than picophytoplankton (20-27%).

Eight autonomous profiling "Bio-Argo" floats were deployed offshore during about 2 years (2008-2010) in Pacific, Atlantic, and Mediterranean zones. They were equipped with miniaturized bio-optical sensors, namely a radiometer measuring within the upper layer the downward irradiance at 412, 490, and 555 nm, and two fluorometers for detection of chlorophyll-a (Chla) and colored dissolved organic matter (CDOM; profiles from 400 m to surface). A first study dealt with the interpretation of the Chla fluorescence signal in terms of concentration, using for this purpose the diffuse attenuation coefficient for irradiance at 490 nm, K d (490), taken as a proxy for the Chla absorption. The present study examines the possibility of similarly using the K d (412) values combined with retrieved Chla profiles to convert the CDOM fluorometric qualitative information into a CDOM absorption coefficient (a y). The rationale is to take advantage of the fact that K d is more sensitive to CDOM presence at 412 nm than at 490 nm. A validation of this method is tested through its application to field data, collected from a ship over a wide range of trophic conditions (Biogeochemistry and Optics South Pacific Experiment (BIOSOPE) cruise); these data include both in situ fluorescence profiles and CDOM absorption as measured on discrete samples. In addition, near-surface a y values retrieved from the floats agree with those derivable from ocean color imagery (Moderate Resolution Imaging Spectroradiometer (MODIS-A)). The low sensitivity of commercially available CDOM fluorometers presently raises difficulties when applying this technique to open ocean waters. It was nevertheless possible to derive from the floats records meaningful time series of CDOM vertical distribution.

Eight autonomous profiling floats equipped with miniaturized radiometers and fluorimeters have collected data in Pacific, Atlantic, and Mediterranean offshore zones. They measured in particular 0-400 m vertical profiles of the downward irradiance at three wavelengths (412, 490, and 555 nm) and of the chlorophyll a fluorescence. Such autonomous sensors collect radiometric data regardless of sky conditions and collect essentially uncalibrated fluorescence data. Usual processing and calibration techniques are no longer usable in such remote conditions and have to be adapted. The proposition here is an interwoven processing by which missing parts of irradiance profiles (due to intermittent cloud occurrence) are interpolated by accounting for possible changes in optical properties (detected by the fluorescence signal) and by which the attenuation coefficient for downward irradiance, used as proxy for [Chl a] (the chlorophyll a concentration), allows the fluorescence signal to be calibrated in absolute units (mg m −3). This method is successfully applied to about 600 irradiance and fluorescence profiles. Validation of the results in terms of [Chl a] is made by matchup with satellite (MODIS-A) chlorophyll (24.3% RMSE, N = 358). Validation of the method is obtained by applying it on similar field data acquired from ships, which, in addition to irradiance and fluorescence profiles, include the [Chl a] HPLC determination, used for final verification.

In vivo fluorescence of Chlorophyll-a (Chl-a) is a potentially useful property to study the vertical distribution of phytoplankton biomass. However the technique is presently not fully exploited as it should be, essentially because of the difficulties in converting the fluorescence signal into an accurate Chl-a concentration. These difficulties arise noticeably from natural variations in the Chl-a fluores-cence relationship, which is under the control of community composition as well as of their nutrient and light status. As a consequence, although vertical profiles of fluores-cence are likely the most recorded biological property in the open ocean, the corresponding large databases are underex-ploited. Here with the aim to convert a fluorescence profile into a Chl-a concentration profile, we test the hypothesis that the Chl-a concentration can be gathered from the sole knowledge of the shape of the fluorescence profile. We analyze a large dataset from 18 oceanographic cruises conducted in case-1 waters from the highly stratified hyperoligotrophic waters (surface Chl-a = 0.02 mg m −3) of the South Pacific Gyre to the eutrophic waters of the Benguela upwelling (sur-face Chl-a = 32 mg m −3) and including the very deep mixed waters in the North Atlantic (Mixed Layer Depth = 690 m). This dataset encompasses more than 700 vertical profiles of Chl-a fluorescence as well as accurate estimations of Chl-a by High Performance Liquid Chromatography (HPLC). Two typical fluorescence profiles are identified, the uniform profile , characterized by a homogeneous layer roughly corresponding to the mixed layer, and the non-uniform profile, characterized by the presence of a Deep Chlorophyll Maximum. Using appropriate mathematical parameterizations, a fluorescence profile is subsequently represented by 3 or 5 shape parameters for uniform or non-uniform profiles, respectively. For both situations, an empirical model is de-Correspondence to: A. Mignot () veloped to predict the "true" Chl-a concentration from these shape parameters. This model is then used to calibrate a flu-orescence profile in Chl-a units. The validation of the approach provides satisfactory results with a median absolute percent deviation of 33 % when comparing the HPLC Chl-a profiles to the Chl-a-calibrated fluorescence. The proposed approach thus opens the possibility to produce Chl-a clima-tologies from uncalibrated fluorescence profile databases that have been acquired in the past and to which numerous new profiles will be added, thanks to the recent availability of autonomous platforms (profiling floats, gliders and animals) in-strumented with miniature fluorometers.

The silicon biogeochemical cycle has been studied in the Mediterranean Sea during late summer/early autumn 1999 and summer 2008. The distribution of nutrients, particulate carbon and silicon, fucoxanthin (Fuco), and total chlorophyll-a (TChl-a) were investigated along an eastward gradient of oligotrophy during two cruises (PROSOPE and BOUM) encompassing the entire Mediterranean Sea during the stratified period. At both seasons, surface waters were depleted in nutrients and the nutriclines gradually deepened towards the East, the phosphacline being the deepest in the easternmost Levantine basin. Following the nutriclines, parallel deep maxima of biogenic silica (DSM), fucoxanthin (DFM) and TChl-a (DCM) were evidenced during both seasons with maximal concentrations of 0.45 μmol L−1 for BSi, 0.26 μg L−1 for Fuco, and 1.70 μg L−1 for TChl-a, all measured during summer. Contrary to the DCM which was a persistent feature in the Mediterranean Sea, the DSM and DFMs were observed in discrete areas of the Alboran Sea, the Algero-Provencal basin, the Ionian sea and the Levantine basin, indicating that diatoms were able to grow at depth and dominate the DCM under specific conditions. Diatom assemblages were dominated by Chaetoceros spp., Leptocylindrus spp., Pseudonitzschia spp. and the association between large centric diatoms (Hemiaulus hauckii and Rhizosolenia styliformis) and the cyanobacterium Richelia intracellularis was observed at nearly all sites. The diatom's ability to grow at depth is commonly observed in other oligotrophic regions and could play a major role in ecosystem productivity and carbon export to depth. Contrary to the common view that Si and siliceous phytoplankton are not major components of the Mediterranean biogeochemistry, we suggest here that diatoms, by persisting at depth during the stratified period, could contribute to a large part of the marine primary production as observed in other oligotrophic areas.

We apply an innovative approach to time series data of surface chlorophyll from satellite observations with SeaWiFS (Sea-viewing Wide Field-of-view Sensor) to estimate the primary production associated with three major phytoplankton classes (micro-, nano-, and picophytoplankton) within the world's oceans. Statistical relationships, determined from an extensive in situ database of phytoplankton pigments, are used to infer class-specific vertical profiles of chlorophyll a concentration from satellite-derived surface chlorophyll a. This information is combined with a primary production model and class-specific photophysiological parameters to compute global seasonal fields of class-specific primary production over a 10-year period from January 1998 through December 2007. Microphytoplankton (mostly diatoms) appear as a major contributor to total primary production in coastal upwelling systems (70%) and temperate and subpolar regions (50%) during the spring-summer season. The contribution of picophytoplankton (e.g., prokaryotes) reaches maximum values (45%) in subtropical oligotrophic gyres. Nanophytoplankton (e. g., prymnesiophytes) provide a ubiquitous, substantial contribution (30-60%). Annual global estimates of class-specific primary production amount to 15 Gt C yr(-1) (32% of total), 20 Gt C yr(-1) (44%) and 11 Gt C yr(-1) (24%) for micro-, nano-, and picophytoplankton, respectively. The analysis of interannual variations revealed large anomalies in class-specific primary production as compared to the 10-year mean cycle in both the productive North Atlantic basin and the more stable equatorial Pacific upwelling. Microphytoplankton show the largest range of variability of the three phytoplankton classes on seasonal and interannual time scales. Our results contribute to an understanding and quantification of carbon cycle in the ocean.

The cores of the subtropical anticyclonic gyres are characterized by their oligotrophic status and minimal chlorophyll concentration, compared to that of the whole ocean. These zones are unambiguously detected by space borne ocean color sensors thanks to their typical spectral reflectance, which is that of extremely clear and deep blue waters. Not only the low chlorophyll (denoted [Chl]) level, but also a reduced amount of colored dissolved organic matter (CDOM or ``yellow substance'') account for this clarity. The oligotrophic waters of the North and South Pacific gyres, the North and South Atlantic gyres, and the South Indian gyre have been comparatively studied with respect to both [Chl] and CDOM contents, by using 10-year data (1998-2007) of the Sea-viewing Wide field-of-view Sensor (Sea-WiFS, NASA). Albeit similar these oligotrophic zones are not identical regarding their [Chl] and CDOM contents, as well as their seasonal cycles. According to the zone, the averaged [Chl] value varies from 0.026 to 0.059 mg m(-3), whereas the ay(443) average (the absorption coefficient due to CDOM at 443 nm) is between 0.0033 and 0.0072 m(-1). The CDOM-to-[Chl] relative proportions also differ between the zones. The clearest waters, corresponding to the lowest [Chl] and CDOM concentrations, are found near Easter Island and near Mariana Islands in the western part of the North Pacific Ocean. In spite of its low [Chl], the Sargasso Sea presents the highest CDOM content amongst the six zones studied. Except in the North Pacific gyre (near Mariana and south of Hawaii islands), a conspicuous seasonality appears to be the rule in the other 4 gyres and affects both [Chl] and CDOM; both quantities vary in a ratio of about 2 (maximum-to-minimum). Coinciding [Chl] and CDOM peaks occur just after the local winter solstice, which is also the period of the maximal mixed layer depth in these latitudes. It is hypothesized that the vertical transport of unbleached CDOM from the subthermocline layers is the main process enhancing the CDOM concentration within the upper layer in winter. In summer, the CDOM experiences its minimum which is delayed with respect to the [Chl] minimum; apparently, the solar photo-bleaching of CDOM is a slower process than the post-bloom algal Chl decay. Where they exist, the seasonal cycles are repeated without notable change from year to year. Long term (10 y) trends have not been detected in these zones. These oligotrophic gyres can conveniently be used for in-flight calibration and comparison of ocean color sensors, provided that their marked seasonal variations are accounted for.

The subtropical gyres represent 40% of the total ocean. They have been considered, since a long time, as steady state biological desert due to low nutrients and phytoplankton biomass throughout the year. Primary production (carbon uptake by phytoplankton) is low too in these regions, but as these structures are huge, it makes the total contribution to ocean primary production important. The South Pacific gyre is considered to be the place where the phytoplankton biomass is the lowest in the global ocean. This area remains poorly sampled and studied, that's why the seasonal variability of phytoplankton, primary production and physical forcing remains unclear. By using in-situ bio-optical profiling float capable to measure physical parameters (temperature and salinity) and proxy of the phytoplankton biomass in the water column, we acquired one year of data near the eastern islands (in the middle of the South Pacific gyre). We found a seasonal variability of the phytoplankton biomass and the carbon flux. This variability is driven by the vertical structure density of the ocean, from very low phytoplankton biomass and carbon uptake in winter (due to the mixing of the water column) to low phytoplankton biomass and carbon uptake in summer (associated with strong stratification). Studying the interaction between the physical forcing and phytoplankton biomass in the subtropical gyres is essential. Any increase in sea water temperature will change the stratification of the water column and thus affect the annual carbon fluxes budget.

Absorption coefficients of phytoplankton, nonalgal particles (NAPs), and colored dissolved organic matter (CDOM), and their relative contributions to total light absorption, are essential variables for bio-optical and biogeochemical models. However, their actual variations in the open ocean remain poorly documented, particularly for clear waters because of the difficulty in measuring very low absorption coefficients. The Biogeochemistry and Optics South Pacific Experiment (BIOSOPE) cruise investigated a large range of oceanic regimes, from mesotrophic waters around the Marquesas Islands to hyperoligotrophic waters in the subtropical gyre and eutrophic waters in the upwelling area off Chile. The spectral absorption coefficients of phytoplankton and NAPs were determined using the filter technique, while the CDOM absorption coefficients were measured using a 2 m capillary waveguide. Over the whole transect, the absorption coefficients of both dissolved and particulate components covered approximately two orders of magnitude; in the gyre, they were among the lowest ever reported for open ocean waters. In the oligotrophic and mesotrophic waters, absorption coefficients of phytoplankton and NAPs were notably lower than those measured in other oceanic areas with similar chlorophyll contents, indicating some deviation from the standard chlorophyll-absorption relationships. The contribution of absorption by NAPs to total particulate absorption showed large vertical and horizontal variations. CDOM absorption coefficients covaried with algal biomass, albeit with a high scatter. The spectral slopes of both NAP and CDOM absorption revealed structured spatial variability in relation with the trophic conditions. The relative contributions of each component to total nonwater absorption were (at a given wavelength) weakly variable over the transect, at least within the euphotic layer.

We recorded vertical profiles of size distributions of large particles (> 100 mu m) to a 1000-m depth in the Atlantic, Indian, and Pacific Oceans and in the Mediterranean Sea with the Underwater Video Profiler. Of the 410 profiles used in our analysis, 193 also included temperature, salinity, and high-performance liquid chromatography (HPLC)-resolved pigments, which were used to characterize the size structure of the phytoplankton community. Classification analysis identified six clusters of vertical profiles of size distributions of particles. Each cluster was characterized by the size distribution of its particles in the mesopelagic layer and the change of the particle-size distribution with depth. Clusters with large particles in the mesopelagic layer corresponded to surface waters dominated by microphytoplankton, and those with small particles corresponded to surface waters dominated by picophytoplankton. We estimated the mass flux at 400 m using a relationship between particle size and mass flux. Principal-component regression analysis showed that 68% of the variance of the mass flux at 400 m was explained by the size structure of the phytoplankton community and integrated chlorophyll a in the euphotic zone. We found that coefficient k in the Martin power relationship, which describes the decrease in the vertical mass flux with depth, varies between 0.2 and 1.0 in the world ocean, and we provided an empirical relationship to derive k from the size structure of phytoplankton biomass in the euphotic zone. Biogeochemists and modelers could use that relationship to obtain a realistic description of the downward particle flux instead of using a constant k value as often done.

