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 contains about 40 times more carbon than the atmosphere, storing 38,000 Pg C as dissolved inorganic carbon (DIC) versus 900 Pg C as carbon dioxide (CO$_2$) in the present atmosphere. The biological carbon pump contributes to ocean carbon storage by moving organic carbon out of the surface ocean into deeper waters in sinking particles, vertically migrating organisms and physical circulation. Century-scale (≥100 years) storage of the resulting biogenic DIC is commonly assumed to occur exclusively in the deep ocean, typically below 1,000 m. However, recent work has shown that carbon can be sequestered at century scales above 1,000 m in many ocean regions, in what we call ‘continuous vertical sequestration’. Here we calculate the century-scale carbon sequestration flux driven by the biological pump throughout the water column by combining previously published estimates of organic carbon flux and modelled values of water-mass sequestration time distributions. We estimate that the flux of organic carbon that is sequestered for ≥100 years in the contemporary ocean by the combined action of various biological pump pathways is 0.9–2.6 Pg C yr$^{−1}$, which is up to six times larger than previous estimates based on the organic carbon flux reaching the deep 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.

Microbial degradation of dissolved organic carbon (DOC) in aquatic environments can cause oxygen depletion, water acidification, and CO2 emissions. These problems are caused by labile DOC (LDOC) and not refractory DOC (RDOC) that resists degradation and is thus a carbon sink. For nearly a century, chemical oxygen demand (COD) has been widely used for assessment of organic pollution in aquatic systems. Here, we show through a multicountry survey and experimental studies that COD is not an appropriate proxy of microbial degradability of organic matter because it oxidizes both LDOC and RDOC, and the latter contributes up to 90% of DOC in high-latitude forested areas. Hence, COD measurements do not provide appropriate scientific information on organic pollution in natural waters and can mislead environmental policies. We propose the replacement of the COD method with an optode-based biological oxygen demand method to accurately and efficiently assess organic pollution in natural aquatic environments.

Earth is, to our knowledge, the only life-bearing body in the Solar System. This extraordinary characteristic dates back almost 4 billion years. How to explain that Earth is teeming with organisms and that this has lasted for so long? What makes Earth different from its sister planets Mars and Venus? The habitability of a planet is its capacity to allow the emergence of organisms. What astronomical and geological conditions concurred to make Earth habitable 4 billion years ago, and how has it remained habitable since? What have been the respective roles of non-biological and biological characteristics in maintaining the habitability of Earth? This unique book answers the above questions by considering the roles of organisms and ecosystems in the Earth System, which is made of the non-living and living components of the planet. Organisms have progressively occupied all the habitats of the planet, diversifying into countless life forms and developing enormous biomassesover the past 3.6 billion years. In this way, organisms and ecosystems "took over" the Earth System, and thus became major agents in its regulation and global evolution. There was co-evolution of the different components of the Earth System, leading to a number of feedback mechanisms that regulated long-term Earth conditions. For millennia, and especially since the Industrial Revolution nearly 300 years ago, humans have gradually transformed the Earth System. Technological developments combined with the large increase in human population have led, in recent decades, to major changes in the Earth's climate, soils, biodiversity and quality of air and water. After some successes in the 20th century at preventing internationally environmental disasters, human societies are now facing major challenges arising from climate change. Some of these challenges are short-term and others concern the thousand-year evolution of the Earth's climate. Humans should become the stewards of Earth.

In this Food for Thought, I use my experience of writing scientific publications to stress some aspects of the process that were especially significant for me, and from which I try to derive some general suggestions. These aspects include: strong interactions (co-evolution) between paper writing and some of my research directions; the pleasure of writing with co-authors; writing as a tool of scientific creativity; long scientific quests through several publications; the importance of writing books, if possible starting early in the career; being published, reaching readers, and contributing to the advancement of knowledge; and giving in to the pleasure of writing. I explain that I often seized unexpected opportunities that led me to develop ideas and write publications that influenced the course of my career, but I do not necessarily suggest that anyone proceed as I did. My motivation was the enjoyment of exploring new topics, and I wholeheartedly recommend that everyone give in to the pleasure of writing.

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.

In recent years, autonomous platforms have been improved with the objective of observing the deep ocean. Many prototypes have reached 6,000 m, but only a few are commercially available. Imaging sensors have been developed to study the numbers, sizes and shapes of particles and plankton. These have been deployed independently or mounted on CTD rosettes (e.g. the UVP5 Underwater Vision Profiler). In some areas, they observed high concentrations of large particles down to the bottom of the ocean, showing intense carbon export. They also documented the presence of fragile organisms, such as rhizarians, whose key ecological roles in the ocean was unknown until then. The new miniaturized UVP6-LP (Low Power) sensor, developed to be mounted on autonomous platforms, is complementary to the larger sensors deployed on CTD rosettes. It also records and identifies particles and plankton, using imagery. It counts and sizes particles \textgreater80 µM ESD and identifies large aggregates and plankton \textgreater700µM ESD. This, at low cost and with very low power. Six prototypes of the sensor have been inter-calibrated with the reference UVP5. The instrument can be deployed down to 6,000 m, which corresponds to 97% of the ocean surface. A UVP6-LP was used to quantify particles released from a deep-sea mining experiment at 4,300 m. Others performed transects on gliders and vertical profiles on a float. A UVP6-LP was moored for one year at 50 m at 82°N. Other UVP6-LP will be deployed on the a cabled observatory in the Mediterranean Sea at 2,200 m, and on the SeaCycler mooring in the Labrador Sea. The very low power required for the operation of the UVP6-LP allows to optimize its use on profiling floats and gliders. Autonomous platforms cannot transmit images due to the limitation of satellite bandwidth or acoustic telemetry. To overcome this limitation, the UVP6-LP includes an embedded algorithm for the automatic classification of large aggregates and plankton images, which provides data that are accurate enough for monitoring programs and scientific studies. Because scientists, policy makers and the public require easy access to data, a complete software ecosystem is used to pilot the instrument, record the data, and make them freely available to the scientists and the public. When the instruments are recovered after deployment, their data include classified images.

Despite widespread climate-driven reductions of coral cover on tropical reefs, little attention has been paid to the possibility that changes in the geographic distribution of coral recruitment could facilitate beneficial responses to the changing climate through latitudinal range shifts. To address this possibility, we compiled a global database of normalized densities of coral recruits on settlement tiles (corals m-2) deployed from 1974 to 2012, and used the data therein to test for latitudinal range shifts in the distribution of coral recruits. In total, 92 studies provided 1253 records of coral recruitment, with 77% originating from settlement tiles immersed for 3-24 mo, herein defined as long-immersion tiles (LITs); the limited temporal and geographic coverage of data from short-immersion tiles (SITs; deployed for <3 mo) made them less suitable for the present purpose. The results from LITs show declines in coral recruitment, on a global scale (i.e. 82% from 1974 to 2012) and throughout the tropics (85% reduction at <20° latitude), and increases in the sub-tropics (78% increase at >20° latitude). These trends indicate that a global decline in coral recruitment has occurred since 1974, and the persistent reduction in the densities of recruits in equatorial latitudes, coupled with increased densities in sub-tropical latitudes, suggests that coral recruitment may be shifting poleward.

Carbon is a keystone element in global biogeochemical cycles. It plays a fundamental role in biotic and abiotic processes in the ocean, which intertwine to mediate the chemistry and redox status of carbon in the ocean and the atmosphere. The interactions between abiotic and biogenic carbon (e.g. CO2, CaCO3, organic matter) in the ocean are complex, and there is a half-century-old enigma about the existence of a huge reservoir of recalcitrant dissolved organic carbon (RDOC) that equates to the magnitude of the pool of atmospheric CO2. The concepts of the biological carbon pump (BCP) and the microbial loop (ML) shaped our understanding of the marine carbon cycle. The more recent concept of the microbial carbon pump (MCP), which is closely connected to those of the BCP and the ML, explicitly considers the significance of the ocean's RDOC reservoir and provides a mechanistic framework for the exploration of its formation and persistence. Understanding of the MCP has benefited from advanced `omics' and novel research in biological oceanography and microbial biogeochemistry. The need to predict the ocean's response to climate change makes an integrative understanding of the BCP, ML and MCP a high priority. In this review, we summarize and discuss progress since the proposal of the MCP in 2010 and formulate research questions for the future.