The current paradigm holds that cyanobacteria, which evolved oxygenic photosynthesis more than 2 billion years ago, are still the major light harvesters driving primary productivity in open oceans. Here we show that tiny unicellular eukaryotes belonging to the photosynthetic lineage of the Haptophyta are dramatically diverse and ecologically dominant in the planktonic photic realm. The use of Haptophyta-specific primers and PCR conditions adapted for GC-rich genomes circumvented biases inherent in classical genetic approaches to exploring environmental eukaryotic biodiversity and led to the discovery of hundreds of unique haptophyte taxa in 5 clone libraries from subpolar and subtropical oceanic waters. Phylogenetic analyses suggest that this diversity emerged in Paleozoic oceans, thrived and diversified in the permanently oxygenated Mesozoic Panthalassa, and currently comprises thousands of ribotypic species, belonging primarily to low-abundance and ancient lineages of the ``rare biosphere.'' This extreme biodiversity coincides with the pervasive presence in the photic zone of the world ocean of 19'-hexanoyloxyfucoxanthin (19-Hex), an accessory photosynthetic pigment found exclusively in chloroplasts of haptophyte origin. Our new estimates of depth-integrated relative abundance of 19-Hex indicate that haptophytes dominate the chlorophyll a-normalized phytoplankton standing stock in modern oceans. Their ecologic and evolutionary success, arguably based on mixotrophy, may have significantly impacted the oceanic carbon pump. These results add to the growing evidence that the evolution of complex microbial eukaryotic cells is a critical force in the functioning of the biosphere.

Chemical and biological sensor technologies have advanced rapidly in the past five years. Sensors that require low power and operate for multiple years are now available for oxygen, nitrate, and a variety of bio-optical properties that serve as proxies for important components of the carbon cycle (e.g., particulate organic carbon). These sensors have all been deployed successfully for long periods, in some cases more than three years, on platforms such as profiling floats or gliders. Technologies for pH, pCO(2), and particulate inorganic carbon are maturing rapidly as well. These sensors could serve as the enabling technology for a global biogeochemical observing system that might operate on a scale comparable to the current Argo array. Here, we review the scientific motivation and the prospects for a global observing system for ocean biogeochemistry.

In the frame of the BIOSOPE cruise in 2004, the spatial distribution and structure of phytoplankton pigments was investigated along a transect crossing the ultra-oligotrophic South Pacific Subtropical Gyre (SPSG) between the Marquesas Archipelago (141° W–8° S) and the Chilean upwelling (73° W–34° S). A High Performance Liquid Chromatography (HPLC) method was improved in order to be able to accurately quantify pigments over such a large range of trophic levels, and especially from strongly oligotrophic conditions. Seven diagnostic pigments were associated to three phytoplankton size classes (pico-, nano and microphytoplankton). The total chlorophyll-a concentrations [TChla] in surface waters were the lowest measured in the centre of the gyre, reaching 0.017 mg m^-3. Pigment concentrations at the Deep Chlorophyll Maximum (DCM) were generally 10 fold the surface values. Results were compared to predictions from a global parameterisation based on remotely sensed surface [TChla]. The agreement between the in situ and predicted data for such contrasting phytoplankton assemblages was generally good: throughout the oligotrophic gyre system, picophytoplankton (prochlorophytes and cyanophytes) and nanophytoplankton were the dominant classes. Relative bacteriochlorophyll-a concentrations varied around 2%. The transition zone between the Marquesas and the SPSG was also well predicted by the model. However, some regional characteristics have been observed where measured and modelled data differ. Amongst these features is the extreme depth of the DCM (180 m) towards the centre of the gyre, the presence of a deep nanoflagellate population beneath the DCM or the presence of a prochlorophyte-enriched population in the formation area of the high salinity South Pacific Tropical Water. A coastal site sampled in the eutrophic upwelling zone, characterised by recently upwelled water, was significantly and unusually enriched in picoeucaryotes, in contrast with an offshore upwelling site where a more typical senescent diatom population prevailed.

We analyzed a large dataset of simultaneous measurements of phytoplankton pigments, spectral specific absorption coefficient for phytoplankton [a*(lambda)], and photosynthesis versus irradiance (P versus E) curve parameters to examine the possible relationships between phytoplankton community structure and photophysiological properties at large spatial scales. Data were collected in various regions, mostly covering the trophic gradient encountered in the world's open ocean. The community composition is described in terms of biomass of three phytoplankton classes, determined using specific biomarker pigments. We present a general empirical model that describes the dependence of algal photophysiology on both the community composition and the relative irradiance within the water column (essentially reflecting photoacclimation). The application of the model to the in situ dataset enables the identification of vertical profiles of photophysiological properties for each phytoplankton class. The class-specific a*(lambda) obtained are consistent with results from the literature and with previous models developed for small and large cells, both in terms of the absolute values and the vertical patterns. Similarly, for the class-specific P versus E curve parameters, the magnitude and vertical distribution obtained with this method are coherent with previous observations. Large cells (mainly diatoms) may be more efficient in carbon storage than smaller cells, whereas their yield of light absorption is lower. We anticipate that such photophysiological parameterizations can improve primary production models by providing estimates of primary production that are specific to different phytoplankton classes on large scale.

Spatial variation of heterotrophic bacterial production and phytoplankton primary production were investigated across the eastern South Pacific Ocean (-141° W, -8° S to -72° W, -35° S) in November–December 2004. Bacterial production (³H leucine incorporation) integrated over the euphotic zone encompassed a wide range of values, from 43 mg C m^-2 d^-1 in the hyper-oligotrophic South Pacific Gyre to 392 mg C m^-2 d^-1 in the upwelling off Chile. In the gyre (120° W, 22° S) records of low phytoplankton biomass (7 mg Total Chl<i>a</i> m^-2) were obtained and fluxes of in situ ¹⁴C-based particulate primary production were as low as 153 mg C m^-2 d^-1, thus equal to the value considered as a limit for primary production under strong oligotrophic conditions. Average rates of ³H leucine incorporation rates, and leucine incorporation rates per cell (5–21 pmol l^-1 h^-1 and 15–56×10^-21 mol cell^-1 h^-1, respectively) determined in the South Pacific gyre, were in the same range as those reported for other oligotrophic subtropical and temperate waters. Fluxes of dark community respiration, determined at selected stations across the transect varied in a narrow range (42–97 mmol O₂ m^-2 d^-1), except for one station in the upwelling off Chile (245 mmol O₂ m^-2 d^-1). Bacterial growth efficiencies varied between 5 and 38%. Bacterial carbon demand largely exceeded ¹⁴C particulate primary production across the South Pacific Ocean, but was lower or equal to gross community production.

The objectives of the BIOSOPE (BIogeochemistry and Optics SOuth Pacific Experiment) project was to study, during the austral summer, the biological, biogeochemical and bio-optical properties of different trophic regimes in the South East Pacific: the eutrophic zone associated with the upwelling regime off the Chilean coast, the mesotrophic area associated with the plume of the Marquises Islands in the HNLC (High Nutrient Low Chlorophyll) waters of this subequatorial area, and the extremely oligotrophic area associated with the central part of the South Pacific Gyre (SPG). At the end of 2004, a 55-day international cruise with 32 scientists on board took place between Tahiti and Chile, crossing the SPG along a North-West South-East transect. This paper describes in detail the objectives of the BIOSOPE project, the implementation plan of the cruise, the main hydrological entities encountered along the ~8000 km South East Pacific transect, and ends with a general overview of the 32 other papers published in this special issue.

Large sinking particles transport organic and inorganic matter into the deeper layers of the oceans. Between 70 and 90% of the aggregates exported from the surface mixed layer are disaggregated within the upper 1000 m. This decrease with depth indicates that fragmentation and remineralization processes are intense during sedimentation. Generally, the estimates of vertical flux rely on sediment trap data but difficulties inherent in their design limit the reliability of this information. During the BIOSOPE study in the southeastern Pacific, 76 vertical casts using the Underwater Video Profiler (UVP) and deployments of drifting sediment traps provided an opportunity to fit the UVP data to sediment trap flux measurements. We applied the calculated UVP flux in the upper 1000 m to the whole 8000 km BIOSOPE transect. Comparison between the large particulate material (LPM) abundance and the estimated fluxes from both UVP and sediment traps showed different patterns in different regions. On the western end of the BIOSOPE section the standing stock of particles in the surface layer was high but the export between 150 and 250 m was low. Below this layer the flux values increased. High values of about 30% of the calculated UVP maximum surface zone flux were observed below 900 m at the HNLC station. The South Pacific Gyre exported about 2 mg m(-2) d(-1). While off Chilean coast 95% of the surface mixed layer matter was disaggregated, remineralized or advected in the upper kilometer, 20% of the surface zone flux was observed below 900 m near the Chilean coast. These results suggest that the export to deep waters is spatially heterogeneous and related to the different biotic and abiotic factors.

Large sinking particles transport organic and inorganic matter into the deeper layers of the oceans. From 70 to 90% of the superficial particulate material is disaggregated within the upper 1000 m. This decrease with depth indicates that remineralization processes are intense during sedimentation. Generally, the estimates of vertical flux rely on the sediment trap data but difficulties inherent in their design, limit the reliability of this information. During the BIOSOPE study in the southeastern Pacific, 76 vertical casts using the Underwater Video Profiler (UVP) and deployments of a limited number of drifting sediment traps provided an opportunity to fit the UVP data to sediment trap flux measurements. We applied than the calculated UVP flux in the upper 1000 m to the whole 8000 km BIOSOPE transect. Comparison between the large particulate material (LPM) abundance and the estimated fluxes from both UVP and sediment traps showed different patterns in different regions. On the western end of the BIOSOPE section the standing stock of particles in the superficial layer was high but the export between 150 and 250 m was low. Below this layer the flux values increased. High values of about 30% of the calculated UVP maximum superficial flux were observed below 900 m at the HNLC station. The South Pacific Gyre exported about 2 mg m^-2 d^-1. While off Chilean coast 95% of the superficial matter was remineralized or advected in the upper kilometer, 20% of the superficial flux was observed below 900 m near the Chilean coast. These results suggest that the export to deep waters is spatially heterogeneous and related to the different biotic and abiotic factors.

Iron is an essential nutrient involved in a variety of biological processes in the ocean, including photosynthesis, respiration and dinitrogen fixation. Atmospheric deposition of aerosols is recognized as the main source of iron for the surface ocean. In high nutrient, low chlorophyll areas, it is now clearly established that iron limits phytoplankton productivity but its biogeochemical role in low nutrient, low chlorophyll environments has been poorly studied. We investigated this question in the unexplored southeast Pacific, arguably the most oligotrophic area of the global ocean. Situated far from any continental aerosol source, the atmospheric iron flux to this province is amongst the lowest of the world ocean. Here we report that, despite low dissolved iron concentrations (~0.1 nmol l^-1) across the whole gyre (3 stations located in the center and at the western and the eastern edges), primary productivity are only limited by iron availability at the border of the gyre, but not in the center. The seasonal stability of the gyre has apparently allowed for the development of populations acclimated to these extreme oligotrophic conditions. Moreover, despite clear evidence of nitrogen limitation in the central gyre, we were unable to measure dinitrogen fixation in our experiments, even after iron and/or phosphate additions, and cyanobacterial nif H gene abundances were extremely low compared to the North Pacific Gyre. The South Pacific gyre is therefore unique with respect to the physiological status of its phytoplankton populations.

Empirical algorithms based on first order relationships between ocean color and the chlorophyll concentration ([ChI]; mg m(-3)) are widely used, but cannot explain the statistical dispersion or ``anomalies'' around the mean trends. We use an empirical approach that removes the first order effects of [ChI] from satellite ocean color, thus allowing us to quantify the impact on the ocean color signal of optical anomalies that vary independently of the global mean trends with remotely sensed [ChI]. We then present statistical and modeling analyses to interpret the observed anomalies in terms of their optical sources (i.e. absorption and backscattering coefficients). We identify two main sources of second order variability for a given [ChI]: 1) the amount of non-algal absorption, especially due to colored dissolved organic matter; and 2) the amplitude of the backscattering coefficient of particles. The global distribution of the anomalies displays significant regional and seasonal trends, providing important information for characterizing the marine optical environment and for inferring biogeochemical influences. We subsequently use our empirically determined anomalies to estimate the backscattering coefficient of particles and the combined absorption coefficient for colored detrital and dissolved materials. This purely empirical approach provides an independent assessment of second order optical variability for comparison with existing methods that are generally based on semi-analytical models. (C) 2008 Elsevier Inc. All rights reserved.

The very clear waters of the South Pacific Gyre likely constitute an end-member of oligotrophic conditions which remain essentially unknown with respect to its impact on carbon fixation and exportation. We describe a non-intrusive bio-optical method to quantify the various terms of a production budget (Gross community production, community losses, net community production) in this area. This method is based on the analysis of the diel cycle in Particulate Organic Carbon (POC), derived from high frequency measurements of the particle attenuation coefficient <i>c</i>_p. We report very high integrated rates of Gross Community Production within the euphotic layer (average of 846±484 mg C m^-2 d^-1 for 17 stations) that are far above any rates determined using incubation techniques for such areas. Furthermore we show that the daily production of POC is essentially balanced by the losses so that the system cannot be considered as net heterotrophic. Our results thus agree well with geochemical methods, but not with incubation studies based on oxygen methods. We stress to the important role of deep layers, below the euphotic layer, in contributing to carbon fixation when incident irradiance at the ocean surface is high (absence of cloud coverage). These deep layers, not considered up to know, might fuel part of the heterotrophic processes in the upper layer, including through dissolved organic carbon. We further demonstrate that, in these extremely clear and stratified waters, integrated gross community production is proportional to the POC content and surface irradiance via an efficiency index ? _GCP^*, the water column cross section for Gross Community Production. We finally discuss our results in the context of the role of oligotrophic gyre in the global carbon budget and of the possibility of using optical proxies from space for the development of growth community rather than primary production global models.

The distribution of lipid biomarkers and their stable carbon isotope composition was investigated on suspended particles from different contrasting trophic environments at six sites in the South East Pacific. High algal biomass with diatom-related lipids (24-methylcholesta-5,24(28)-dien-3β-ol, C25 HBI alkenes, C16:4 FA, C20:5 FA) was characteristic in the upwelling zone, whereas haptophyte lipids (long-chain (C37-C39) unsaturated ketones) were proportionally most abundant in the nutrient-poor settings of the centre of the South Pacific Gyre and on its easter edge. The dinoflagellate–sterol, 4α-23,24-trimethylcholest-22(E)-en-3β-ol, was a minor contributor in all of the studied area and the cyanobacteria-hydrocarbon, C17n-alkane, was at maximum in the high nutrient low chlorophyll regime of the subequatorial waters near the Marquesas archipelago. The taxonomic and spatial variability of the relationships between carbon photosynthetic fractionation and environmental conditions for four specific algal taxa (diatoms, haptophytes, dinoflagellates and cyanobacteria) was also investigated. The carbon isotope fractionation factor (εp) of the 24-methylcholesta-5,24(28)-dien-3β-ol diatom marker, varied over a range of 16% along the different trophic systems. In contrast, εp of dinoflagellate, cyanobacteria and alkenone markers varied only by 7–10‰. The low fractionation factors and small variations between the different phytoplankton markers measured in the upwelling area likely reveals uniformly high specific growth rates within the four phytoplankton taxa, and/or that transport of inorganic carbon into phytoplankton cells may not only occur by diffusion but also by other carbon concentrating mechanisms (CCM). In contrast, in the oligotrophic zone, i.e. gyre and eastgyre, relatively high εp values, especially for the diatom marker, indicate diffusive CO2 uptake by the eukaryotic phytoplankton. At these nutrient-poor sites, the lower εp values for haptophytes, dinoflagellates and cyanobacteria indicate higher growth rates or major differences on the carbon uptake mechanisms compared to diatoms.

Due to the low atmospheric input of phosphate into the open ocean, it is one of the key nutrients that could ultimately control primary production and carbon export into the deep ocean. The observed trend over the last 20 years has shown a decrease in the dissolved inorganic phosphate (DIP) pool in the North Pacific gyre, which has been correlated to the increase in di-nitrogen (N₂) fixation rates. Following a NW-SE transect, in the Southeast Pacific during the early austral summer (BIOSOPE cruise), we present data on DIP, dissolved organic phosphate (DOP) and particulate phosphate (PP) pools along with DIP turnover times (T_DIP) and N₂ fixation rates. We observed a decrease in DIP concentration from the edges to the centre of the gyre. Nevertheless the DIP concentrations remained above 100 nmol L^-1 and T_DIP was more than 6 months in the centre of the gyre; DIP availability remained largely above the level required for phosphate limitation to occur and the absence of Trichodesmium spp and low nitrogen fixation rates were likely to be controlled by other factors such as temperature or iron availability. This contrasts with recent observations in the North Pacific Ocean at the ALOHA station and in the western Pacific Ocean at the same latitude (DIAPALIS cruises) where lower DIP concentrations (<20 nmol L^-1) and T_DIP <50 h were measured during the summer season in the upper layer. The South Pacific gyre can be considered a High Phosphate Low Chlorophyll (HPLC) oligotrophic area, which could potentially support high N₂ fixation rates and possibly carbon dioxide sequestration, if the primary ecophysiological controls, temperature and/or iron availability, were alleviated.

We have examined several approaches for estimating the surface concentration of particulate organic carbon, POC, from optical measurements of spectral remote-sensing reflectance, <i>R_rs</i>(?), using field data collected in tropical and subtropical waters of the eastern South Pacific and eastern Atlantic Oceans. These approaches include a direct empirical relationship between POC and the blue-to-green band ratio of reflectance, <i>R_rs</i>(?_<i>B</i>)/<i>R_rs</i>(555), and two-step algorithms that consist of relationships linking reflectance to an inherent optical property IOP (beam attenuation or backscattering coefficient) and POC to the IOP. We considered two-step empirical algorithms that exclusively include pairs of empirical relationships and two-step hybrid algorithms that consist of semianalytical models and empirical relationships. The surface POC in our data set ranges from about 10 mg m^-3 within the South Pacific Subtropical Gyre to 270 mg m^-3 in the Chilean upwelling area, and ancillary data suggest a considerable variation in the characteristics of particulate assemblages in the investigated waters. The POC algorithm based on the direct relationship between POC and <i>R_rs</i>(?_<i>B</i>)/<i>R_rs</i>(555) promises reasonably good performance in the vast areas of the open ocean covering different provinces from hyperoligotrophic and oligotrophic waters within subtropical gyres to eutrophic coastal upwelling regimes characteristic of eastern ocean boundaries. The best error statistics were found for power function fits to the data of POC vs. <i>R_rs</i>(443)/<i>R_rs</i>(555) and POC vs. <i>R_rs</i>(490)/<i>R_rs</i>(555). For our data set that includes over 50 data pairs, these relationships are characterized by the mean normalized bias of about 2% and the normalized root mean square error of about 20%. We recommend that these algorithms be implemented for routine processing of ocean color satellite data to produce maps of surface POC with the status of an evaluation data product for continued work on algorithm development and refinements. The two-step algorithms also deserve further attention because they can utilize various models for estimating IOPs from reflectance, offer advantages for developing an understanding of bio-optical variability underlying the algorithms, and provide flexibility for regional or seasonal parameterizations of the algorithms.

Probably because it is a readily available ocean color product, almost all models of primary productivity use chlorophyll as their index of phytoplankton biomass. As other variables become more readily available, both from remote sensing and in situ autonomous platforms, we should ask if other indices of biomass might be preferable. Herein, we compare the accuracy of different proxies of phytoplankton biomass for estimating the maximum photosynthetic rate (<i>P</i>_max) and the initial slope of the production versus irradiance (P vs. E) curve (a). The proxies compared are: the total chlorophyll a concentration (Tchla, the sum of chlorophyll a and divinyl chlorophyll), the phytoplankton absorption coefficient, the phytoplankton photosynthetic absorption coefficient, the active fluorescence in situ, the particulate scattering coefficient at 650 nm (<i>b_p</i> (650)), and the particulate backscattering coefficient at 650 nm (<i>b_bp</i> (650)). All of the data (about 170 P vs. E curves) were collected in the South Pacific Ocean. We find that when only the phytoplanktonic biomass proxies are available, <i>b_p</i> (650) and Tchla are respectively the best estimators of <i>P</i>_max and alpha. When additional variables are available, such as the depth of sampling, the irradiance at depth, or the temperature, Tchla becomes the best estimator of both <i>P</i>_max and a. From a remote sensing perspective, error propagation analysis shows that, given the current algorithms errors for estimating <i>b_bp</i>(650), Tchla remains the best estimator of <i>P</i>_max.

During the BIOSOPE field campaign October–December 2004, measurements of inherent optical properties from the surface to 500 m depth were made with a ship profiler at stations covering over ~8000 km through the Southeast Pacific Ocean. Data from a ~3000 km section containing the very clearest waters in the central gyre are reported here. The total volume scattering function at 117°, ß<i>_t</i>(117°), was measured with a WET Labs ECO-BB3 sensor at 462, 532, and 650 nm with estimated uncertainties of 2×10^-5, 5×10^-6, and 2×10^-6 m^-1 sr^-1, respectively. These values were approximately 6%, 3%, and 3% of the scattering by pure seawater at their respective wavelengths. From a methodological perspective, there were several results: – <i>b_bp</i> distributions were resolvable even though some of the values from the central gyre were an order of magnitude lower than the lowest previous measurements in the literature; – Direct in-situ measurements of instrument dark offsets were necessary to accurately resolve backscattering at these low levels; – accurate pure seawater backscattering values are critical in determining particulate backscattering coefficients in the open ocean (not only in these very clear waters); the pure water scattering values determined by Buiteveld et al. (1994) with a [1 + 0.3<i>S</i>/37] adjustment for salinity based on Morel (1974) appear to be the most accurate estimates, with aggregate accuracies as low as a few percent; and – closure was demonstrated with subsurface reflectance measurements reported by Morel et al. (2007) within instrument precisions, a useful factor in validating the backscattering measurements. This methodology enabled several observations with respect to the hydrography and the use of backscattering as a biogeochemical proxy: – The clearest waters sampled were found at depths between 300 and 350 m, from 23.5° S, 118° W to 26° S, 114° W, where total backscattering at 650 nm was not distinguishable from pure seawater; – Distributions of particulate backscattering <i>b_bp</i> across the central gyre exhibited a broad particle peak centered ~100 m; – The particulate backscattering ratio typically ranged between 0.4% and 0.6% through the majority of the central gyre from the surface to ~210 m, indicative of "soft" water-filled particles with low bulk refractive index; and – <i>b_bp</i> at 532 and 650 nm showed a distinct secondary deeper layer centered ~230 m that was absent in particulate attenuation <i>c_p</i> data. The particulate backscattering ratio was significantly higher in this layer than in the rest of the water column, reaching 1.2% in some locations. This high relative backscattering, along with the pigment composition and ecological niche of this layer, appear to be consistent with the coccolithophorid <i>F. profunda</i>. Moreover, results were consistent with several expectations extrapolated from theory and previous work in oceanic and coastal regions, supporting the conclusion that particulate and total backscattering could be resolved in these extremely clear natural waters.

BIOSOPE cruise achieved an oceanographic transect from the Marquise Islands to the Peru-Chili upwelling (PCU) via the centre of the South Pacific Gyre (SPG). Water samples from 6 depths in the euphotic zone were collected at 20 stations. The concentrations of suspended calcite particles, coccolithophores cells and detached coccoliths were estimated together with size and weight using an automatic polarizing microscope, a digital camera, and a collection of softwares performing morphometry and pattern recognition. Some of these softwares are new and described here for the first time. The coccolithophores standing stocks are usually low and reach maxima west of the PCU. The coccoliths of <I>Emiliania huxleyi</I>, <I>Gephyrocapsa</I> spp. and <I>Crenalithus</I> spp. (Order Isochrysidales) represent 50% of all the suspended calcite particles detected in the size range 0.1–46 µm (21% of PIC in term of the calcite weight). The latter species are found to grow preferentially in the Chlorophyll maximum zone. In the SPG their maximum concentrations was found to occur between 150 and 200 m, which is very deep for these taxa. The weight and size of coccoliths and coccospheres are correlated. Large and heavy coccoliths and coccospheres are found in the regions with relative higher fertility in the Marquises Island and in the PCU. Small and light coccoliths and coccospheres are found west of the PCU. This distribution may correspond to that of the concentration of calcium and carbonate ions.

Due to the low atmospheric input of phosphate into the open ocean, it is one of the key nutrients that could ultimately control primary production and carbon export into the deep ocean. The observed trend over the last 20 years, has shown a decrease in the dissolved inorganic phosphate (DIP) pool in the North Pacific gyre, which has been correlated to the increase in di-nitrogen (N₂) fixation rates. Following a NW-SE transect, in the Southeast Pacific during the early austral summer (BIOSOPE cruise), we present data on DIP, dissolved organic phosphate (DOP), and particulate phosphate (PP) pools and DIP turnover times (T_DIP) along with N₂ fixation rates. We observed a decrease in DIP concentration from the edges to the centre of the gyre. Nevertheless the DIP concentrations remained above 100 nmol L^-1 and T_DIP were more than a month in the centre of the gyre: DIP availability remained largely above the level required for phosphate limitation. This contrasts with recent observations in the western Pacific Ocean at the same latitude (DIAPALIS cruises) where lower DIP concentrations (<20 nmol L^-1) and T_DIP<50 h were measured during the summer season. During the BIOSOPE cruise, N₂ fixation rates were higher within the cold water upwelling near the Chilean coast. This observation contrasts with recently obtained model output for N₂ fixation distribution in the South Pacific area and emphasises the importance of studying the main factors controlling this process. The South Pacific gyre can be considered a High P Low Chlorophyll (HPLC) oligotrophic area, which could potentially support high N₂ fixation rates, and possibly carbon dioxide sequestration, if the primary ecophysiological controls, temperature and/or iron availability, were alleviated.

<I>Prochlorococcus</i>, <I>Synechococcus</I>, picophytoeukaryotes and bacterioplankton abundances and contributions to the total particulate organic carbon concentration, derived from the total particle beam attenuation coefficient (<I>c</I>_p), were determined across the eastern South Pacific between the Marquesas Islands and the coast of Chile. All flow cytometrically derived abundances decreased towards the hyper-oligotrophic centre of the gyre and were highest at the coast, except for <I>Prochlorococcus</I>, which was not detected under eutrophic conditions. Temperature and nutrient availability appeared important in modulating picophytoplankton abundance, according to the prevailing trophic conditions. Although the non-vegetal particles tended to dominate the <I>c</I>_p signal everywhere along the transect (50 to 83%), this dominance seemed to weaken from oligo- to eutrophic conditions, the contributions by vegetal and non-vegetal particles being about equal under mature upwelling conditions. Spatial variability in the vegetal compartment was more important than the non-vegetal one in shaping the water column particle beam attenuation coefficient. Spatial variability in picophytoplankton biomass could be traced by changes in both total chlorophyll <I>a</I> (i.e. mono + divinyl chlorophyll <I>a</I>) concentration and <I>c</I>_p. Finally, picophytoeukaryotes contributed ~38% on average to the total integrated phytoplankton carbon biomass or vegetal attenuation signal along the transect, as determined by size measurements (i.e. equivalent spherical diameter) on cells sorted by flow cytometry and optical theory. Although there are some uncertainties associated with these estimates, the new approach used in this work further supports the idea that picophytoeukaryotes play a dominant role in carbon cycling in the upper open ocean, even under hyper-oligotrophic conditions.

<i>Prochlorococcus</i>, <i>Synechococcus</i>, picophytoeukaryotes and bacterioplankton abundances and contributions to the total particulate organic carbon concentration (POC), derived from the total particle beam attenuation coefficient (<i>c</i>_p), were determined across the eastern South Pacific between the Marquesas Islands and the coast of Chile. All flow cytometrically derived abundances decreased towards the hyper-oligotrophic centre of the gyre and were highest at the coast, except for <i>Prochlorococcus</i>, which is not detected under eutrophic conditions. Temperature and nutrient availability appeared important in modulating picophytoplankton abundance, according to the prevailing trophic conditions. Although the non-vegetal particles tended to dominate the <i>c</i>_p signal everywhere along the transect (50 to 83%), this dominance seemed to weaken from oligo- to eutrophic conditions, the contributions by vegetal and non-vegetal particles being about equal under mature upwelling conditions. Spatial variability in the vegetal compartment was more important than the non-vegetal one in shaping the water column particulate attenuation coefficient. Spatial variability in picophytoplankton biomass could be traced by changes in both Tchl<i>a</i> and <i>c</i>_p. Finally, picophytoeukaryotes contributed with ~38% on average to the total integrated phytoplankton carbon biomass or vegetal attenuation signal along the transect, as determined by direct size measurements on cells sorted by flow cytometry and optical theory. The role of picophytoeukaryotes in carbon and energy flow would therefore be very important, even under hyper-oligotrophic conditions.