Dissolved oxygen (O2) is a relevant tracer to interpret variations of both water mass properties in the open ocean and biological production in the surface layer of both coastal and open waters. Deep-water formation is very active in the northwestern Mediterranean Sea, where it influences intermediate and deep waters properties, nutrients replenishment and biological production. This study analyses, for the first time, the 20-year time series of monthly O2 concentrations at the DYFAMED long-term sampling site in the Ligurian Sea. Until the winters of 2005 and 2006, a thick and strong oxygen minimum layer was present between 200 and 1300 m because dense water formation was then local, episodic and of low intensity. In 2005-2006, intense and rapid deep convection injected 24 mol O2 m-2 between 350 and 2000 m from December 2005 to March 2006. Since this event, the deep layer has been mostly ventilated during winter time by newly formed deep water spreading from the Gulf of Lion 250 km to the west and by some local deep mixing in early 2010, 2012 and 2013. In the context of climate change, it is predicted that the intensity of deep convection will become weaker in the Mediterranean, which could potentially lead to hypoxia in intermediate and deep layers with substantial impact on marine ecosystems. With the exception of winters 2005 and 2006, the O2 changes in surface waters followed a seasonal trend that reflected the balance between air-sea O2 exchanges, changes in the depth of the mixed layer and phytoplankton net photosynthesis. We used the 20-year O2 time series to estimate monthly and annual net community production. The latter was 7.1 mol C m-2 yr-1, consistent with C-14 primary production determinations and sediment-trap carbon export fluxes at DYFAMED.

Atmospheric input of anthropogenic lead increased globally over the last centuries. The present study shows that the concentrations of lead in sediment cores from low-productivity Hudson Bay, northern Canada, remained relatively constant over the last centuries. The lack of imprint of the increased anthropogenic lead input in this marine environment is not consistent with the increased lead concentrations in nearby lakes over the same period. In addition, the observed trend in lead isotopic composition in our cores suggests an apparent progressive overprint of anthropogenic lead during the 1900's. In other words, isotopes clearly registered the increasingly anthropogenic nature of lead in the sedimentary record, but total lead concentrations remained constant, indicating that some process limited the export of lead to the sediment. These observations point to a long-term limitation of the downward export of particles in Hudson Bay. Given that the source of lead was the same for both Hudson Bay and neighboring highproductivity lakes, we hypothesize that the very low primary productivity of Hudson Bay waters was responsible for the low vertical export of lead to marine sediments. We further propose that primary productivity is the most important factor that generally drives the vertical export of particulate matter, and thus hydrophobic contaminants, in near-oligotrophic marine environments.

Large macroalgal blooms (i.e. green tides of Ulva prolifera) occurred in the southern Yellow Sea, China, yearly from 2007 to 2016. They were among the largest of such outbreaks around the world, and these blooms likely originated along the coast of the Jiangsu Province, China. Understanding the roles of nutrients in the onset of these macroalgal blooms is needed to identify their origin. This study analyzes the spatiotemporal variations in dissolved inorganic nitrogen and phosphorus (DIN and PO4-P) and the N/P ratio along the Jiangsu coast from 1996 to 2014 during late-March to April, the months which corresponds to the pre-bloom period of green tides since 2007. A zone of high DIN and PO4-P concentrations has developed along the Jiangsu coast, between the cities of Sheyang and Nantong, since 1996. There was an 18-year trend of increasing DIN concentrations during the pre-bloom period as well as a positive correlation between the U. prolifera biomass and DIN concentrations. Nutrient inputs from rivers and mariculture in the Jiangsu Province may have provided nitrogen that contributed the magnitude of macroalgal blooms that subsequently spread into the southern Yellow Sea. (C) 2017 Elsevier B.V. All rights reserved.

Transparent Exopolymer Particles (TEP) have received considerable attention since they were first described in the ocean more than 20 years ago. This is because of their carbon-rich composition, their high concentrations in ocean’s surface waters, and especially because of their ability to promote aggregation due to their high stickiness (i.e. biological glue). As large aggregates contribute significantly to vertical carbon flux, TEP are commonly seen as a key factor that drives the downward flux of particulate organic carbon (POC). However, the density of TEP is lower than that of seawater, which causes them to remain in surface waters and even move upwards if not ballasted by other particles, which often leads to their accumulation in the sea surface microlayer. Hence we question here the generally accepted view that TEP always increase the downward flux of POC via gravitational settling. In the present reassessment of the role of TEP, we examine how the presence of a pool of non-sinking carbon-rich particulate organic matter in surface waters influences the cycling of organic carbon in the upper ocean at daily to decadal time scales. In particular, we focus on the role of TEP in the retention of organic carbon in surface waters versus downward export, and discuss the potential consequences of climate change on this process and on the efficiency of the biological carbon pump. We show that TEP sink only when ballasted with enough high-density particles to compensate their low density, and hence that their role in vertical POC export is not solely linked to their ability to promote aggregation, but also to their contribution to the buoyancy of POC. It follows that the TEP fraction of POC determines the degree of retention and remineralization of POC in surface waters versus its downward export. A high TEP concentration may temporally decouple primary production and downward export. We identify two main parameters that affect the contribution of TEP to POC cycling; TEP stickiness, and the balance between TEP production and degradation rates. Because stickiness, production and degradation of TEP vary with environmental conditions, the role of TEP in controlling the balance between retention versus export, and hence the drawdown of atmospheric CO2 by the biological carbon pump, can be highly variable, and is likely to be affected by climate change.

Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Herewe present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs): six types of phytoplankton, three types of zooplankton, and heterotrophic procaryotes. We improved the representation of zooplanktondynamics in our model through (a) the explicit inclusion of large, slow-growing macrozooplankton (e.g. krill), and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean high-nutrient lowchlorophyll (HNLC) region during summer. When model simulations do not include macrozooplankton grazing explicitly, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when thereis no iron deposition from dust. When model simulations include a slow-growing macrozooplankton and trophic cascades among three zooplankton types, the high-chlorophyll summer bias in the Southern Ocean HNLC region largely disappears. Our model results suggest that the observed lowphytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community, despite iron limitation of phytoplankton community growth rates. This result has implications for the representation of global biogeochemicalcycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean.

The marine macrophyte Ulva prolifera is the dominant green-tide-forming seaweed in the southern Yellow Sea, China. Here we assessed, in the laboratory, the growth rate and nutrient uptake responses of U. prolifera to different nutrient treatments. The growth rates were enhanced in incubations with added organic and inorganic nitrogen [i.e. nitrate (NO3−), ammonium (NH4+), urea and glycine] and phosphorus [i.e. phosphate (PO43−), adenosine triphosphate (ATP) and glucose 6-phosphate (G-6-P)], relative to the control. The relative growth rates of U. prolifera were higher when enriched with dissolved organic nitrogen (urea and glycine) and phosphorus (ATP and G-6-P) than inorganic nitrogen (NO3− and NH4+) and phosphorus (PO43−). In contrast, the affinity was higher for inorganic than organic nutrients. Field data in the southern Yellow Sea showed significant inverse correlations between macroalgal biomass and dissolved organic nutrients. Our laboratory and field results indicated that organic nutrients such as urea, glycine and ATP, may contribute to the development of macroalgal blooms in the southern Yellow Sea.

This study analyzes the pH of surface-sediment porewater (i.e. 2-3 cm below the water-sediment interface), and concentrations of CaCO3 and organic carbon (OC) in 1192 sediment cores from the northern South China Sea, in water depths ranging from 137 to 3702 m. This is the first study in the literature to analyze the large-scale spatial variability of deep-water surface-sediment pH over a large ocean basin. The data showed strong spatial variations in pH. The lowest pH values (<7.3) were observed south of Hainan Island, an area that is affected by summer upwelling and freshwater runoff from the Pearl and Red Rivers. Moderately low pH values (generally 7.3-7.5) occurred in two other areas: a submarine canyon, where sediments originated partly from the Pearl River and correspond to a paleo-delta front during the last glacial period; and southwest of Taiwan Island, where waters are affected by the northern branch of the Kuroshio intrusion current (KIC) and runoff from Taiwan rivers. The surface sediments with the highest pH (>= 7.5, and up to 8.3) were located in a fourth area, which corresponded to the western branch of the KIC where sediments have been intensively eroded by bottom currents. The pH of surface sediment porewater was significantly linearly related to water depth, bottom-water temperature, and CaCO3 concentration (p < 0.05 for the whole sampling area). This study shows that the pH of surface sediment porewater can be sensitive to characteristics of the overlying water column, and suggests that it will respond to global warming as changes in surface-ocean temperature and pH progressively reach deeper waters. (C) 2016 Elsevier Ltd. All rights reserved.

Ecological succession provides a widely accepted description of seasonal changes in phy-toplankton and mesozooplankton assemblages in the natural environment, but concurrent changes in smaller (i.e. microbes) and larger (i.e. macroplankton) organisms are not included in the model because plankton ranging from bacteria to jellies are seldom sampled and analyzed simultaneously. Here we studied, for the first time in the aquatic literature, the succession of marine plankton in the whole-plankton assemblage that spanned 5 orders of magnitude in size from microbes to macroplankton predators (not including fish or fish lar-vae, for which no consistent data were available). Samples were collected in the northwestern Mediterranean Sea (Bay of Villefranche) weekly during 10 months. Simultaneously collected samples were analyzed by flow cytometry, inverse microscopy, FlowCam, and ZooScan. The whole-plankton assemblage underwent sharp reorganizations that corresponded to bottom-up events of vertical mixing in the water-column, and its development was top-down controlled by large gelatinous filter feeders and predators. Based on the results provided by our novel whole-plankton assemblage approach, we propose a new comprehensive conceptual model of the annual plankton succession (i.e. whole plankton model) characterized by both stepwise stacking of four broad trophic communities from early spring through summer, which is a new concept, and progressive replacement of ecological plankton categories within the different trophic communities, as recognised traditionally.

The biological carbon pump causes carbon sequestration in deep waters by downward transfer of organic matter, mostly as particles. This mechanism depends to a great extent on the uptake of CO2 by marine plankton in surface waters and subsequent sinking of particulate organic carbon (POC) through the water column. Most of the sinking POC is remineralized during its downward transit, and modest changes in remineralization have substantial feedback on atmospheric CO2 concentrations, but little is known about global variability in remineralization. Here we assess this variability based on modern underwater particle imaging combined with field POC flux data and discuss the potential sources of variations. We show a significant relationship between remineralization and the size structure of the phytoplankton assemblage. We obtain the first regionalized estimates of remineralization in biogeochemical provinces, where these estimates range between -50 and +100% of the commonly used globally uniform remineralization value. We apply the regionalized values to satellite-derived estimates of upper ocean POC export to calculate regionalized and ocean-wide deep carbon fluxes and sequestration. The resulting value of global organic carbon sequestration at 2000m is 0.33PgCyr(-1), and 0.72PgCyr(-1) at the depth of the top of the permanent pycnocline, which is up to 3 times higher than the value resulting from the commonly used approach based on uniform remineralization and constant sequestration depth. These results stress that variable remineralization and sequestration depth should be used to model ocean carbon sequestration and feedback on the atmosphere.

Using a previous model based on the microbial hub (HUB; consists of heterotrophic bacteria and microzooplankton, the latter being heterotrophic protists), we investigated the effects of competition for inorganic and organic resources in planktonic food webs by proposing and developing the concept of `competition switches'. A competition switch controls the flow of carbon toward either the HUB or other food web compartments. The 3 switches are PB: competition for inorganic nutrients between bacteria and phytoplankton; MB: competition for detritus between bacteria and mesozooplankton; and M mu: competition for large-sized phytoplankton production between microzooplankton and mesozooplankton. Here, we explored the novel hypothesis that competition for resources between the HUB and other food web compartments plays a crucial role in controlling the flows of biogenic carbon in the euphotic zone. We ran a numerical model to determine the potential effects of the 3 competition switches and found that the most important switch is MB, followed by PB and M mu. Comparison of our model results with field data indicated that the strong effects of HUB competition for resources with phytoplankton and mesozooplankton exist both in our model as well as in the world ocean. Finally, comparison of our model results with carbon flows estimated by the linear inverse approach showed that the competition switches can determine large changes in the flows of carbon in marine pelagic food webs. The focus of our study was the propagation of competition effects that occur at the core of the food web.

Compiled abundances of juvenile corals revealed no change over time in the Pacific, but a decline in the Caribbean. Using these analyses as a rationale, we explored recruitment and post-settlement success in determining coral cover using studies in the Caribbean (St John, Bonaire) and Pacific (Moorea, Okinawa). Juvenile corals, coral recruits, and coral cover have been censused in these locations for years, and the ratio of juvenile (J) to recruiting (R) corals was used to measure post-settlement success. In St John and Bonaire, coral cover was stable but different between studies, with the ratio of the density of juveniles to density of recruits (J:R) 0.10; in Moorea, declines in coral cover were followed by recovery that was related to the density of juvenile corals 3 years before, with J:R 0.40; and in Okinawa, a decline in coral cover in 1998 was followed by a slow recovery with J/R 0.01. Coral cover was associated positively with juvenile corals in St John, and in Okinawa, the density of juvenile corals was associated positively with recruits the year before. J:R varied among studies, and standardised densities of juvenile corals declined in the Caribbean, but increased in the Pacific. These results suggest that differences in the post-settlement success may drive variation in coral community structure.

This work is based on a long time series of data collected in the well-preserved Bay of Calvi (Corsica island, Ligurian Sea, NW Mediterranean) between 1979 and 2011, which include physical characteristics (31 years), chlorophyll a (chl a, 15 years), and inorganic nutrients (13 years). Because samples were collected at relatively high frequencies, which ranged from daily to biweekly during the winter-spring period, it was possible to (1) evidence the key role of two interacting physical variables, i.e. water temperature and wind intensity, on nutrient replenishment and phytoplankton dynamics during the winter-spring period, (2) determine critical values of physical factors that explained interannual variability in the replenishment of surface nutrients and the winter-spring phytoplankton bloom, and (3) identify previously unrecognised characteristics of the planktonic ecosystem. Over the >30 year observation period, the main driver of nutrient replenishment and phytoplankton (chl a) development was the number of wind events (mean daily wind speed >5 m s(-1)) during the cold-water period (subsurface water <= 13.5 degrees C). According to winter intensity, there were strong differences in both the duration and intensity of nutrient fertilisation and phytoplankton blooms (chl a). The trophic character of the Bay of Calvi changed according to years, and ranged from very oligotrophic (i.e. subtropical regime, characterised by low seasonal variability) to mesotrophic (i.e. temperate regime, with a well-marked increase in nutrient concentrations and chl a during the winter-spring period) during mild and moderate winters, respectively. A third regime occurred during severe winters characterised by specific wind conditions (i.e. high frequency of northeasterly winds), when Mediterranean ``high nutrient - low chlorophyll'' conditions occurred as a result of enhanced crossshore exchanges and associated offshore export of the nutrient-rich water. There was no long-term trend (e.g. climatic) in either nutrient replenishment or the winter-spring phytoplankton bloom between 1979 and 2011, but both nutrients and chl a reflected interannual and decadal changes in winter intensity. (C) 2015 Elsevier Ltd. All rights reserved.

Nitrogen and iron largely determine phytoplankton production in the ocean, yet there are few direct incubation studies on nutrient limitation of phytoplankton in the western tropical North Pacific Ocean. Here, we assessed the effects of N and Fe amendments on both photosynthesis and biomass of phytoplankton (including final chlorophyll a biomass and taxonomic composition) in that region. Overall, the carrying capacity of phytoplankton biomass was primarily constrained by N, and Fe was a limiting resource only near the equator where phytoplankton was co-limited by N and Fe. Positive growth responses in nutrient-receiving treatments were predominantly caused by diatoms, sometimes together with Synechococcus. The often strong stimulation of large phytoplankton in these treatments substantially modified the phytoplankton assemblage from picoplankton-to diatom-dominated, which was often accompanied by a significant increase in photosynthetic competence. This is the first report showing that phytoplankton in the tropical western North Pacific are primarily limited by N, and co-limited by Fe in some areas. We combined our results with those from comparable studies in the tropical and subtropical Pacific Ocean to propose a general typology that describes and predicts the long-term effects of rate-limiting N and Fe on phytoplankton final biomass and changes in community structure.