The phytoplankton (>15 µm) composition and abundance was investigated along a ~8000 km transect between the Marquesas Islands Archipelago and the Chilean coasts off Concepción. In the southern limit of the central Equatorial Pacific (at 8° S, 141° W), in High-Nutrient Low-Chlorophyll (HNLC) warm waters, the micro-phytoplankton assemblage was dominated by the lightly silicified diatoms <i>Pseudo-nitzschia delicatissima</i> and <i>Rhizosolenia bergonii</i>. The morphology of these species, a small pennate diatom that exhibited a tendency to form "ball of needles" clusters and large centric diatom (>500 µm long), are interpreted as two anti-grazing strategies in an environment dominated by small micro-grazers. Surprisingly, this a priori typical HNLC phytoplankton assemblage was also found in the temperate offshore waters of the Perú-Chile Current between 2000 and 600 km off Chile. This observation suggests that a common set of environmental factors (obviously other than temperature and salinity) are responsible for the establishment and maintaining of this distinctive phytoplankton in these geographically and hydrologically distant regions. Both regions are characterized by a surface nitrate-silicate ratio ranging from 1–3. Occasionally <i>Rhizosolenia bergonii</i> showed frustules anomalously fragmented, likely the result of extreme weakly silicified phytoplankton. We suggest that silicate-deficiency may be responsible of the occurrence of HNLC phytoplankton assemblage in the tropical central Pacific as well as offshore Perú-Chile Current during the austral summer.

Predicting heterotrophic bacteria and phytoplankton growth rates (µ) is of great scientific interest. Many methods have been developed in order to assess bacterial or phytoplankton µ. One widely used method is to estimate µ from data obtained on biomass or cell abundance and rates of biomass or cell production. According to Kirchman (2002), the most appropriate approach for estimating µ is simply to divide the production rate by the biomass or cell abundance estimate. Most of the methods using this approach are expressed using carbon (C) data. Nevertheless it is also possible to estimate µ using phosphate (P) data. We showed that particulate phosphate (PartP) can be used to estimate biomass and that the phosphate uptake rate to PartP ratio can be employed to assess µ. Contrary to other methods using C, this estimator does not need conversion factors and provides an evaluation of µ for both autotrophic and heterotrophic organisms. We report values of P-based µ in three size fractions (0.2–0.6; 0.6–2 and >2 µm) along a Southeast Pacific transect, over a wide range of P-replete trophic status. P-based µ values were higher in the 0.6–2 µm fraction than in the >2 µm fraction, suggesting that picoplankton-sized cells grew faster than the larger cells, whatever the trophic regime encountered. Picoplankton-sized cells grew significantly faster in the deep chlorophyll maximum layer than in the upper part of the photic zone in the oligotrophic gyre area, suggesting that picoplankton might outcompete >2 µm cells in this particular high-nutrient, low-light environment. P-based µ attributed to free-living bacteria (0.2–0.6 µm) and picoplankton (0.6–2 µm) size-fractions were relatively low (0.11±0.07 d^-1 and 0.14±0.04 d^-1, respectively) in the Southeast Pacific gyre, suggesting that the microbial community turns over very slowly.

Iron is an essential nutrient involved in a variety of biological processes in the ocean, including photosynthesis, respiration and nitrogen fixation. Atmospheric deposition of aerosols is recognized as the main source of iron for the surface ocean. In high nutrient, low chlorophyll areas, it is now clearly established that iron limits phytoplankton productivity but its biogeochemical role in low nutrient, low chlorophyll environments has been poorly studied. We investigated this question in the unexplored southeast Pacific, arguably the most oligotrophic area of the global ocean. Situated far from any continental aerosol source, the atmospheric iron flux to this province is amongst the lowest of the world ocean. Here we report that, despite low dissolved iron concentrations (~0.1 nmol l^-1) measured across the whole gyre (3 stations situated in the center, the western and the eastern edge), photosynthesis and primary productivity are only limited by iron availability at the border of the gyre, but not in the center. The seasonal stability of the gyre has apparently allowed for the development of populations acclimated to these extreme oligotrophic conditions. Moreover, despite clear evidence of nitrogen limitation in the central gyre, we were unable to measure nitrogen fixation in our experiments, even after iron and/or phosphate additions, and cyanobacterial <i>nifH</i> gene abundances were extremely low compared to the North Pacific Gyre. The South Pacific gyre is therefore unique with respect to the physiological status of its phytoplankton populations.

Natural populations of the marine cyanobacterium Prochlorococcus exist as two main ecotypes, inhabiting different layers of the ocean's photic zone. These socalled high light- (HL-) and low light (LL-) adapted ecotypes are both physiologically and genetically distinct. HL strains can be separated into two major clades (HLI and HLII), whereas LL strains are more diverse. Here, we used several molecular techniques to study the genetic diversity of natural Prochlorococcus populations during the Prosope cruise in the Mediterranean Sea in the summer of 1999. Using a dot blot hybridization technique, we found that HLI was the dominant HL group and was confined to the upper mixed layer. In contrast, LL ecotypes were only found below the thermocline. Secondly, a restriction fragment length polymorphism analysis of PCR-amplified pcb genes (encoding the major light-harvesting proteins of Prochlorococcus) suggested that there were at least four genetically different ecotypes, occupying distinct but overlapping light niches in the photic zone. At comparable depths, similar banding patterns were observed throughout the sampled area, suggesting a horizontal homogenization of ecotypes. Nevertheless, environmental pcb gene sequences retrieved from different depths at two stations proved all different at the nucleotide level, suggesting a large genetic microdiversity within those ecotypes.

During the BIOSOPE field campaign October–December 2004, measurements of inherent optical properties from the surface to 500 m depth were made with a ship profiler at stations covering over 8000 km through the Southeast Pacific Ocean. Data from a ~3000 km section containing the very clearest waters in the central gyre are reported here. The total volume scattering function at 117°, ß_t(117°), was measured with a WET Labs ECO-BB3 sensor at 462, 532, and 650 nm with estimated uncertainties of 2×10^-5, 5×10^-6, and 2×10^-6 m^-1 sr^-1, respectively. These values were approximately 6%, 3%, and 3% of the volume scattering by pure seawater at their respective wavelengths. From a methodological perspective, there were several results: – distributions were resolvable even though some of the values from the central gyre were an order of magnitude lower than the lowest previous measurements in the literature; – Direct in-situ measurements of instrument dark offsets were necessary to accurately resolve backscattering at these low levels; – accurate pure seawater backscattering values are critical in determining particulate backscattering coefficients in the open ocean (not only in these very clear waters); the pure water scattering values determined by Buiteveld et al. (1994) with a [1+0.3S/37] adjustment for salinity based on Morel (1974) appear to be the most accurate estimates, with aggregate accuracies as low as a few percent; and – closure was demonstrated with subsurface reflectance measurements reported by Morel et al. (2007) within instrument precisions, a useful factor in validating the backscattering measurements. This methodology enabled several observations with respect to the hydrography and the use of backscattering as a biogeochemical proxy: –The clearest waters sampled were found at depths between 300 and 350 m, from 23.5° S, 118° W to 26° S, 114° W, where total backscattering at 650 nm was not distinguishable from pure seawater; –Distributions of particulate backscattering b_bp across the central gyre exhibited a broad particle peak centered ~100 m; –The particulate backscattering ratio typically ranged between 0.4% and 0.6% at 650 nm through the majority of the central gyre from the surface to ~210 m, indicative of "soft" water-filled particles with low bulk refractive index; and – b_bp showed a distinct secondary deeper layer centered ~230 m that was absent in particulate attenuation c_p data. The particulate backscattering ratio was significantly higher in this layer than in the rest of the water column, reaching 1.2% in some locations. This high relative backscattering, along with the pigment composition and ecological niche of this layer, appear to be consistent with the coccolithophorid <i>Florisphaera profunda</i>. Moreover, results were consistent with several expectations extrapolated from theory and previous work in oceanic and coastal regions, supporting the conclusion that particulate and total backscattering could be resolved in these extremely clear natural waters.

Spatial variations of heterotrophic bacterial production and phytoplankton primary production were investigated across South East Pacific Ocean (–141° W, –8° S to –72° W, –35° S) in November–December 2004. Bacterial production (³H leucine incorporation) integrated over the euphotic zone encompassed a wide range of values, from 43 mg C m^-2 d^-1 in the hyper-oligotrophic South Pacific Gyre to 392 mg C m^-2 d^-1 in the upwelling off Chile. Within the gyre (120° W, 22° S) records of low phytoplankton biomass (7 mg TChl<i>a</i> m^-2) were obtained and in situ ¹⁴C based particulate primary production rates were as low as 153 mg C m^-2 d^-1, thus equal to the value considered as a limit for primary production under strong oligotrophic conditions. In the South Pacific gyre average rates of ³H leucine incorporation rates, and leucine incorporation rates per cell (5–21 pmol L^-1 h^-1 and 15–56×10^-21 mol cell^-1 h^-1, respectively), were in the same range as those reported for other oligotrophic sub tropical and temperate waters. Rates of dark community respiration, determined at selected stations across the transect varied in a narrow range (42–97 mmol O₂ m^-2 d^-1), except for one station in the upwelling off Chile (245 mmol O₂ m^-2 d^-1). Bacterial growth efficiencies varied between 5 and 38% and bacterial carbon demand largely exceeded ¹⁴C particulate primary production across the South Pacific Ocean. Net community production also revealed negative values in the South Pacific Gyre (–13±20 to –37±40 mmol O₂ m^-2 d^-1). Such imbalances being impossible in this area far from any external input, we discuss the techniques involved for determining the coupling between primary production and bacterial heterotrophic production.

The optical properties of Case 1 waters have been empirically related to the chlorophyll concentration, [Chl], historically used as an index of the trophic state and of the abundance of the biological materials. The natural variability around the mean statistical relationships is here examined by comparing the apparent optical properties (spectral downward irradiance attenuation and reflectance as a function of [Chl]) which were determined in two environments, the Pacific and Mediterranean waters. These oceanic zones apparently form two extremes of the bio-optical variability range. The systematic deviations, in both directions with respect to the average laws, mainly result from the differing contents in non-algal detrital materials and dissolved colored substance for a given [Chl] level. These contents are higher and lower than the average, in the Mediterranean Sea and Pacific Ocean, respectively. The divergences between the two water bodies, detected in the visible spectral domain, are considerably accentuated in the UV domain. The bio-optical properties in this spectral domain (310–400 nm) are systematically explored. Their prediction based on the sole [Chl] index is problematic; although it is probably possible on a regional scale, an ubiquitous relationship does not seem to exist for the global scale.

Spectral absorption coefficients of phytoplankton can now be derived, under some assumptions, from hyperspectral ocean color measurements and thus become accessible from space. In this study, multilayer perceptrons have been developed to retrieve information on the pigment composition and size structure of phytoplankton from these absorption spectra. The retrieved variables are the main pigment groups (chlorophylls a, b, c, and photosynthetic and nonphotosynthetic carotenoids) and the relative contributions of three algal size classes (pico-, nano-, and microphytoplankton) to total chlorophyll a. The networks have been trained, tested, and validated using more than 3700 simultaneous absorption and pigment measurements collected in the world ocean. Among pigment groups, chlorophyll a is the most accurately retrieved (average relative errors of 17% and 16% for the test and validation data subsets, respectively), while the poorest performances are found for chlorophyll b (average relative errors of 51% and 40%). Relative contributions of algal size classes to total chlorophyll a are retrieved with average relative errors of 19% to 33% for the test subset and of 18% to 47% for the validation subset. The performances obtained for the validation data, showing no strong degradation with respect to test data, suggest that these neural networks might be operated with similar performances for a large variety of marine areas. (c) 2007 Optical Society of America.

Predicting heterotrophic bacteria and phytoplankton specific growth rates (µ ) is of great scientific interest. Many methods have been developed in order to assess bacterial or phytoplankton µ. One widely used method is to estimate µ from data obtained on biomass or cell abundance and rates of biomass or cell production. According to Kirchman (2002), the most appropriate approach for estimating µ is simply to divide the production rate by the biomass or cell abundance estimate. Most methods using this approach to estimate µ are based on carbon (C) incorporation rates and C biomass measurements. Nevertheless it is also possible to estimate µ using phosphate (P) data. We showed that particulate phosphate (PartP) can be used to estimate biomass and that the P uptake rate to PartP ratio can be employed to assess µ. Contrary to other methods using C, this estimator does not need conversion factors and provides an evaluation of µ for both autotrophic and heterotrophic organisms. We report values of P-based µ in three size fractions (0.2–0.6; 0.6–2 and >2 µm) along a Southeast Pacific transect, over a wide range of P-replete trophic status. P-based µ values were higher in the 0.6–2 µm fraction than in the >2 µm fraction, suggesting that picoplankton-sized cells grew faster than the larger cells, whatever the trophic regime encountered. Picoplankton-sized cells grew significantly faster in the deep chlorophyll maximum layer than in the upper part of the photic zone in the oligotrophic gyre area, suggesting that picoplankton might outcompete >2 µm cells in this particular high-nutrient, low-light environment. P-based µ attributed to free-living bacteria (0.2-0.6 µm) and picoplankton (0.6–2 µm) size-fractions were relatively low (0.11±0.07 d^-1 and 0.14±0.04 d^-1, respectively) in the Southeast Pacific gyre, suggesting that the microbial community turns over very slowly.

Optical measurements within both the visible and near ultraviolet (UV) parts of the spectrum (305-750 nm) were recently made in hyperoligotrophic waters in the South Pacific gyre (near Easter Island). The diffuse attenuation coefficients for downward irradiance, K-d(lambda), and the irradiance reflectances, R(lambda), as derived from hyperspectral (downward and upward) irradiance measurements, exhibit very uncommon values that reflect the exceptional clarity of this huge water body. The K-d(lambda) values observed in the UV domain are even below the absorption coefficients found in current literature for pure water. The R(A) values (beneath the surface) exhibit a maximum as high as 13% around 390 nm. From these apparent optical properties, the absorption and backscattering coefficients can be inferred by inversion and compared to those of (optically) pure seawater. The total absorption coefficient (a(tot)) exhibits a flat minimum (similar to 0.007 m(-1)) around 410-420 nm, about twice that of pure water. At 310 nm, a(tot) may be as low as 0.045 m(-1), i.e., half the value generally accepted for pure water. The particulate absorption is low compared to those of yellow substance and water and represents only similar to 15% of a(tot) in the 305-420-nm domain. The backscattering coefficient is totally dominated by that of water molecules in the UV domain. Because direct laboratory determinations of pure water absorption in the UV domain are still scarce and contradictory, we determine a tentative upper bound limit for this elusive coefficient as it results from in situ measurements.

The phytoplankton (>15 µm) composition and abundance was investigated along a ~8000 km transect between the Marquesas Islands Archipelago and the Chilean coasts off Concepción. In the southern limit of the central Equatorial Pacific (at 8° S, 141° W), in High-Nutrient Low-Chlorophyll (HNLC) warm waters, the microphytoplankton assemblage was dominated by the lightly silicified diatoms <i>Pseudo-nitzschia delicatissima</i> and <i>Rhizosolenia bergonii</i>. The morphology of these species, a small pennate diatom that exhibited a tendency to form "ball of needles" clusters and large centric diatom (>500 µm long), are interpreted as two anti-grazing strategies in an environment dominated by small micrograzers. Surprisingly, this a priori typical HNLC phytoplankton assemblage was also found in the temperate offshore waters of the Perú-Chile Current between 2000 and 600 km off Chile. This observation suggests that a common set of environmental factors (obviously other than temperature and salinity) are responsible for the establishment and maintaining of this distinctive phytoplankton in these geographically and hydrologically distant regions. Both regions are characterized by a surface nitrate-silicic acid ratio ranging from 1–3. Occasionally <i>Rhizosolenia bergonii</i> showed frustules anomalously fragmented, likely the result of extreme weakly silicified phytoplankton. We suggest that silicon deficiency may be responsible of the occurrence of HNLC phytoplankton assemblage in the tropical central Pacific as well as the offshore Perú-Chile Current during the austral summer.