Three vertical ocean carbon pumps have been known for almost three decades to sequester atmospheric carbon in the deep-water and sediment reservoirs, i.e. the solubility pump, the carbonate pump, and the soft-tissue (also known as organic, or biological) carbon pump (BCP). These three pumps maintain the vertical gradient in total dissolved inorganic carbon between the surface and deep waters. The more recently proposed microbial carbon pump (MCP) would maintain a gradient between short- and long-lived dissolved organic carbon (DOC; average lifetimes of <100 and >100 years, respectively). Long-lived DOC is an additional proposed reservoir of sequestered carbon in the ocean. This review: examines critically aspects of the vertical ocean carbon pumps and the MCP, in particular their physical dimensions and their potential roles in carbon sequestration; normalises the dimensions of the MCP to allow direct comparisons with the three vertical ocean carbon pumps; compares the MCP and vertical ocean carbon pumps; organises in a coherent framework the information available in the literature on refractory DOC; explores the potential effects of the globally changing ocean on the MCP; and identifies the assumptions that generally underlie the MCP studies, as bases for future research. The study: proposes definitions of terms, expressions and concepts related to the four ocean carbon pumps (i.e. three vertical pumps and MCP); defines the magnitude for the MCP as the rate of production of DOC with an average lifetime of >100 years and provides its first estimate for the World Ocean, i.e. 0.2 Pg C year−1; and introduces an operational “first-time-sequestration” criterion that prevents organic carbon fluxes from being assigned to both the BCP and the MCP. In our review of the potential effects of predicted climate-related changes in the ocean environment on the MCP, we found that three of the seven predicted changes could potentially enhance carbon sequestration by the MCP, and three could diminish it.

This paper reviews progress on understanding biological carbon sequestration in the ocean with special reference to the microbial formation and transformation of recal-citrant dissolved organic carbon (RDOC), the microbial carbon pump (MCP). We propose that RDOC is a concept with a wide continuum of recalcitrance. Most RDOC compounds maintain their levels of recalcitrance only in a specific environmental context (RDOC t). The ocean RDOC pool also contains compounds that may be inaccessible to microbes due to their extremely low concentration (RDOC c). This differentiation allows us to appreciate the linkage between microbial source and RDOC composition on a range of temporal and spatial scales. Analyses of biomarkers and isotopic records show intensive MCP processes in the Proterozoic oceans when the MCP could have played a significant role in regulating climate. Understanding the dynamics of the MCP in conjunction with the better constrained biological pump (BP) over geological timescales could help to predict future climate trends. Integration of the MCP and the BP will require new research approaches and opportunities. Major goals include understanding the interactions between particulate organic carbon (POC) and RDOC that contribute to sequestration efficiency, and the concurrent determination of the chemical composition of organic carbon, microbial community composition and enzymatic activity. Molecular biomarkers and isotopic tracers should be employed to link water column processes Published by Copernicus Publications on behalf of the European Geosciences Union. 5286 N. Jiao et al.: Mechanisms of microbial carbon sequestration in the ocean to sediment records, as well as to link present-day observations to paleo-evolution. Ecosystem models need to be developed based on empirical relationships derived from bioassay experiments and field investigations in order to predict the dynamics of carbon cycling along the stability continuum of POC and RDOC under potential global change scenarios. We propose that inorganic nutrient input to coastal waters may reduce the capacity for carbon sequestration as RDOC. The nutrient regime enabling maximum carbon storage from combined POC flux and RDOC formation should therefore be sought.

Identification of the trophic pathway that dominates a given planktonic assemblage is generally based on the distribution of biomasses among food-web compartments, or better, the flows of materials or energy among compartments. These flows are obtained by field observations and a posteriori analyses, including the linear inverse approach. In the present study, we re-analysed carbon flows obtained by inverse analysis at 32 stations in the global ocean and one large lake. Our results do not support two "classical" views of plankton ecology, i.e. that the herbivorous food web is dominated by mesozooplankton grazing on large phytoplankton, and the microbial food web is based on microzooplankton significantly consuming bacteria; our results suggest instead that phytoplankton are generally grazed by microzooplankton, of which they are the main food source. Furthermore, we identified the "phyto-microbial food web", where microzooplankton largely feed on phytoplankton, in addition to the already known "poly-microbial food web", where microzooplankton consume more or less equally various types of food. These unexpected results led to a (re)definition of the conceptual models corresponding to the four trophic pathways we found to exist in plankton, i.e. the herbivorous, multivorous, and two types of microbial food web. We illustrated the conceptual trophic pathways using carbon flows that were actually observed at representative stations. The latter can be calibrated to correspond to any field situation. Our study also provides researchers and managers with operational criteria for identifying the dominant trophic pathway in a planktonic assemblage, these criteria being based on the values of two carbon ratios that could be calculated from flow values that are relatively easy to estimate in the field. © 2013 Elsevier Ltd.

Invasion of the carnivorous ctenophore Mnemiopsis leidyi in the Black Sea in the 1980s disrupted the ecosystem, which started to recover with the arrival of the predatory ctenophore Beroe ovata in 1997. We used the results of 25 yr of field observations and experiments in the northeastern Black Sea to assess 3 hypotheses that should explain most of the population dynamics of M. leidyi and B. ovata. The first hypothesis is that since its arrival, B. ovata has controlled the period of the year during which M. leidyi was present in sizable concentrations. This hypothesis is supported by the observation that M. leidyi abundance was sizable almost year-round (spring, summer, autumn) before the arrival of B. ovata but was sizable only for a period of 1.3 to 3.1 mo (mostly summer) after its arrival. The second hypothesis is that the same sequence of predator prey mechanisms that led B. ovata to shorten the duration of a sizable M. leidyi population occurred every year irrespective of interannual environmental variability. This is supported by the repetition of the same reproductive sequences of the 2 ctenophores yearly since 1999 despite differences in environmental factors. The third hypothesis (i.e. environmental conditions influenced the joint abundances of the 2 species) is supported by the observed covariability between the 2 species every year. Experimental and field results identified temperature, food and wind as the key factors influencing M. leidyi, which suggested that the interannual environmental variations that affect M. leidyi abundance cause proportional changes in B. ovata abundance. Some aspects of these hypotheses have been previously examined in the literature, but this is the first study in which they are assessed using a consistent set of data.

Large amounts of soot are continuously deposited on the global ocean. Even though significant concentrations of soot particles are found in marine waters, the effects of these aerosols on ocean ecosystems are currently unknown. Using a combination of in situ and experimental data, and results from an atmospheric transport model, we show that the deposition of soot particles from an oil-fired power plant impacted biogeochemical properties and the functioning of the pelagic ecosystem in tropical oligotrophic oceanic waters off New Caledonia. Deposition was followed by a major increase in the volume concentration of suspended particles, a change in the particle size spectra that resulted from a stimulation of aggregation processes, a 5% decrease in the concentration of dissolved organic carbon (DOC), a decreases of 33 and 23% in viral and free bacterial abundances, respectively, and a factor~2 increase in the activity of particle-attached bacteria suggesting that soot introduced in the system favored bacterial growth. These patterns were confirmed by experiments with natural seawater conducted with both soot aerosols collected in the study area and standard diesel soot. The data suggest a strong impact of soot deposition on ocean surface particles, DOC, and microbial processes, at least near emission hot spots.

Ship measurements made 2 days after the passage of a tropical depression (TD) in the South China Sea (SCS, April 2011) showed two contrasted responses of the partial pressure of CO2 at sea surface (pCO(2,sw)). In low sea-surface salinity (SSS) water, pCO(2,sw) was low (3497 atm), and the area was a carbon sink (-4.71.8 mmol CO2 m(-2) d(-1)), whereas in water with high SSS and chlorophyll a and low dissolved oxygen and sea surface temperature, pCO(2,sw) was higher than for normal SCS water (3768 versus 3624 atm) and the area was a carbon source (1.2 +/- 3.1 mmol CO2 m(-2) d(-1)). Satellite data showed two large areas of low SSS before the TD, which were likely influenced by rainfall, and these areas were considered to have low pCO(2,sw) because of their low SSS. The high pCO(2,sw) after the TD is explained by the uplifting to the surface of deeper and CO2-rich water, due to winds accompanied by the TD. The difference in sea-air CO2 flux between the TD-affected area and the lower-SSS water was 1.99+4.70= 6.7 mmol CO2 m(-2) d(-1), indicating a 100% change caused by the TD compared to the average seasonal value in spring in southern SCS (3.3 +/- 0.3 mmol CO2 m(-2) d(-1)). Undersaturation of CO2 prior to the TD due to dilution by freshwater and the preexisting cold eddy, and slow translation speed of the TD, are considered to account for the CO2 flux change.