Probably because it is a readily available ocean color product, almost all models of primary productivity use chlorophyll as their index of phytoplankton biomass. As other variables become more readily available, both from remote sensing and in situ autonomous platforms, we should ask if other indices of biomass might be preferable. Herein, we compare the accuracy of different proxies of phytoplankton biomass for estimating the maximum photosynthetic rate (<I>P</I>_max) and the initial slope of the production versus irradiance (P vs. E) curve (a). The proxies compared are: the total chlorophyll <I>a</I> concentration (Tchl<I>a</I>, the sum of chlorophyll <I>a</I> and divinyl chlorophyll), the phytoplankton absorption coefficient, the phytoplankton photosynthetic absorption coefficient, the active fluorescence in situ, the particulate scattering coefficient at 650 nm (<I>b_p</I>(650)), and the particulate backscattering coefficient at 650 nm (<I>b_bp</I>(650)). All of the data (about 170 P vs. E curves) were collected in the South Pacific Ocean. We find that when only the phytoplanktonic biomass proxies are available, <I>b_p</I>(650) and Tchl<I>a</I> are respectively the best estimators of <I>P</I>_max and a. When additional variables are available, such as the depth of sampling, the irradiance at depth, or the temperature, Tchl<I>a</I> is the best estimator of both <I>P</I>_max and a.

Little is known about the abundance, distribution, and ecology of aerobic anoxygenic phototrophic (AAP) bacteria, particularly in oligotrophic environments, which represent 60% of the ocean. We investigated the abundance of AAP bacteria across the South Pacific Ocean, including the center of the gyre, the most oligotrophic water body of the world ocean. AAP bacteria, Prochlorococcus, and total prokaryotic abundances, as well as bacteriochlorophyll a (BChl a) and divinyl-chlorophyll a concentrations, were measured at several depths in the photic zone along a gradient of oligotrophic conditions. The abundances of AAP bacteria and Prochlorococcus were high, together accounting for up to 58% of the total prokaryotic community. The abundance of AAP bacteria alone was up to 1.94 ؋ 10 5 cells ml ؊1 and as high as 24% of the overall community. These measurements were consistent with the high BChl a concentrations (up to 3.32 ؋ 10 ؊3 g liter ؊1) found at all stations. However, the BChl a content per AAP bacterial cell was low, suggesting that AAP bacteria are mostly heterotrophic organisms. Interestingly, the biovolume and therefore biomass of AAP bacteria was on average twofold higher than that of other prokaryotic cells. This study demonstrates that AAP bacteria can be abundant in various oligotrophic conditions, including the most oligotrophic regime of the world ocean, and can account for a large part of the bacterioplanktonic carbon stock.

A method based on spectral information is used to derive spectral absorption coefficients of phytoplankton a(phi)(lambda) and colored detrital material, CDM, which includes non-algal particle and colored dissolved organic matter, a(CDM)(lambda), from total minus water absorption coefficients measurements. This method is first validated over a dataset of more than 300 simultaneous measurements of phytoplankton, non-algal particle, and colored dissolved organic matter absorption coefficients spectra acquired with a laboratory spectrophotometer in various oceanic and coastal European waters. The validation is presented for measurements made with a high spectral resolution (hyper-spectral case), and a limited spectral resolution (multi-spectral case)-the case of most devices routinely used for in situ profiling, such as the WETLabs ac-9. In order to examine the various sources of error in the method, we test its performance considering various levels of a priori knowledge of phytoplankton absorption properties over the study area: for each site, over each region, or over a global dataset only. When the method is applied to the multi-spectral case without introducing any ``local'' information on phytoplankton absorption properties, we obtain a good performance with a relative Root Mean Square Error equal to 17.8%, 14.6%, and 40.7% for a(CDM)(412), the CDM exponential slope, and a(phi)(440), respectively. Finally, the partitioning method is directly applied to in situ profiles of total minus water spectral absorption coefficient measured with an ac-9 in various oceanic Mediterranean waters, allowing the in situ description of CDM and phytoplankton absorption coefficients with a high spatial resolution.

The distribution of lipid biomarkers and their carbon isotope composition was investigated on suspended particles from different contrasting trophic environments at six sites in the South East Pacific. High algal biomass with diatom-related lipids was characteristic in the upwelling zone, whereas haptophyte lipids were proportionally most abundant in the nutrient-poor settings of the centre of the South Pacific Gyre and on its easter edge. Dinoflagellate–sterols were minor contributors in all of the studied area and cyanobacteria-hydrocarbons were at maximum in the high nutrient low chlorophyll regime of the subequatorial waters at near the Marquesas archipelago. The taxonomic and spatial variability of the relationships between carbon photosynthetic fractionation and environmental conditions for four specific algal taxa (diatoms, haptophytes, dinoflagellates and cyanobacteria) was also investigated. The carbon isotope fractionation factor (e_<i>p</i>) of the diatom marker varied over a range of 16‰ along the different trophic systems. In contrast, e_<i>p</i> of dinoflagellate, cyanobacteria and alkenone markers varied only by 7–10‰. The low fractionation factors and small variations between the different phytoplankton markers measured in the upwelling area likely reveals uniformly high specific growth rates within the four phytoplankton taxa, and/or that transport of inorganic carbon into phytoplankton cells may not only occur by diffusion but by other carbon concentrating mechanisms (CCM). In contrast, in the oligotrophic zone, i.e. gyre and eastgyre, relatively high e_<i>p</i> values, especially for the diatom marker, indicate diffusive CO₂ uptake by the eukaryotic phytoplankton. At these nutrient-poor sites, the lowest e_<i>p</i> values for haptophytes, dinoflagellates and cyanobacteria infer higher growth rates compared to diatoms.

The optical properties of Case 1 waters have been empirically related to the chlorophyll concentration, [Chl], historically used as an index of the trophic state and of the abundance of the biological materials. The well-known natural variability around the mean statistical relationships is here examined by comparing the apparent optical properties (spectral downward irradiance attenuation and reflectance) as a function of [Chl] in two Case 1 environments, the Pacific and Mediterranean waters. These oceanic zones apparently represent two extremes of the possible bio-optical variability range around the mean. The systematic deviations, in both directions with respect to the average laws, mainly result from the differing contents in non-algal detrital materials and dissolved colored substance for a given [Chl] level. These contents are higher than the average in the Mediterranean Sea, and lower in the Pacific Ocean, respectively. These divergences between the two water bodies, detectable in the visible spectral domain, are considerably accentuated in the UV domain. The bio-optical properties in this spectral domain (310–400 nm) are systematically explored. They are more varying for a given [Chl] than those in the visible domain. Their prediction based on the sole [Chl] index is thus problematic, although it is probably possible on a regional scale if reliable field data are available. It does not seem, however, that ubiquitous relationships exist for this spectral domain for all Case 1 waters at global scale.

In the frame of the BIOSOPE cruise in 2004, the spatial distribution and structure of phytoplankton pigments was investigated along a transect crossing the ultra-oligotrophic South Pacific Subtropical Gyre (SPSG) between the Marquesas Archipelago (141° W–8° S) and the Chilean upwelling (73° W–34° S). A High Performance Liquid Chromatography (HPLC) method was improved in order to be able to accurately quantify pigments over such a large range of trophic levels, and especially the strongly oligotrophic conditions. Seven diagnostic pigments were associated to three phytoplankton size classes (pico-, nano and microphytoplankton). The total chlorophyll <I>a</I> (TChl<I>a</I>) concentrations in surface waters were the lowest measured in the centre of the gyre, reaching 0.017 mg m^-3. Pigment concentrations at the Deep Chlorophyll Maximum (DCM) were generally 10 fold the surface values. Results were compared to predictions from a global parameterisation based on remotely sensed surface TChl<I>a</I> concentrations. The agreement between the in situ and predicted data for such contrasting phytoplankton assemblages was generally good: throughout the oligotrophic gyre system, picophytoplankton (prochlorophytes and cyanophytes) was a dominant class, the nanophytoplankton signature was also significant and relative bacteriochlorophyll <I>a</I> concentrations varied around 2%. The transition zone between the Marquesas and the SPSG was also well predicted by the model. However, some regional particularities have been observed where measured and modelled data differ. Amongst these features is the extreme depth of the DCM (180 m) towards the centre of the gyre, the presence of a deep nanoflagellate population beneath the DCM or the presence of a prochlorophyte-enriched population in the high salinity formation area of the South Pacific Tropical Water. A coastal site sampled in the eutrophic upwelling zone, characterised by recently upwelled water, was significantly and unusually enriched in picoeucaryotes, in contrast with the offshore upwelling site where a more typical senescent diatom population was dominant.

The very clear waters of the South Pacific Gyre likely constitute an end-member of oligotrophic conditions which remain essentially unknown with respect to its impact on carbon fixation and exportation. We describe a non-intrusive bio-optical method to quantify the various terms of a production budget (Gross Community Production, community losses, net community production) in this area. This method is based on the analysis of the diel cycle in Particulate Organic Carbon (POC), derived from high frequency measurements of the particle attenuation coefficient <i>c_p</i>. We report very high integrated rates of Gross Community Production within the euphotic layer (average of 846±484 mg C m^-2 d^-1 for 17 stations) that are far above any rates determined using incubation techniques for such areas. Furthermore we show that the daily production of POC is essentially balanced by the losses so that the system cannot be considered as net heterotoph. Our results thus agree well with geochemical methods, but not with incubation studies based on oxygen methods. We stress to the important role of deep layers, below the euphotic layer, in contributing to carbon fixation when incident irradiance at the ocean surface is high (absence of cloud coverage). These deep layers, not considered up to now, might fuel a part of the heterotrophic processes in the upper layer, in particular through dissolved organic carbon release. We further demonstrate that, in these extremely clear and stratified waters, integrated Gross Community Production is proportional to the POC content and surface irradiance via an efficiency index ?_GCP^*, the water column cross section for Gross Community Production. We finally discuss our results in the context of the role of oligotrophic gyre in global carbon budget and of the possibility of using optical proxy from space for the development of gross community rather than primary production global models.

The present study examines the potential of using the near-surface chlorophyll a concentration ([Chla](surf)), as it can be derived from ocean color observation, to infer the column-integrated phytoplankton biomass, its vertical distribution, and ultimately the community composition. Within this context, a large High-Performance Liquid Chromatography (HPLC) pigment database was analyzed. It includes 2419 vertical pigment profiles, sampled in case 1 waters with various trophic states (0.03-6 mg Chla m(-3)). The relationships between [Chla](surf) and the chlorophyll a vertical distribution, as previously derived by Morel and Berthon (1989), are fully confirmed. This agreement makes it possible to go further and to examine if similar relationships between [Chla](surf) and the phytoplankton assemblage composition along the vertical can be derived. Thanks to the detailed pigment composition, and use of specific pigment biomarkers, the contribution to the local chlorophyll a concentration of three phytoplankton groups can be assessed. With some cautions, these groups coincide with three size classes, i.e., microplankton, nanoplankton and picoplankton. Corroborating previous regional findings (e.g., large species dominate in eutrophic environments, whereas tiny phytoplankton prevail in oligotrophic zones), the present results lead to an empirical parameterization applicable to most oceanic waters. The predictive skill of this parameterization is satisfactorily tested on a separate data set. With such a tool, the vertical chlorophyll a profiles of each group can be inferred solely from the knowledge of [Chla](surf). By combining this tool with satellite ocean color data, it becomes possible to quantify on a global scale the phytoplankton biomass associated with each of the three algal assemblages.

We examined the mechanisms related to the diel variations in the parameters of the relationship between the rate of carbon fixation of phytoplankton and irradiance (P vs. E curve). Our goal was to understand what determines the phase of these variations relative to that of the light cycle. We grew the marine prokaryote Prochlorococcus in an axenic cyclostat culture system under a light-dark cycle that mimicked natural conditions at sea surface and followed changes in cell physiology with a 2-h resolution. Individual cells divide mostly in phase with each other, once a day at the beginning of the dark period. The quantum yields of chlorophyll fluorescence, the maximum quantum yield of carbon fixation and the maximum rate of carbon fixation (P-max(B)) exhibited diel variations over about factors of 2, 4, and 4, respectively, with maxima at the beginning of the light period. The morning drop in phi(Cmax) and the quantum yield of fluorescence, which was accompanied by only a small decrease (< 15%) of photochemial efficiency of PSII (F-v/F-m), suggests regulation by light and preceded the drop in P-max(B) by 4 h. The decrease in P-max(B) during the day matched a decrease in the transcription level of Rubisco. The quantum yield of fluorescence, phi(Cmax), and P-max(B) increased again during the dark period, but this recovery was slowed at the time of cell division. Our results suggest that the pattern of diel variations in the photosynthetic parameters is determined both by photoacclimation and the cell-division cycle.

In the nearshore coastal waters along the Antarctic Peninsula, a recurrent shift in phytoplankton community structure, from diatoms to cryptophytes, has been documented. The shift was observed in consecutive years (1991-1996) during the austral summer and was correlated in time and space with glacial melt-water runoff and reduced surface water salinities. Elevated temperatures along the Peninsula will increase the extent of coastal melt-water zones and the seasonal prevalence of cryptophytes. This is significant because a change from diatoms to cryptophytes represents a marked shift in the size distribution of the phytoplankton community, which will, in turn, impact the zooplankton assemblage. Cryptophytes, because of their small size, are not grazed efficiently by Antarctic krill, a keystone species in the food web. An increase in the abundance and relative proportion of cryptophytes in coastal waters along the Peninsula will likely cause a shift in the spatial distribution of krill and may allow also for the rapid asexual proliferation of carbon poor gelatinous zooplankton, salps in particular. This scenario may account for the reported increase in the frequency of occurrence and abundance of large swarms of salps within the region. Salps are not a preferred food source for organisms that occupy higher trophic levels in the food web, specifically penguins and seals, and thus negative feedbacks to the ecology of these consumers can be anticipated as a consequence of shifts in phytoplankton community composition.