Large-scale regional marine ecosystems can be compared for various processes that include their structure and biodiversity, functioning, services, and effects on biogeochemical processes. The comparisons can proceed from data up, or from conceptual models down, or from a combination of models and data. This study proposes a typology of methods and approaches that are currently used, or could possibly be used for making large-scale ecosystem comparisons. The various methods and approaches are illustrated with examples drawn from the literature. © 2011 Elsevier B.V.

There is growing interest in linking marine biogeochemistry with marine ecosystems research in response to the increasing need to understand and predict the effect of global change on the marine ecosystem. Such a holistic approach combines oceanographic and biogeochemical processes and information on organisms, ranging from microbes to higher-trophic-levels. Comparative studies offer a means to improve understanding of critical mechanisms that influence marine systems by showing differences in ecosystem response to changing ocean conditions. Comparing similar biomes that differ in a particular set of physical or biological characteristics can provide insight into the susceptibility of the key features of a system to perturbation. Also, comparative studies based on long-term observations at fixed time-series stations enable the evaluation of long-term changes in the physical and biological environment, such as those driven by climate patterns. Moreover, the comparative approach provides a feasible alternative to costly and complex research programs designed to provide detailed end-to-end evaluations of marine systems. Planned and unplanned perturbations allow the investigation of the sensitivity of ecosystems and their biogeochemical processes to change at different time and space scales. In well-studied regions where sufficient data are available, models can provide comprehensive syntheses, mechanistic insights and even predictions. We present examples of successful comparative studies that incorporate both biogeochemical and ecosystems aspects. A framework for a basic approach for comparative studies is proposed that considers the interactions between biogeochemical cycles and ecosystems. This approach is based on constructing a minimalistic observational framework grounded within a conceptual model. (C) 2012 Elsevier B.V. All rights reserved.

A general model of species diversity predicts that the latter is maximized when productivity and disturbance are balanced. Based on this model, we hypothesized that the response of bacterial diversity to the ratio of viral to bacterial production (VP/BP) would be dome-shaped. In order to test this hypothesis, we obtained data on changes in bacterial communities (determined by terminal restriction fragment length polymorphism of 16S rRNA gene) along a wide VP/BP gradient (more than two orders of magnitude), using seawater incubations from NW Mediterranean surface waters, i.e., control and treatments with additions of phosphate, viruses, or both. In December, one dominant Operational Taxonomic Unit accounted for the major fraction of total amplified DNA in the phosphate addition treatment (75 +/- 20%, +/- S.D.), but its contribution was low in the phosphate and virus addition treatment (23 +/- 19%), indicating that viruses prevented the prevalence of taxa that were competitively superior in phosphate-replete conditions. In contrast, in February, the single taxon predominance in the community was held in the phosphate addition treatment even with addition of viruses. We observed statistically robust dome-shaped response patterns of bacterial diversity to VP/BP, with significantly high bacterial diversity at intermediate VP/BP. This was consistent with our model-based hypothesis, indicating that bacterial production and viral-induced mortality interactively affect bacterial diversity in seawater.

Temperate symbiotic corals, such as the Mediterranean species Cladocora caespitosa, live in seasonally changing environments, where irradiance can be ten times higher in summer than winter. These corals shift from autotrophy in summer to heterotrophy in winter in response to light limitation of the symbiont's photosynthesis. In this study, we determined the autotrophic carbon budget under different conditions of irradiance (20 and 120 µmol photons m(-2) s(-1)) and feeding (fed three times a week with Artemia salina nauplii, and unfed). Corals were incubated in H(13)CO(3) (-)-enriched seawater, and the fate of (13)C was followed in the symbionts and the host tissue. The total amount of carbon fixed by photosynthesis and translocated was significantly higher at high than low irradiance (ca. 13 versus 2.5-4.5 µg cm(-2) h(-1)), because the rates of photosynthesis and carbon fixation were also higher. However, the percent of carbon translocation was similar under the two irradiances, and reached more than 70% of the total fixed carbon. Host feeding induced a decrease in the percentage of carbon translocated under low irradiance (from 70 to 53%), and also a decrease in the rates of carbon translocation per symbiont cell under both irradiances. The fate of autotrophic and heterotrophic carbon differed according to irradiance. At low irradiance, autotrophic carbon was mostly respired by the host and the symbionts, and heterotrophic feeding led to an increase in host biomass. Under high irradiance, autotrophic carbon was both respired and released as particulate and dissolved organic carbon, and heterotrophic feeding led to an increase in host biomass and symbiont concentration. Overall, the maintenance of high symbiont concentration and high percentage of carbon translocation under low irradiance allow this coral species to optimize its autotrophic carbon acquisition, when irradiance conditions are not favourable to photosynthesis.

Corals live in symbiosis with dinoflagellates of the genus Symbiodinum. These dinoflagellates translocate a large part of the photosynthetically fixed carbon to the host, which in turn uses it for its own needs. Assessing the carbon budget in coral tissue is a central question in reef studies that still vexes ecophysiologists. The amount of carbon fixed by the symbiotic association can be determined by measuring the rate of photosynthesis, but the amount of carbon translocated by the symbionts to the host and the fate of this carbon are more difficult to assess. In the present study, we propose a novel approach to calculate the budget of autotrophic carbon in the tissue of scleractinian corals, based on a new model and measurements made with the stable isotope C-13. Colonies of the scleractinian coral Stylophora pistillata were incubated in (HCO3-)-C-13-enriched seawater, after which the fate of C-13 was followed in the symbionts, the coral tissue and the released particulate organic carbon (i.e. mucus). Results obtained showed that after 15. min, ca. 60% of the carbon fixed was already translocated to the host, and after 48. h, this value reached 78%. However, ca. 48% of the photosynthetically fixed carbon was respired by the symbiotic association, and 28% was released as dissolved organic carbon. This is different from other coral species, where <1% of the total organic carbon released is from newly fixed carbon. Only 23% of the initially fixed carbon was retained in the symbionts and coral tissue after 48. h. Results show that our C-13-based model could successfully trace the carbon flow from the symbionts to the host, and the photosynthetically acquired carbon lost from the symbiotic association.

Temperature is a powerful correlate of large-scale terrestrial and marine diversity patterns but the mechanistic links remain unclear. Whilst many explanations have been proposed, quantitative predictions that allow them to be tested statistically are often lacking. As an important exception, the metabolic theory of ecology (MTE) provides a rather robust technique using the relationship between diversity, temperature and metabolic rate in order to elucidate the ultimate underlying mechanisms driving large-scale diversity patterns. We tested if the MTE could explain geographic variations in marine copepod diversity on both ocean-wide and regional scales (East Japan Sea and North East Atlantic). The values of the regression slopes of diversity (ln taxonomic richness) over temperature (1/kT) across all spatial scales were lower than the range predicted by the metabolic scaling law for species richness (i.e. -0.60 to -0.70).We therefore conclude that the MTE in its present form is not suitable for predicting marine copepod diversity patterns. These results further question the applicability of the MTE for explaining diversity patterns and, despite the relative lack of comparable studies in the marine environment, the generality of the MTE across systems.

We have investigated the relationships between covariations in environmental variables and variations in distributions of marine copepod diversity over an extensive latitudinal range from 86.5°N to 46.5°S. For this purpose, 7 data sets (representing 13,713 samples) of copepod species composition data and 11 environmental data sets were assembled. Principal components analysis was applied to investigate the relationships among the mean and seasonal variations in environmental descriptors (ocean temperature, chlorophyll a [Ch1a], net primary production, and other physical and chemical properties of the ocean) and their relationships with spatial variations in copepod diversity. High copepod diversity corresponded to a combination of high ocean temperature and salinity and low Ch1a and nutrient concentrations (nitrate, silicate, phosphate). To a lesser extent, high-diversity regions were also correlated to low seasonal variability in oxygen, ocean temperature, and mixed-layer depth. Regression on principal components provided a robust prediction of global copepod diversity (R2 = 0.45, p < 0.001) as our subset of environmental data was representative of the full range of environmental variability that occurs globally

We have investigated the relationships between covariations in environmental variables and variations in distributions of marine copepod diversity over an extensive latitudinal range from 86.5 degrees N to 46.5 degrees S. For this purpose, 7 data sets (representing 13,713 samples) of copepod species composition data and 11 environmental data sets were assembled. Principal components analysis was applied to investigate the relationships among the mean and seasonal variations in environmental descriptors (ocean temperature, chlorophyll a [ Chl a], net primary production, and other physical and chemical properties of the ocean) and their relationships with spatial variations in copepod diversity. High copepod diversity corresponded to a combination of high ocean temperature and salinity and low Chl a and nutrient concentrations (nitrate, silicate, phosphate). To a lesser extent, high-diversity regions were also correlated to low seasonal variability in oxygen, ocean temperature, and mixed-layer depth. Regression on principal components provided a robust prediction of global copepod diversity (R-2 = 0.45, p < 0.001) as our subset of environmental data was representative of the full range of environmental variability that occurs globally.

Although recent studies suggest that climate change may substantially accelerate the rate of species loss in the biosphere, only a few studies have focused on the potential consequences of a spatial reorganization of biodiversity with global warming. Here, we show a pronounced latitudinal increase in phytoplanktonic and zooplanktonic biodiversity in the extratropical North Atlantic Ocean in recent decades. We also show that this rise in biodiversity paralleled a decrease in the mean size of zooplanktonic copepods and that the reorganization of the planktonic ecosystem toward dominance by smaller organisms may influence the networks in which carbon flows, with negative effects on the downward biological carbon pump and demersal Atlantic cod (Gadus morhua). Our study suggests that, contrary to the usual interpretation of increasing biodiversity being a positive emergent property promoting the stability/resilience of ecosystems, the parallel decrease in sizes of planktonic organisms could be viewed in the North Atlantic as reducing some of the services provided by marine ecosystems to humans.

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.

We tested the hypothesis that viruses can control bacterial growth efficiency (BGE) in marine pelagic environments. In the Bay of Villefranche, Northwestern Mediterranean, three experiments were conducted on different months to determine bacterial and viral variables in seawater cultures. In December, phosphorus ( P) addition enhanced bacterial growth 6-9-fold with concomitant increase in viral production (4-15-fold), but little enhancement of bacterial respiration ( BR). In other months, P enrichment increased BR 2-6-fold and viral production 2-5-fold, but did not increase in bacterial abundance (Aug, Feb) or growth ( Feb). BGE depended on the fraction of bacterial production destroyed by viruses (shunting efficiency, nu; i.e., when nu was low, nutrient enrichment enhanced BGE, whereas when nu was high, nutrient enrichment mainly led to low BGE). Viral production and bacterial production and respiration in the Western North Pacific and other data from the literature showed that BGE was negatively correlated with shunting efficiency. Predictions from a carbon flow model were consistent with the above results showing that decreased BGE over a broad range of values ( from 0.7 to 0.001) could be largely explained by viral-induced conversion of bacterial biomass to dissolved organic carbon. Viruses exert the major influence on patterns in carbon fluxes mediated by bacteria in marine pelagic environments.

Our review consolidates published information on the functioning of the microbial heterotrophic components of pelagic food webs, and extends this into a novel approach: the `microbial hub' (HUB). Crucial to our approach is the identification and quantification of 2 groups of organisms, each with distinct effects on food-web flows and biogeochemical cycles: microbes, which are generally responsible for most of the organic carbon respiration in the euphotic zone, and metazoans, which generally account for less respiration than microbes. The key characteristics of the microbialhub approach are: all heterotrophic microbes are grouped together in the HUB, whereas larger heterotrophs are grouped into a metazoan compartment (METAZ); each food-web flow is expressed as a ratio to community respiration; summary respiration flows through, between, and from the HUB and METAZ are computed using flows from observations or models; both the HUB and METAZ receive organic carbon from several food-web sources, and redirect this carbon towards other foodweb compartments and their own respiration. By using the microbial-hub approach to analyze a wide range of food webs, different zones of the world ocean, and predicted effects of climate change on food-web flows, we conclude that heterotrophic microbes always dominate respiration in the euphotic zone, even when most particulate primary production is grazed by metazoans. Furthermore, climate warming will increase HUB respiration and channeling of primary production toward heterotrophic community respiration and decrease the corresponding METAZ flows. The microbial-hub approach is a significant evolution and extension of the microbial loop and food web, and provides a new, powerful tool for exploring pelagic community metabolism.

This study provides and discusses the spatial distributions of abundances and sizes of marine-snow aggregates across the Ligurian Sea frontal system. A cross-front transect was sampled 34 times between 1992 and 1996, using the Underwater Video Profiler (UVP). Atlantic Water flows parallel to the Ligurian coast in the NW Mediterranean Sea, where that current creates a quasi-permanent front that separates the central and coastal waters. The horizontal distribution of aggregates (> 150 mu m ESD, Equivalent Spherical Diameter) in the upper 1000 m shows two main features. First, the smaller aggregates (150 mu m <ESD< 1 mm) are more abundant in coastal waters, as a result of continental input, cross-slope export, and re-suspension along the slope. The layers that contain very high concentrations of small aggregates are observed from surface down to 1000 m, and extend from the continental slope to the front. Second, the concentrations of large aggregates (ESD > 1 mm) are highest in and under the frontal zone, probably as a result of physical coagulation, and/or biological transformations. The seasonal intensity of large aggregate accumulations in and under the frontal structure seems to be more related to the autumn-winter increase in sub-mesoscale and mesoscale activity of the current flow than to the surface phytoplankton biomass. Interestingly, the horizontal distribution of aggregates is affected not only in the frontal zone (0-300 m depth), but also deeper down to 1000 m, probably as a consequence of rapid sinking or vertical transport. Results suggest that the settling of large aggregates under the frontal zone may limit the cross-slope transport of fine-grained particles by coagulation due to differential settling between the small particles suspended in the continental nepheloid layer and the large aggregates. This process, which takes place in sub-mesoscale zones (5-10 km wide), was also observed in one other front in the Western Mediterranean Sea. This led us to hypothesize that the impact of frontal processes on particle and aggregate dynamics might be generalized. Since fronts exist in many other coastal regions, the vertical fluxes at sub-mesoscale may have consequences for the transport of continental particles to the ocean's interior. (C) 2007 Elsevier B.V. All rights reserved.

Spatial and temporal variability in the distribution of marine aggregates (> 110 mu m) was studied using underwater video profilers in an area off the Iberian Peninsula and Azores Islands dominated by mesoscale and submesocale hydrodynamics in winter, spring, and summer 2001. In the 0-200-m layer, aggregates were most abundant in spring (100-120 mg dry weight [dry wt] m(-3)) and lowest during summer and winter (1-10 mg dry wt (-3)). In the deeper layers,(down to 1,000 m), the seasonal pattern was different, with concentrations highest in spring and summer, and lowest in winter (e.g., at 800 in, 5-10 mg dry wt m(-3) in spring and summer; 1-5 mg dry wt m(-3) in winter). The seasonal change in the abundance of aggregates in the upper 1,000 m was consistent with changes in the composition and intensity of the particulate flux recorded in sediment traps and with seasonal changes in the surface phytoplankton community. In an area dominated by eddies, surface accumulation of aggregates and export down to 1,000 in occur at mesoscale distances (< 100 km). The occurrence of a rich aggregate layer may be related to mesoscale activity in water flow that drives nutrient inputs, phytoplankton production, and the formation of large aggregates. Such spatially constrained zones of massive export may be typical of frontal open-sea systems, and may have been missed by conventional sediment trap moorings, which cannot resolve export at this mesoscale level.

SUMMARY 1. A steady-state model of carbon flows was developed to describe the summer planktonic food web in the surface mixed-layer of the North Basin in Lake Biwa, Japan. This model synthesised results from numerous studies on the plankton of Lake Biwa. 2. An inverse analysis procedure was used to estimate missing flow values in a manner consistent with known information. Network analysis was applied to characterise emergent properties of the resulting food web. 3. The system strongly relied on flows related to detrital particles. Whereas primary production was mainly by phytoplankton >20 lm, microzooplankton were active and mainly ingested detritus and bacteria. 4. The main emergent property of the system was strong recycling, through either direct ingestion of non-living material by zooplankton, or ingestion of bacteria after degradation of detritus to release dissolved organic carbon.