A study of the biogeochemical properties of the Almeria-Oran front was carried out in December 1997 to January 1998. A strong salinity gradient between Atlantic and Mediterranean waters in the Alboran Sea allowed the differentiation of several subsystems: the Mediterranean waters, the frontal zone, and the anticyclonic gyre. Si and C biomass and production were clearly enhanced by the frontal dynamics on the Atlantic side of the jet while Mediterranean waters, which encountered severe nutrient depletion in the mixed layer, exhibited a typical oligotrophic regime. The distribution of particulate matter was controlled by a cross-frontal downwelling along the isopycnal slopes, that shoaled to the surface on the dense Mediterranean side and deepened toward the Atlantic side of the jet. A strong decoupling of production and biomass maximums occurred between the frontal limit, where particulate matter was produced, and the gyre, where it was accumulated. Export fluxes at 300 m were low at the frontal limit, representing 1-2% of surface Si and C production, and it is hypothesized that advective fluxes rather than grazing were the main factor limiting the accumulation of biomass. The adjacent systems, namely the associated anticyclonic gyre and the Mediterranean waters, were exporting Si to depth more efficiently than the frontal zone. The Si and C decoupling with depth appeared higher in the Almeria-Oran frontal system than in other open-ocean zones. The integrated Si production at the Almeria-Oran Front was 0.83 mmol Si m−2 d−1, which was closest to the production rates of mid-ocean oligotrophic gyres than of other frontal systems, and may be explained by the sampling period, which occurred in the winter season.

The spectral absorption coefficients of phytoplankton in oceanic waters were previously shown to vary with chlorophyll a concentration according to nonlinear relationships with a great deal of noise. We analyzed this biological noise on a data set of 596 simultaneous absorption and high-pressure liquid chromatography (HPLC) pigment measurements acquired within the surface layer (first optical depth) from various regions of the world's oceans. We observed systematic deviations from the average relationships for some oceanic areas and also seasonally within given areas. Using the detailed HPLC measurements, the influences of pigment composition and package effect (the two main sources of variability in algal absorption for a given chlorophyll a concentration) were explicitly separated for each sample. It was found that while the pigment composition experiences large variations, even within a restricted chlorophyll range, it is often not (at least within the first optical depth) the dominant source of the biological noise. Instead, these deviations mostly result from variability in the pigment packaging effect (for a given chlorophyll a concentration) due to variations in algal community size structure. This conclusion is fully confirmed by an independent approach, which consists of estimating a ``size index'' of algal populations from the relative concentrations of taxonomic pigments.

Because of recent findings that Fe is a limiting factor for phytoplankton activity even at relatively high dissolved iron (DFe) concentrations, the potential importance of Fe limitation was revisited in the northeast Atlantic Ocean (39-458N, 17-218W). We report data gathered during deck incubation experiments performed at three stations in February-March 2001 with surface seawater containing DFe concentrations of ;0.40 nmol L21. At all stations, Fe addition enhanced phytoplankton growth. Fe limitation was moderate and occurred simultaneously with limitation by major nutrients. This was clearly demonstrated for diatoms that were colimited by orthosilicic acid. Micro-, nano-, and picoplankton benefited from Fe enrichment. Experiments performed with the trihydroxamate siderophore desferrioxamine mesylate B (DFOB) indicated that Fe reserves exist within the cells, especially within the larger cells. This reserve could result from luxurious storage of Fe by colimited cells during episodic atmospheric deposition of Saharan dust. Simulating concentrations of dust resulting from aerosol deposition in well-stratified surface waters, we determined that the solubility of Saharan dust was very low (,0.1% w/w) but the amount of DFe released in seawater was sufficient to relieve the Fe limitation of the ambient phytoplankton community.

Whether for biogeochemical studies or ocean color validation activities, high-performance liquid chromatography (HPLC) is an established reference technique for the analysis of chlorophyll a and associated phytoplankton pigments. The results of an intercomparison exercise of HPLC pigment determination, performed for the first time on natural samples and involving four laboratories (each using a different HPLC procedure), are used to address three main objectives: (a) estimate (and explain) the level of agreement or discrepancy in the methods used, (b) establish whether or not the accuracy requirements for ocean color validation activities can be met, and (c) establish how higher order associations in individual pigments (i.e., sums and ratios) influence the uncertainty budget while also determining how this information can be used to minimize the variance within larger pigment databases. The round-robin test samples (11 different samples received in duplicate by each laboratory) covered a range of total chlorophyll a concentration, [TChl a], representative of open ocean conditions from 0.045 mg m(-3), typical of the highly oligotrophic surface waters of the Ionian Sea, to 2.2 mg m(-3), characteristic of the upwelling regime off Morocco. Despite the diversity in trophic conditions and HPLC methods, the agreement between laboratories, defined here as the absolute percent difference (APD), was approximately 7.0% for [TChl a], which is well within the 25% accuracy objective for remote sensing validation purposes. For other pigments (mainly chemotaxinomic carotenoids), the agreement between methods was 21.5% on average (ranging from 11.5% for fucoxanthin to 32.5% for peridinin), and inversely depended on pigment concentration (with large disagreements for pigments close to the detection limits). It is shown that better agreement between methods can be achieved if some simple procedures are employed: (a) disregarding results less than the effective limit of quantitation (LOQ, an alternative to the method detection limit, MDL), (b) standardizing the manner in which the concentration of pigment standards are determined, and (c) accurately accounting for divinyl chlorophyll a when computing [TChl a] for those methods which do not chromatographically separate it from monovinyl chlorophyll a. The use of these quality-assurance procedures improved the agreement between methods, with average APD values dropping from 7.0% to 5.5% for [TChl a] and from 21.5% to 13.9% for the principal carotenoids. Additionally, it is shown that subsequent grouping of individual pigment concentrations into sums and ratios significantly reduced the variance and, thus, improved the agreement between laboratories. This grouping, therefore, provides a simple mechanism for decreasing the variance within databases composed of merged data from different origins. Among the recommendations for improving database consistency in the future, it is suggested that submissions to a database should include the relevant information related to the limit of detection for the HPLC method. (C) 2003 Elsevier B.V. All rights reserved.

The presence of two dinoflagellate species of the genus Asterodinium, which are a priori representative of warm waters, is reported for the first time in the western Mediterranean Sea. Asterodinium libanum was identified in the Bay of Villefranche-surMer (Ligurian Sea), while Asterodinium gracile is reported in the Tyrrhenian Sea. These findings are discussed in the context of the progressive warming of Mediterranean waters.

We measured the absorption properties of phytoplankton, nonalgal particles (NAP), and colored dissolved organic matter (CDOM) at about 350 stations in various coastal waters around Europe including the English Channel, Adriatic Sea, Baltic Sea, Mediterranean Sea, and North Sea. For comparison, we also collected data in the open ocean waters of North Atlantic. The exponential slope of the CDOM absorption spectrum varied within a narrow range around 0.0176 nm(-1) (SD=0.0020 nm(-1)). When data from all the regions were considered altogether, the relationship between phytoplankton absorption and chlorophyll concentration was generally similar to the one previously established for open oceanic waters. Our coastal data, however, show that significant departures from the general trend may occur due to peculiar pigment composition and cell size. In some coastal areas, high phaeopigment concentrations gave rise to especially high blue-to-red ratio of phytoplankton absorption. The NAP absorption covaried with the particle dry weight. Most absorption spectra of these particles were well described by an exponential function with a slope averaging 0.0123 nm(-1) (SD=0.0013 nm(-1)). In some highly turbid waters, the spectra exhibited a signature possibly associated with iron oxides. In the Baltic Sea, NAP absorption systematically showed lower values at wavelengths shorter than 440 nm than predicted from the fitted exponential function. Overall, the variability in the absorption properties of European coastal waters showed some consistent patterns despite the high diversity of the examined waters. Distinct features were identified in the phytoplankton and NAP components. An absorption budget is presented and parameterizations are proposed.

Experimental work during a cruise along a W‐E transect in the Mediterranean Sea suggests that (1) orthophosphate concentrations in the upper photic zone show a decreasing trend from the west to the east reaching levels well below 1 nM and (2) microorganisms in the 0.6–2ߝµm size fraction, probably Synechococcus, have, in addition to high affinity for orthophosphate, significantly higher maximum uptake rates than heterotrophic bacteria or eukaryotic algae. These specific advantages concerning orthophosphate uptake at low (<5 nM) as well as at relatively high (5–25 nM) concentrations could explain both general Synechococcus abundance in P‐depleted environments and transient blooms of this species in the open ocean where episodic orthophosphate nanopulse events are likely to occur.

Photoacclimatization of zooxanthellae extracted from the coral Pocillopora verrucoso was studied through the determination of pigments, light absorption and photosynthetic parameters. for samples collected in summer and winter between 1 and 40 m on a northwestern reef of Tahiti (French Polynesia). The same measurements were also performed on phytoplanktonic samples collected at a stable oceanic site north of the island. For the zooxanthellae, the variations with depth of all the parameters were generally of small amplitude. Seasonal differences were also observed. The photosynthetic to non-photosynthetic pigments ratio was higher at depth in both seasons and was higher in winter. The intracellular concentration of chlorophyll a and photosynthetic pigments was higher in winter, as was the photosynthetic pigments/chlorophyll a ratio, whereas the non-photosynthetic pigments/chlorophyll a ratio was higher in summer. Variations in the light absorption properties were also small. The photosynthetic parameters showed limited changes with depth with the largest variations (a factor of similar to2) observed for (B)(max). The trends observed for the phytoplankton assemblage were generally of much higher amplitudes than for the zooxanthellae (e.g. for photosynthetic to non-photosynthetic pigments ratio or the saturation parameter, E.). These results suggest that, in the very clear Polynesian waters, the amount of energy that reaches the zooxanthellae of P. verrucosa not variable enough in the 1-40 m depth range to result in a drastic modification of the photosynthetic apparatus of the algae. (C) 2002 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS. All rights reserved.

In situ optical measurements provide evidence that oligotrophic waters of the Mediterranean Sea have a greener color than would result from their phytoplankton content alone. This anomaly, detectable in low chlorophyll waters, remains unnoticed in the chlorophyll-rich waters of the nearby waters of the Moroccan upwelling zone. It is due to enhanced absorption in the blue and enhanced backscattering in the green parts of the visible spectrum likely resulting from the presence of submicron Saharan dust in suspension within the upper layer. This result implies that regional estimations of carbon fixation from ocean color images might be significantly overestimated, not only in the Mediterranean Sea, but also in other oligotrophic areas of the Northern hemisphere, subjected to desert dust deposition.

Prochlorococcus is an important component of phototrophic biomass in oligotrophic areas. In Such systems, diel variations in optical properties have been reported. In order to better understand these natural variations, the optical (absorption, attenuation, and scattering) and biochemical (carbon and pigment) properties of an axenic clone of Prochlorococcus, PCC 9511, grown in a turbidostat set under light conditions that simulated those found near the ocean surface, were monitored every 2 h for several consecutive days. All optical parameters showed pronounced diel patterns except the divinyl-chlorophyll a (Dv-Chl a) specific absorption coefficient at 676 nm, a*(676), which remained stable (0.019 m(2) mg Dv-Chl a(-1)). The diel oscillations of the Dv-Chl a specific absorption coefficient at 440 nm, a*(440) (43% increase between sunrise and sunset), were essentially governed by variations in the zeaxanthin to Dv-Chl a ratio (52% increase of this ratio between sunrise and sunset). The scattering cross section at 555 nm, sigma(b)(555), showed oscillations with the largest amplitude (182% increase between sunrise and sunset). Finally, the carbon-specific attenuation coefficient at 660 nm, c(c)*(660) (1.04 m(2) gC(-1)) is less than half that of the other algal groups. This estimation, together with the diel fluctuations in c(c)*(660) highlighted in the present study, is discussed in the context of using in situ measurements of optical properties to infer biogeochemical stocks (vegetal or detrital biomass) or processes (primary production).

Effects of nitrogen limitation on Photosystem II (PSII) activities and on phycoerythrin were studied in batch cultures of the marine oxyphotobacterium Prochlorococcus marinus. Dramatic decreases in photochemical quantum yields (F-V/F-M), the amplitude of thermoluminescence (TL) B-band, and the rate of QA reoxidation were observed within 12 h of growth in nitrogen-limited conditions. The decline in F-V/F-M paralleled changes in the TL B-band amplitude, indicative of losses in PSII activities and formation of non-functional PSII centers. These changes were accompanied by a continuous reduction in D1 protein content. In contrast, nitrogen deprivation did not cause any significant reduction in phycoerythrin content. Our results refute phycoerythrin as a nitrogen storage complex in Prochlorococcus. Regulation of phycoerythrin gene expression in Prochlorococcus is different from that in typical phycobilisome-containing cyanobacteria and eukaryotic algae investigated so far. (C) 2001 Elsevier Science B.V. All rights reserved.

A cyclostat was designed for growing the oceanic oxyphotobacterium Prochlorococcus PCC 9511. Culture of this organism, known to bedifficult to grow, was mastered for a large volume. Prochlorococcusgrew well and axenic conditions were maintained for up to 15 days. Wedesigned an illumination system allowing a smooth bell-shaped irradiancecurve reaching almost 1000 μmol quanta m-2 s-1 tobe obtained. Cell division was strongly synchronised under theseillumination conditions, which were close to those found at low latitude inthe upper layer of ocean. The described device is particularly well suited tomake experiments requiring up to 6 L per day of well synchronised,exponentially-growing Prochlorococcus culture.

Using a sampling grid of 67 stations, the influence of basin-wide and subbasrascale circulation features on phytoplankton community composition and primary and new productions-was investigated m the eastern Mediterranean during winter. Taxonomic pigments were used as size class markers of phototroph groups (picophytoplankton, nanophytoplankton and microphytoplankton). Primary production rates-were computed using a light photosynthesis model that makes use of the total chlorophyll a (Tchl a) concentration profile as an input variable. New production was estimated as the product of primary production by a pigment-based proxy of thef ratio (new production/total production). For the whole eastern Mediterranean, Tchl a concentration was 20.4 mg m-2, and estimated primary and new production were 0.27 and 0.04 g C m-2 d '1 respectively, when integrated between the surface and the depth of the productive zone (1.5 times the euphotic layer). Nanophytoplankton and picophytoplankton (determined from the pigment-derived criteria) were the dominant size classes and contributed to 60 and 27%, respectively, of Tchl a, While microphytoplankton contributed only to 13%. Subbasra and, to a certain extent, mesoscale structures (cyclonic and anticyclonic gyres)-were exceptions to this general trend. Anticyclonic gyres-were characterized by low Tchl a concentrations (18.8 + 4.2 mg m-2, with the lowest value being 12.4 mg m-2) and the highest picophytoplankton contribution (40% of Tchl a•. In contrast, cyclonic gyres contained the highest Tchl a concentration (40.3 + 15.3 mg m-k) with the highest microphytoplankton contribution (up to 26% of Tchl a). Observations conducted at a mesoscale in the Rhode gyre (cyclonic) region show that the core of the gyre is dominated by microphytoplankton (mainly diatoms), while adjacent areas are characterized by high chlorophyll concentration dominated by picophytoplankton and nanophytoplankton. We estimate that the Rhodes gyre is a zone of enhanced new production,-which is 9 times higher than that m adjacent oligotrophic areas of the Levantine basin. Our results confirm the predominance of oligotrophic conditions in the eastern Mediterranean and emphasize the role of subbasra and mesoscale dynamics m driving phytoplankton biomass and composition and, finally, biogeochemical cycling in this area.