The carbon to nitrogen (C:N) stoichiometry of phytoplankton production varied significantly during the spring-summer bloom in the North Water Polynya (NOW), from April through July 1998. The molar ratio of particulate organic carbon (POC) to nitrogen (PON) production by phytoplankton (ΔPOC:ΔPON) increased from 5.8 during April through early June to 8.9 in late June and July. The molar dissolved inorganic carbon (DIC) to nitrate+nitrite (NO ₃) drawdown ratio (ΔDIC: ΔNO ₃) increased from 6.7 in April and May, to 11.9 in June (no estimate for July because of ice melting). The discrepancy between ΔPOC:ΔPON and ΔDIC:ΔNO ₃ was likely due to dissolved organic carbon (DOC) production. Increased ΔPOC:ΔPON of phytoplankton and surface water ΔDIC:ΔNO ₃ throughout the phytoplankton blooms resulted from changes in physical properties of the upper water column, such as reduced thickness of the surface mixed layer that exposed phytoplankton to increased photosynthetically available radiation (PAR), accompanied by NO ₃ depletion. This is expected to have significant effects on the cycling of carbon (C) and nitrogen (N) in pelagic ecosystems, as the increased C:N ratio of organic matter decreases its quality as substrate for grazers and microbial communities. Based on ΔPOC:ΔPON, the ratio of POC to chlorophyll a (Chl) production (ΔPOC:ΔChl) and the relationship between Chl yields and NO ₃ depletion, we estimate that 71±17% and 46±20% of the depleted NO ₃ went to PON production in the euphotic zone over the polynya from April to early June, and late June to July, respectively. The remaining NO ₃ was likely channelled to dissolved organic nitrogen (DON) and heterotrophic bacteria, which were not returned to the dissolved inorganic nitrogen (DIN) pool through recycling during the course of the study. Hence, the autotrophic production of organic N and its recycling by the microbial food web were not coupled temporally.

The growth and dynamics of plankton in the ocean vary with natural cycles, global climate change and the long-term evolution of ecosystems. The ocean is a large reservoir for CO2 and the food webs in the upper ocean play critical roles in regulating the global carbon cycle, changes in atmospheric CO2 and associated global warming. Microheterotrophs are a key component of the upper ocean food webs. Here, we report on the results of an analysis of the distribution of bacteria and related properties in the World Ocean. We found that, for the data set as a whole, there is a significant latitudinal gradient in all field-measured and computed bacterial properties, except growth rate. Gradients were, for the most part, driven by an equator-ward increase in the Southern Hemisphere. The biomass, rates of production and respiration and dissolved organic carbon concentrations were significantly higher in the Northern than the Southern hemispheres. In contrast, growth rates were the same in the two hemispheres. We conclude that the lower biomass and production in the Southern Hemisphere reflects greater top-down control by microbial grazers, which would be due to a lower abundance or activity of omnivorous zooplankton in the Southern than Northern Hemispheres. These large spatial differences in dynamics, structure and activity of the bacterial community and the microbial food web will be reflected in different patterns of carbon cycling, export and air-sea exchange of CO2 and the potential ability of the ocean to sequester carbon.

Changes in both the environment and environmental research have led to the development of new protocols and approaches. These new approaches consider both the effects of changes in the global environment on living organisms (i.e. the responses of ecosystems to environmental processes) and the feedback responses of these organisms and ecosystems (i.e. the effects of living organisms on the environment). The present paper focuses on pelagic food webs in aquatic ecosystems. We examine three major effects of global environmental changes on aquatic organisms: (i) the release of pollutants and biological agents in lakes and nearshore marine waters; (ii) the loss of biodiversity and the collapse of commercially exploited resources that were heretofore renewable. We develop detailed examples of the effects of human activities on marine organisms (i.e. the effects of nutrient supply on the structure of pelagic food webs in marine systems. Finally, we examine (iii) the food-web-controlled exchanges of CO(2) between the atmosphere and the ocean, as a feedback effect of pelagic ecosystems on the global environment with respect to the ongoing climate change.

We present a new approach to assess the role of upper ocean pelagic food webs on the partitioning of phytoplankton production (P-T) into its 3 principal component fluxes: remineralization to CO2 (i.e. respiration, R), transfer to the pelagic food web (F), and downward export (ET); ET is the sum of its particulate (POC) and dissolved (DOC) organic carbon components (E-T = E-DOC + E-POC). Although it is well known that there are relationships between the size and trophic structure of the planktonic community on the one hand, and the export of organic carbon (OC) from the euphotic zone and its potential sequestration below the permanent pycnocline on the other hand, the causative mechanisms for these relationships are not well understood, Here, we propose that the fluxes of OC in the upper ocean depend on the coexistence of a relatively small POC pool, which is responsible for the fluxes P-T, R, F and E-POC, and a much larger DOC pool, which sustains both bacterial production and E-DOC. In our model, phytoplankton, microbial heterotrophic plankton, and large zooplankton are the 3 food-web control nodes of the 5 carbon fluxes (P-T, R, F, E-DOC and E-DOC). The phytoplankton node controls the downward flux of phytodetritus (mostly from large phytoplankton), which is often the major component of Epoc. The microbial heterotrophic plankton node is responsible for most of the remineralization of OC to CO2 and the uptake and release of DOC. This node therefore controls the size of the DOC pool that can be exported downwards. The large zooplankton node controls both the transfer of POC to large metazoans and part of the downward POC flux (Epoc; faecal pellets and vertically migrating organisms). We implemented our model by estimating export as E-T = P-T - R at 8 sites in different regions of the World Ocean. The functional relationship between E-T and POC was highly significant (r(2) = 0.85): In contrast to other approaches, where export is calculated as a fraction of P-T, we estimate E-T as the difference between 2 independent variables (i.e. PT and R); hence our approach produces some regional values E-T < 0 - these regions are net heterotrophic. Overall, our approach improves our understanding of carbon cycling and export in the upper ocean.

Specific taxa and community structure of the phytoplankton and other protists in the North Water, a productive polynya ecosystem between Ellesmere Island (Canada) and Greenland, were determined from April to July 1998. Distinct temporal and spatial trends were observed within different geographic regions, influenced by water masses determined by temperature and salinity (TS) characteristics. In April, the northernmost communities were primarily flagellates and dinotlagellates, resembling those of the Arctic Basin, while large centric diatoms dominated in May and persisted through July. Throughout the rest of the polynya in April, centric diatoms and alveolates (ciliates and dinoflagellates) dominated. In the eastern part of the polynya in May and June, centric diatoms, especially Thalassiosira spp., reached very high levels, comprising 50-80% of biomass carbon there, while in July the eastern community was primarily Chaetoceros spp. and large alveolates. The community in the western region in April was more mixed, with Actinocyclus spp./Coscinodiscus spp. and alveolates as the dominant groups. In May and June, pennate-ribbon diatoms were mixed with Thalassiosira and Chaetoceros spp., while in July western biomass was primarily alveolates and Chaetoceros spp. Throughout the polynya and the 4-month sampling period, the community of phytoplankton and other protists was dominated by microplankton (20-200 mum). Records of this size class from the current study were comparable to historical records collected since 1876. The planktonic communities along the southern border of the North Water, which was ice-covered until mid-June and had TS characteristics of the West Greenland Current (WGC), differed from those in the rest of the polynya. They contained few planktonic diatoms and had a higher proportion of nanoplankton size (2-20 mum) flagellates and gymnodinoid dinoflagellates. The WGC microplankton were dominated by thecate dinoflagellates such as Ceratium arcticum. Overall, the diatom-dominated phytoplankton bloom in the North Water was 2-3 months earlier than blooms in the southern region. In addition to blooming sooner, large cells persisted, were a dominant biomass component from May to July, and thus were major contributors to the overall productivity of the North Water. (C) 2002 Elsevier Science Ltd. All rights reserved.

The structure and functioning of the planktonic food web were characterized in the lagoon of Takapoto Atoll (French Polynesia). Several variables were estimated over several weeks in April 1996 and 1997: carbon stocks of different planktonic compartments, phytoplankton particulate net production (PPNP), release of dissolved organic carbon (DOC) from phytoplankton, bacterial production and metabolic activity, and sinking of partides. The carbon stocks and flows were used to build up a quantitative description of the planktonic food web. The heterotrophic bacterial community was characterized by high biomass, low production and a small proportion of actively respiring cells. This was consistent with the observed low consumption of bacteria by protozoa. The relatively high PPNP was mostly due to cyanobacteria \textless1 μm in size. DOC release from phytoplankton represented, on average, 48% of the total photoassimilated carbon. The planktonic food web was characterized by a high biomass of small protozoa (82% were ≤14 μm). The contribution of metazoa to the carbon stock of zooplankton was smaller than that of protozoa, but their rates of production and ingestion were high. Detritus had a high sinking rate and were largely consumed by metazoan plankton. The carbon budget of the planktonic system shows that 70% of phytoplankton total net production (PTNP: particulate and dissolved) is lost through heterotrophic respiration. This moderate value, for oligotrophic warm waters, is due to the low activity of bacteria. The remaining 30% of the PTNP is exported from the planktonic system through food-web transfers and sinking of detritus (20 and 10% of PTNP, respectively). It follows that the lagoonal planktonic food web, despite the small size of primary producers, is quite efficient at exporting biogenic carbon. Protozoan grazers play a key role in carbon export, as they exert a strong grazing pressure on phytoplankton, and are themselves largely consumed by metazoan zooplankton.