The diversity and abundance of the Bolidophyceae (Heterokonta), a newly described picoplanktonic algal class which is a sister group to the diatoms, was assessed in the equatorial Pacific Ocean and in the Mediterranean Sea by culture isolation, molecular biology techniques, and pigment analyses. Eight strains of Bolidophyceae were isolated in culture from different mesotrophic and oligotrophic areas. The corresponding small subunit (SSU) rRNA gene sequences allowed us to design two probes specific for the Bolidophyceae. These probes have been used in natural samples (i) to selectively amplify and detect Bolidophyceae sequences and (ii) to quantify the relative abundance of Bolidophyceae within the picoeukaryote community. Sequences available to date indicate that the class Bolidophyceae comprises at least three different clades, two corresponding to the previously described species Bolidomonas pacifica and Bolidomonas mediterranea and the third one corresponding to a subspecies of B. pacifica. Amplification of the SSU rRNA gene from natural samples with universal primers and hybridization using a Bolidomonas-specific probe followed by a eukaryote-specific probe allowed us to estimate the contribution of the Bolidophyceae to the eukaryotic DNA in both Pacific and Mediterranean waters to be lower than 1%. Similarly, high-performance liquid chromatography analyses of fucoxanthin, the major carotenoid present in Bolidophyceae, indicated that less than 4% of the total chlorophyll a in the picoplanktonic fraction in the equatorial Pacific was due to Bolidophyceae. Consequently, although strains of Bolidophyceae have been isolated from samples collected at several stations, this new class seems to have been a minor component of the natural picoeukaryotic populations in the ecosystems investigated, at least during the periods sampled.

We analysed samples taken through the euphotic zone from 18 stations between the Ligurian Sea (6 degrees E) and the Levantin Basin (32 degrees E) from 24 May to 25 June 1996. Both ciliate and chlorophyll concentrations ranged over a factor of about 7, but ciliate concentrations (0.4-2.8 mg C m(3)) varied irregularly compared to a longitudinal decline, west to east, in chlorophyll concentration (0.07-0.48 mg m(3)). The lower chlorophyll concentrations (0.1 mg m(2)) of the eastern basin stations corresponded with a relatively high stock of ciliates (0.5 mg C m(2)). Large mixotrophic ciliates were more abundant, in both absolute and relative terms, in the eastern Mediterranean stations with less chlorophyll. The species diversity of tintinnid ciliates appeared higher in the central and eastern basins compared to the west. Our results suggest a shift from the western to eastern Mediterranean in the planktonic food towards a microbially dominated system. (C) 1999 Elsevier Science Ltd. All rights reserved.

A new algal class, the Bolidophyceae (Heterokonta), is described from one genus, Bolidomonas, gen, nov., and two species, Bolidomonas pacifica, sp, nov and Bolidomonas mediterranea, sp, nov., isolated from the equatorial Pacific Ocean and the Mediterranean Sea, respectively. Both species are approximately 1.2 mu m in diameter and have two unequal flagella; the longer flagellum bears tubular hairs, whereas the shorter is smooth. The flagellar basal apparatus is restricted to two basal bodies, and there is no transitional helix. Cells are naked, devoid of walls or siliceous structures. The internal cellular organization is simple with a single plastid containing a ring genophore and a girdle lamella, one mitochondrion with tubular cristae, and one Golgi apparatus close to the basal bodies. The Mediterranean and the Pacific species differ in the insertion angle between their flagella and their pattern of swimming, these differences possibly being linked to each other. Analyses of the SSU rDNA gene place the two strains as a sister group to the diatoms, Moreover, pigment analyses confirm this position, as fucoxanthin is found as the major carotenoid in both lineages. These data strongly suggest that the ancestral heterokont that gave rise to the diatom lineage was probably a biflagellated unicell.

The variability in particle attenuation (c(p)) and in chlorophyll in situ fluorescence (F-is) was examined in November 1994 along 150 degrees W in the Pacific Ocean. Two main sources of variation in c(p) and F-is profiles are identified by analyzing data from a 16 degrees S-1 degrees N transect, and from two 5 day stations (5 degrees S and 16 degrees S). The first source reflects changes in the trophic status resulting from prevailing hydrodynamical regimes at large scales. By using flow cytometric data and some assumptions about the size distribution of the different biological stocks, a decomposition of c(p) into its vegetal (c(veg)) and nonvegetal (c(nveg)) components is attempted. Within the euphotic layer, c(veg) accounts for 43% of the total c(p) Signal at the equator and for only 20% in the South Pacific gyre. The nonvegetal component is then subdivided into heterotrophic organisms and detritus contributions. The detrital material is an important contributor with 43% of c(p) at 5 degrees S and 55% at 16 degrees S. A further decomposition of F-is and c(veg) into the three dominant phytoplanktonic groups (Prochlorococcus, Synechococcus, and picoeucaryotes) confirms that picoeucaryotes are important contributors of the vegetal biomass, especially within and below the deep chlorophyll maximum (DCM) (>50% of the algal stock) at 16 degrees S. The second, and essentially local, source of variation is related to specific rhythms in biological and physiological processes. The prominent signals detected during the time series occur at the daily scale: besides the pronounced fluorescence depression at noon in upper layers, particle attenuation in all the layers examined and fluorescence in the DCM display conspicuous daily oscillations. They result from the balance between daytime accumulation and night removal of particles, of algal cells in particular. Finally, the estimation of cp-based growth rates points out the surprisingly rapid turnover time of the whole particulate matter stock in oligotrophic waters (16 degrees S), not only in the euphotic zone (0.63 d(-1)) but also within the dimly lit layers of the DCM (0.36 d(-1)). The corresponding growth rate at 5 degrees S, within a quasi-mesotrophic regime, is 0.47 d(-1) within the euphotic zone.

Spectral absorption coefficients of total particulate matter a(p) (lambda) were determined using the in vitro filter technique. The present analysis deals with a set of 1166 spectra, determined in various oceanic (case 1) waters, with field chi a concentrations ([chl]) spanning 3 orders of magnitude (0.02-25 mg m(-3)). As previously shown [Bricaud et al.; 1995] for the absorption coefficients of living phytoplankton a(phi)(lambda), the a(p)(lambda) coefficients also increase nonlinearly with [chl]. The relationships (power laws) that link a(p)(lambda) and a(phi)(lambda) to [chl] show striking similarities. Despite large fluctuations, the relative contribution of nonalgal particles to total absorption oscillates around an average value of 25-30% throughout the [chl] range. The spectral dependence of absorption by these nonalgal particles follows an exponential increase toward short wavelengths, with a weakly variable slope (0.011 +/- 0.0025 nm(-1)). The empirical relationships linking a(p)(lambda) to [chl] can be used in bio-optical models. This parameterization based on in vitro measurements leads to a good agreement with a former modeling of the diffuse attenuation coefficient based on in situ measurements. This agreement is worth noting as independent methods and data sets are compared. It is stressed that for a given [chl], the a(p)(lambda) coefficients show large residual variability around the regression lines (for instance, by a factor of 3 at 440 nm). The consequences of such a variability, when predicting or interpreting the diffuse reflectance of the ocean, are examined, according to whether or not these variations in a(p) are associated with concomitant variations in particle scattering. In most situations the deviations in a(p) actually are not compensated by those in particle scattering, so that the amplitude of reflectance is affected by these variations.

The egestion of particulate material as well as pigment degradation during microzooplankton grazing on phytoplankton are poorly known processes. In an attempt to evaluate these processes, changes in pigment concentrations within various size fractions were monitored in batch cultures of an assemblage of a pelagic ciliate (Strombidium sulcatum) and a heterotrophic flagellate (Paraphysomonas sp.) feeding on a cyanobacterium (Synechococcus sp.) over a 10-day period. Chlorophyll a, carotenoids and phaeopigments were not found in the 0.1-0.7 mu m fraction while the pigments originally in the 0.7-3.0 mu m fraction (prey) were transferred into the > 3.0 mu m size fraction (predator). During this transfer, the carotenoids (zeaxanthin and B-carotene) were not degraded significantly. In contrast, chlorophyll a was degraded into phaeophytin-like compounds which accounted for almost 100 % of the recorded phaeopigments. The destruction of chlorophyll a varied with time ranging from 4 % (day 3) to almost 100 % tend of the experiment) and this destruction was inversely related to micro-grazer ingestion rates. Microscopic examinations of samples did not reveal any large egested particles > 3.0 mu m, suggesting that phaeopigments and carotenoids measured in this size fraction were accumulated inside the protozoa. Zeaxanthin was very stable even when it was within the mice-grazer. (C) Elsevier, Paris.

Using a highly resolved Long Term Ecological Research (LTER) database collected near Palmer Station, Antarctica, from 1991 to 1994, the variability in the column photosynthetic cross section (Psi*, m(2) g Chl a(-1)) was analyzed. The relationship between the daily integrated primary production rates versus the product of surface irradiance (Q(PAR)(O-2(+)) and the integrated chlorophyll content (down to 0.1% Q(PAR)(0(+)) gave a Psi* value of 0.0695 m(2) g Chl a(-1) (r(2) = 0.85, p < 0.001, n = 151) which is similar to those determined for temperate and tropical seas. However, the average value of single Psi* estimates is higher (0.109 +/- 0.075 m(2) g Chl a(-1)) with extreme values extending over a fiftyfold range (0.009-0.488 m g Chi a(-1)). The possible drivers of this variability are analyzed in detail, considering variables which are presently used in biooptical models (e.g., surface irradiance and chlorophyll content) and those which are not (taxonomic composition). A sixfold variation in Psi* was observed with time of year and strongly associated with the high seasonality in incident irradiance characteristic of these polar sampling sites. Variability in daily incident irradiance as influenced by cloudiness and variation in chlorophyll content were responsible for an additional twofold variation in Psi*. Finally, the taxonomic dependency of Psi* was demonstrated for the first time. For identical chlorophyll content and surface irradiance, mean Psi* values of 0.114 +/- 0.051 m(2) g Chl a(-1) were recorded for diatom blooms and 0.053 +/- 0.011 m(2) g Chi a(-1) for cryptophyte-dominated populations. Results illustrate the validity of Psi*-based approaches for estimating primary production for the Southern Ocean but emphasize the need to address taxon-specific photophysiology to better estimate primary production on smaller spatio-temporal scales.

Chlorophyll-specific absorption coefficients of particles, a(p)*(lambda), and of length amplification effect was derived from field measurements. Then a decomposition technique using the high-performance liquid chromatography pigment information and taking into account the package effect was used to partition a(ph)* into the contributions of photosynthetic pigments (a(ps)*) and nonphotosynthetic pigments (a(nps)*). Both a(ph)*, and a values were observed to decrease from the oligotrophic waters of the subequatorial nps area (13 degrees-1 degrees S) to the mesotrophic waters of the equatorial area (1 degrees S-1 degrees N) and from the surface to deep waters. The a(ph)* variations were primarily, but not exclusively, caused by changes in the concentrations of nonphotosynthetic pigments. The level of pigment packaging was also variable both horizontally and vertically, as a result of changes in populations and photoacclimation. In comparison with a(ph)*, a(ps)* exhibited a reduced range of variation with depth and along the latitudinal gradient. The variations in a(ps)* originating from the package effect were partly compensated by variations in the concentrations of photosynthetic pigments. We extended this analysis to include data collected in other areas with different trophic states. The a(ps)* values varied over a factor of 4 at 440 nm, instead of 8 for a(ph)*, for chlorophyll a concentrations covering 2 orders of magnitude (0.02-2 mg m(-3)). In agreement with a previous study performed off California with a different method [Sosik and Mitchell, 1995], we conclude that a(ps)* is less dependent on environmental parameters than a(ps)*. In addition, our results provide evidence that the variability in a(ps)* cannot be neglected. The use of a(ps)* instead of a(ph)* in light-photosynthesis models (in conjunction with a quantum yield for carbon fixation defined with respect to the photosynthetically active absorbed amount of quanta) presents the advantage of removing the variability associated with nonphotosynthetic pigments.

The C-14 labelling of chlorophylls and carotenoids is increasingly used to evaluate phytoplanktonic biomass and growth rates in oceanic systems. Rigorous testing of the technique in the laboratory, however, is necessary prior to its application in the field. A Mediterranean clone of Prochlorococcus, a photosynthetic prokaryote which is an important component of the autotrophic biomass in oligotrophic environments, was subjected to shifts in light intensity. Particulate organic carbon (POC) was monitored by CHN analysis, pigments by HPLC and Prochlorococcus and heterotrophic bacteria concentrations by flow cytometry. Using a combination of HPLC and on-line radioactivity detection, C-14 labelling kinetics of divinyl-chlorophyll a (Dv-chl a) and zeaxanthin were followed. Prochlorococcus changed its Dv-chl a content markedly in response to change in light intensity, but not its zeaxanthin content, which remained nearly constant around 1.07 fg cell(-1) regardless of the irradiance. Pigment synthesis rates were correctly estimated from their C-14 incorporation rates whatever the Light level. From POC measurements and cell concentrations, the Prochlorococcus carbon content was estimated to be 49 fg C cell(-1). Moreover, under both constant and shifted (high to low and vice versa) Light conditions, Prochlorococcus growth rate (as computed from variations in cell. density) was much better estimated from zeaxanthin than from Dv-chl a labelling rates.

Natural variability of the maximum quantum yield of carbon fixation (phi C-max), as determined from the initial slope of the photosynthesis-irradiance curve and from light absorption measurements, was studied at three sites in the northeast tropical Atlantic representing typical eutrophic, mesotrophic and oligotrophic regimes. At the eutrophic and mesotrophic sites, where the mixed layer extended deeper than the euphotic layer, all photosynthetic parameters were nearly constant with depth, and phi C-max averaged between 0.05 and 0.03 mol C (mel quanta absorbed)(-1), respectively. At the oligotrophic site, a deep chlorophyll maximum (DCM) existed and phi C-max varied from ca 0.005 in the upper nutrient-depleted mixed layer to 0.063 below the DCM in stratified waters. Firstly, phi C-max was found roughly to covary with nitrate concentration between sites and with depth at the oligotrophic site, and secondly, it was found to decrease with increasing relative concentrations of non-photosynthetic pigments. The extent of phi C-max variations directly related to nitrate concentration was inferred from variations in the fraction of functional PS2 reaction centers (f), measured using fast repetition rate fluorometry. Covariations between f and nitrate concentration indicate that the latter factor may be responsible for a 2-fold variation in phi C-max. Moreover, partitioning light absorption between photosynthetic and non-photosynthetic pigments suggests that the variable contribution of the non-photosynthetic absorption may explain a 3-fold variation in phi C-max, as indicated by variations in the effective absorption cross-section of photosystem 2 (sigma(PS2)). Results confirm the role of nitrate in phi C-max variation, and emphasize those of light and vertical mixing. Copyright (C) 1996 Elsevier Science Ltd

A rapid reverse-phase HPLC method is presented for the identification and quantification of most of the phytoplankton pigments. This method yields the resolution of divinyl-chlorophyll a and chlorophyll a, as well as the partial resolution of lutein and zeaxanthin, and of divinyl-chlorophyll b and chlorophyll b. In addition, chlorophylls c(1,2) and c(3) are well resolved. The analysis time for one sample is 20 min, which makes this method particularly suited when large numbers of samples have to be processed.

Measurements of in vivo spectral absorption, a(lambda), and fluorescence excitation, F-m(lambda), of phytoplankton were performed in two contrasted situations (oligotrophic and mesotrophic) of the tropical North Atlantic, during October 1991. The vertical and inter-site variability of these properties, and the relative fluorescence yield (F-m/a)(lambda) were investigated, in relation to light conditions and to the pigment and taxonomic composition of the natural populations. The large vertical variations in the chi-specific absorption coefficients (a*) at the oligotrophic site appear to be related to both the decrease of the non-photosynthetic pigment concentration, and the increase of the package effect with depth. At the mesotrophic site, lower a* coefficients are observed, likely originating from the larger average size of the phytoplanktonic organisms, which induces a larger package effect. Both vertical and intersite variation of the chi-specific fluorescence (F-m*) shows opposite trends compared to a*. The variable presence of non-photosynthetic carotenoids (mainly zeaxanthin) is revealed to be the most clearly identifiable source of variation for the relative fluorescence yield, with a drop in the blue-green region. Prochlorococcus, Synechococcus and picoeukaryotes show a strong photoacclimation capacity to low irradiances. In addition there is evidence in the phytoplankton communities for a complementary chromatic adaptation process to the prevailing spectral irradiance conditions at both sites. Copyright (C) 1996 Elsevier Science Ltd

Variability in the chlorophyll (chl) a-specific absorption coefficients of living phytoplankton a(ph)*(lambda) was analyzed using a data set including 815 spectra determined with the wet filter technique in different regions of the world ocean (covering the chlorophyll concentration range 0.02-25 mg m(-3)). The a(ph)* values were observed to decrease rather regularly from oligotrophic to eutrophic waters, spanning over more than 1 order of magnitude (0.18 to 0.01 m(2) mg(-1)) at the blue absorption maximum. The observed covariation between a(ph)*(lambda) and the field chl a concentration (chl) can be explained considering (1) the level of pigment packaging and (2) the contribution of accessory pigments to absorption. Empirical relationships between a(ph)*(lambda) and (chl) were derived by least squares fitting to power functions. These relationships can be used to produce a(ph)* spectra as a function of [chl]. Such a simple parameterization, if confirmed with further data, can be used, e.g., for refining estimates of the carbon fixation rate at global or regional scales, such as those obtained by combining satellite pigment concentration maps with primary production models based on physiological parameters, among which a(ph)* is an important one.

Phytoplankton pigment concentrations and primary production rates were measured in the North Tropical Atlantic Ocean (20 degrees N, 31 degrees W) in September-October 1991 and in May-June 1992 to provide new insights into the phytoplankton biomass and dynamics of oligotrophic environments. The overall biomass standing stocks were remarkably constant during both periods (around 23 mg chlorophyll a m(-2)), despite marked differences in the water column stratification. The structure of the autotrophic community was also stable: prochlorophytes, cyanobacteria and flagellates were the dominant autotrophic groups and contributed to 36, 30 and 34% of the chlorophyll a biomass in May-June and 43, 30 and 27% in September-October. The vertical distribution of these taxa was also stable with cyanobacteria dominating at the surface (100-10% of surface irradiance), prochlorophytes at intermediate depths (10-0.1% of surface irradiance) and flagellates below the euphotic zone (0.1-0.01% of surface irradiance). Despite this qualitative and quantitative stability of the phytoplankton biomass, primary production rates were significantly higher (p < 0.05) in May-June (352 +/- 68 mg C m(-2) d(-1)) than in September-October (267 +/- 53 mg C m(-2) d(-1)). The cross-section for photosynthesis per unit chlorophyll a was constant during both periods (0.063 m(2) g Chla(-1)) suggesting that differences in production rates were mainly governed by variations in irradiance. The photic zone accounted for more than 80% of the integrated production, but less than 50% of the chlorophyll a biomass. Analysis of the photoadaptation characteristics of the dominant populations suggests that cyanobacteria and prochlorophyte distributions are mainly regulated by light, whereas flagellate distribution is mainly linked to nutrient availability. The respective distributions of fucoxanthin, 19'-hexanoyloxyfucoxanthin and 19'-butanoyloxyfucoxanthin suggest that, in such oligotrophic environments, a particular group of 19'-butanoyloxyfucoxanthin-containing flagellates, living close to the nitracline, is responsible for the new production associated with the regular diffusion of nitrate, but that diatoms, generally present at background levels, can be responsible for spatio-temporal events of new production.

A new picoplanktonic alga, Ostreococcus tauri Courties et Chretiennot-Dinet, gen. et sp. nov. (Chlorophyta, Prasinophyceae) is described from the Thau Lagoon on the Mediterranean coast, France. Almost undetectable by light or fluorescence microscopy in field studies, the cells were discovered by their flow cytometric signature and appeared numerically as the main component of the phytoplankton. Their ultrastructure is described, with additional information on cell size, pigment and DNA content. Each cell contains a nucleus, a chloroplast, one mitochondrion, one Golgi body and a very reduced cytoplasmic compartment. A starch granule and Chi a and b demonstrate its affinity to the Chlorophyta and the presence of a Chl c-like pigment, Mg 3,8 DVP (=Mg 2,4 DVP) argues for placing it in the Prasinophyceae. It differs from other coccoid taxa in ultrastructural details, mode of division and detailed pigment composition. The size and DNA content make Ostreococcus tauri the smallest eucaryote known.

Between March 1992 and April 1993, an intensive sampling program was carried out al a coastal station in the Northwestern Mediterranean Sea to study the relationship between phytoplankton distributions, as evaluated by taxonomic pigments, and the hydrographic structures of the water column. The study period covered the range of hydrographic conditions which prevail in the Mediterranean Sea. The 0 to 75 m integrated chlorophyll a concentration averaged 23.3 mg m(-2), with the highest values (above 45 mg m(-2)) restricted to semi-mixed periods. The major phytoplankton signature and water column structure relationships were: (1) phytoplanktonic prokaryotes (cyanobacteria and prochlorophytes) appear sensitive to water column mixing with prochlorophytes being the most sensitive group as strong stratification is associated with the highest biomass found mainly in deeper waters; (2) prymnesiophytes and chrysophytes (19'-BF and 19'-HF) appear the most abundant under a variety of conditions and therefore seem able to adapt to various water column structures; (3) diatoms bloom in semi-mixed conditions, but while these conditions are necessary, they are not sufficient for bloom formation; and (4) green chlorophyll b-containing flagellates appear to require strong mixing. During the stratification period, 2 noticeable wind-induced mixing events occurred, and while the first did not have any marked influence on the phytoplankton community, the second was followed by a subsurface development of green flagellates and diatoms. This second wind-mixing event also altered the vertical prokaryote distribution, but 1 wk alter this perturbation vertical segregation of prochlorophytes and cyanobacteria was reestablished. The results suggest that, while different phytoplankton taxa are generally adapted to specific water column structures, this is not always the case, especially at small scales where specific light/nutrient requirements may have to be met.

Using phytoplankton pigments as biomarkers, we investigated the relationship between the physical forcing and the resulting biological, ecological and biogeochemical properties of the geostrophic front of the Eastern Alboran Sea. (1) Typical frontal sites present biomass levels averaging 60 mg chl a m-2 (up to 100 mg m-2), whereas the adjacent zones (typical Atlantic and Mediterranean) are characterized by an average integrated chlorophyll biomass of 20 mg chl a m-2. (2) The phytoplankton biomass at front is diatom-dominated and differs markedly from the adjacent zones (typical Atlantic and Mediterranean), flagellate- and cyanobacteria-dominated. Therefore, high biomasses at the front do not result from purely physical accumulation but rather from local production. (3) The chlorophyll and diatom biomasses increase from the left to the right side of the Atlantic jet, which supports the hypothesis of a cross-frontal secondary circulation allowing a diatom bloom development. (4) Using assumptions on the carbon/chlorophyll ratio and growth rates for the different phytoplankton taxa, we evaluated the specific productions: diatoms account for 67% of the production at front and only about 10% at adjacent zones. (5) High concentrations of phaeopigments are only found at frontal stations, which points out the pecularities of the food web at the frontal site, compared to adjacent areas. (6) The observations made during this study give a precise picture of that frontal system: autotrophic new production and exportation are enhanced. The implication of this frontal system on the carbon budget at a regional scale may be important.

For various oceanic regimes, a pigment biomarker approach is used to investigate the relationship between the biomass and taxonomic composition of autotrophic communities. It is demonstrated that chlorophyll standing stocks are linearly related to the diatom (fucoxanthin) and dinoflagellate (peridinin) contents; other phytoplankton diagnostic pigments do not present any significant correlation with chlorophyll standing stocks. A pigment ratio, F(p), is proposed as an estimator of the proportion of new producers' biomass in a phytoplankton community. The variation of the F(p)-ratio with chlorophyll a biomass and (modeled) primary production rates suggests strong similarities between F(p) and the f-ratio (new production: total production).

As a part of the interdisciplinary study of the geostrophic front of the eastern Alboran Sea (''Almofront-1'', April-May 1991), several characteristics of phytoplankton biomass have been measured at the regional scale to evaluate the gradients between the frontal jet and the surrounding water masses. Microplanktonic diatoms, chlorophyll a and fucoxanthin were the most abundant in the front by 1-2 orders of magnitude whereas pico- and nanoplankton, which consist mostly of prymnesiophytes, and 19'hexanoyloxufucoxanthin tended to be the most abundant in the adjacent waters. Correlations between the various phytoplankton components and tracers are examined. The Almeria-Oran front behaves typically as a fertilisation site in an otherwise oligotrophic environment. Frontal fertilisation favored the growth of one or a few opportunistic, autochthonous diatom species, the remainder of the Alboran Sea being occupied by a diversified population of the smallest size-classes of phytoplankton.

Three unialgal strains of Prochlorococcus and four of Synechococcus were grown in batch culture at low irradiances. The spectral values of light absorption, scattering and backscattering of intact cells in suspension were determined, together with cell counts, size distribution and pigment composition (via HPLC). The spectral efficienCY factors Q(a), Q(b), Q(bb) for light absorption, scattering and backscattering respectively, were derived, as well as the corresponding chlorophyll-specific coefficients a*, b* and b(b)*. The pigment used when normalizing is `'true'' chlorophyll a for Synechococcus, and divinyl-chlorophyll a for Prochlorococcus. In correspondence with small sizes (0.6 mum, on average) Prochlorococcus exhibits Q(b) values below those of Synechococcus (size about 0.9 mum, on average). In contrast, Q(a) is higher for Prochlorococcus than for Synechococcus, in response to high internal divinyl-chlorophyll content. In the blue part of the spectrum the probability for photons of being absorbed by a Prochlorococcus cell exceeds that of being scattered. Such a combination has never been found before for other algal cells, consistently more efficient as scatterers than as absorbers. The magnitude of the three efficiency Q-factors, as well as their spectral variations, can be understood and reconstructed in the frame of the Mie theory. The impact of these small organisms, dominant in oligotrophic environment, upon the optical properties of such waters are discussed on the basis of their chlorophyll-specific optical coefficients. Their absorption capabilities (per unit of chlorophyll) are not far from being maximum, to the extent that the package effect is rather reduced. With respect to scattering, Prochlorococcus cells have a minute signature compared to that of Synechococcus. This point is illustrated using vertical profiles of fluorescence, attenuation coefficient, cell number, Chl a and divinyl-Chl a concentrations, as observed in an oligotrophic tropical situation. Even if the backscattering-to-scattering ratio is, as theoretically expected, higher for Prochlorococcus than for all other algae (including Synechococcus), their light backscattering capacity definitely remains negligible.

The biochemical composition (carbon, nitrogen, free amino acids, fatty acids, chlorophylls, carotenoids) of copepod faecal pellets was measured and compared with that of the particulate matter collected from three sites, of differing hydrographic regimes, in the Irish Sea during May/June 1988. The sites were: coastal weakly thermally stratified, central strongly thermally stratified and a central mixed isothermal site. Food was not limiting for copepods at the sites, as shown by the maximum concentration of the chlorophyll a which ranged from 4 to 7.5 mug l-1. There were marked differences in the composition and quality of particulates at these three sites. Site I was dominated by diatoms, site II was characterized by a diverse phytoplankton population, while site III was dominated by senescent diatoms and detritus. These differences were reflected in the biochemical composition of copepod faecal pellets. No significant bacterial enrichment occurred in facecal pellets as shown by the level of bacterial biomarkers. The fate of faecal pellets in the water column is discussed in relation to the use of biomarkers.

Particulate samples were collected throughout the water column (0-4200 m) in June/July 1988 at the Biotrans site and their carotenoids and chlorophylls analysed by HPLC. These photosynthetic pigments were used as biomarkers to characterize the autotrophic populations, their utilization by heterotrophs and sedimentation of particles out of the euphotic zone. In the upper 50 m the pico- and nanophytoplankton accounted for 85% of the chlorophyll a biomass. The major pigment of the nanophytoplankton fraction was 19'-hexanoyloxyfucoxanthin (prymnesiophytes), whereas the main pigment in the microphytoplankton was peridinin (dinoflagellates). The peaks in the distributions of phaeophorbide a and nanophytoplankton pigments (19'-hexanoyloxyfucoxanthin, 19'-butanoyloxyfucoxanthin, chlorophyll b, lutein and/or zeaxanthin) coincided between 75 and 100 m, which pointed to an active grazing of nanophytoplankton by zooplankton. These pigments were detected in particles > 20-mu-m from the Double Longhurst Hardy Plankton Recorder down to 1000 m, probably as a consequence of their incorporation into sedimenting faecal material. In contrast, the vertical distributions of phaeophorbide a and peridinin (microphytoplankton pigment) did not coincide, and this carotenoid was not detected below 400 m in particles > 20-mu-m. A vertical profile (0-4200 m) shows, at 2300 m, the presence of nanophytoplanktonic material similar in its pigment pattern and composition to that of surface populations, suggesting fast sedimentation of Prymnesiophyte floc.