Phytoplankton carbon fluxes were studied in the Northeast Water (NEW) Polynya, off the eastern coast of Greenland (79degrees to 81degreesN, 6degrees to 17degreesW), during summer 1993. The downward flux of organic particles was determined during 54 days using a sediment trap moored at a fixed location, below the pycnocline (130 in). The hypothesis of the present study is that wind events were ultimately responsible for the events of diatoms downward flux recorded in the trap. Wind conditions can influence the vertical transport of phytoplankton by affecting (1) the environmental conditions (e.g. hydrostatic pressure, nutrient concentrations, and irradiance) encountered by phytoplankton during their vertical excursion, and (2) the aggregation and disaggregation of phytoplankton flocs, The first mechanism affects the physiological regulation of buoyancy, whereas the second one affects the size and shape of settling particles. Using field data (wind velocity, density profiles and phytoplankton abundance), we assessed the potential aggregation and the vertical excursion of phytoplankton in surface waters. The results show that, upstream from the trap. wind and hydrodynamic conditions were sometimes favourable to the downward export of phytoplankton. Lag-correlation between time series of wind and phytoplankton downward flux shows that flux events lagged wind events by ca. 16 days. Given that the average current velocity in the top 100 in was ca. 10 cm s(-1), a lag of 16 days corresponded to a lateral transport of ca. 130 km, upstream from the sediment trap. where phytoplankton production was lower than at the location of the trap. According to that scenario, 21% to 60% of primary production was exported to depth during wind events. If we had assumed instead a tight spatial coupling between the material collected in the trap and the relatively high phytoplankton production at the location of the trap, we would have concluded that < 7% of primary production was exported to depth. The difference between the two scenarios has great implications for the fate of phytoplankton. Our results stress the importance of investigating the spatial coupling between surface and trap data before assessing the pathways of phytoplankton carbon cycling. (C) 2002 Elsevier Science B.V. All rights reserved.

Data concerning the planktonic food web and the farmed pearl oysters of the lagoon of Takapoto Atoll were assembled into a steady state model of carbon flows. A method of optimisation, using constraints from the literature, called 'inverse analysis' was chosen as the numerical tool for estimating the missing flow values. The resulting food web is characterised by: 1) high primary production, achieved by low phytoplankton biomass, 2) high production of non-living matter, especially as dissolved organic carbon, 3) low bacterial production, 4) zooplankton dominated by protozoa (biomass and processes) and mesozooplankton (processes), and 5) very low consumption of plankton by farmed bivalves compared to planktonic fluxes. When considering the whole lagoon, the farmed oysters (Pinctada margaritifera) and associated bivalves (Pinctada maculata) consume 0.24% of the planktonic gross primary production. In addition, the consumption by natural populations of the main benthic bivalves in this lagoon (Chama iostoma, Arca ventricosa, Pinctada margaritifera and Pinctada maculata) is also low compared to the high planktonic primary production (4.1%). The oyster farming in this lagoon is thus very far from being food-limited. © 2001 Ifremer/CNRS/Inra/IRD/Cemagref/Editions scientifiques et médicales Elsevier SAS.

Background: The present review is based on the identification of four major environmental crises that have been approached from a biological oceanographic viewpoint. These crises are the release of contaminants in nearshore marine waters, the collapse of marine resources that were renewable until recently, the loss of biodiversity, and global climate change Methods: The review examines the contribution of cytometry-based biological oceanography to the resolution of the four environmental crises. Using a database of 302 papers, flow cytometric (FCM) studies in biological oceanography over the 1989-1999 decade are examined. Future biological oceanographic applications of FCM are discussed. Results: Most of the published FCM oceanographic studies focus on phytoplankton and bacterioplankton. Analysis of our 1983-1999 database shows the predominance of studies dedicated to phytoplankton (77%), followed by heterotrophic bacteria (21%). The latter progressively increased over the last decade, together with the improved understanding of the biogeochemical and trophic roles of marine bacteria. Most studies on these two microorganisms were conducted in vitro until 1996, after which the trend reversed in favor of in situ research. The most investigated areas were these with major international sampling efforts, related to the changing climate. Concerning environmental topics, 62% of papers on phytoplankton and bacterioplankton focused on the structure of microbial communities and fluxes (e.g., production grazing); this provides the basis for biological oceanographic studies on resources and climate change. Conclusions: Future progress in the biological oceanographic use of FCM will likely fall into two categories, i.e., applications where FCM will be combined with the development of other methods and those where FCM will be the main analytical tool. It is expected that FCM and other cytometric approaches will improve the ability of biological oceanograhy to address the major environmental challenges that are confronting human societies. (C) 2001 Wiley Liss, Inc.

The characteristics of the phytoplankton of Takapoto Atoll are reviewed from the studies conducted between 1974 and 1998. These studies mainly concerned the biomass and primary production of phytoplankton while the taxonomic composition received far less attention. The mean biomass is 0.2-0.3 mug chi a.L-1. The phytoplankton is homogeneously distributed on a year scale although an higher biomass (0.8 mug chi a.L-1) may temporarily exist in the south part of the atoll under moderate tradewinds or calm weather. The gross primary production reached 0.8 g C.m(-2).day(-1) whereas the net primary production is estimated to be 0.7 g C.m(-2).day(-1). No significant long-term changes of the biomass or primary production can be observed. The implications of this stability are discussed in the context of the mother-of-pearl mariculture. Size fractionated samples revealed the predominance of picophytoplankton which represented more than 60% of the phytoplankton biomass and achieved > 50% of the primary production. The taxonomic composition observed in 1974 showed the predominance of three algal groups: diatoms, dinoflagellates and coccolithophorids. The diatoms were the most diversified group, while the dinoflagellates were the most abundant. No further examination of the phytoplankton was undertaken until 1996. At that time, the microplankton was quite absent, and the phytoplankton communities were dominated by the pico- and nanophytoplankton, mainly chlorophytes, prymnesiophytes and dinoflagellates. This drastic shift of the phytoplankton communities towards smaller size is not clearly understood. It emphasises the need of taxonomic studies for a better understanding of the lagoon ecology. (C) 2001 Ifremer/CNRS/INRA/IRD/Cemagref/Editions scientifiques et medicales Elsevier SAS.

Despite the midnight sun, herbivore copepods Calanus hyperboreus, C. glacialis and Pseudocalanus acuspes displayed a normal diel vertical migration (NDVM) under the ice cover of Barrow Strait in spring, ascending into the chlorophyll-rich under-ice surface layer around maximum relative rate of change in irradiance (DeltaI/I) at dusk but returning to depth a few hours later, well in advance of the dawn maximum DeltaI/I. Nauplii prey being abundant above 50 m, the upward night-time incursions of the omnivore Metridia longa seldom reached beyond <25 m. In the absence of UV-B radiation or a temperature gradient, migration out of the euphotic layer was interpreted as a reaction to visual predators (e.g. Arctic cod Boreogadus saida and the hyperiid amphipod Themisto libellula). Swarms of T. libellula actively preying on copepods accumulated at the ice-water interface at dusk. Low vulnerability to visual predators and a more uniform vertical distribution of their food explained the limited DVM of the small omnivores Microcalanus pygmaeus, Oithona similis and Oncaea borealis. Once the feeding migrations developed, the daytime depth of the centre of mass of the distribution of a copepod was correlated to its size (r(2) = 0.63). Our observations suggest that, under Arctic sea ice, interspecific differences in the pattern and extent of copepod DVM can be related to the vertical distribution of potential food and to vulnerability to visual predators.

Food-web processes are important controls of oceanic biogenic carbon flux and ocean-atmosphere carbon dioxide exchange. Two key controlling parameters are the growth efficiencies of the principal trophic components and the rate of carbon remineralization. We report that bacterial growth efficiency is an inverse function of temperature. This relationship permits bacterial respiration in the euphotic zone to be computed from temperature and bacterial production. Using the temperature-growth efficiency relationship, we show that bacterial respiration generally accounts for most community respiration. This implies that a Larger fraction of assimilated carbon is respired at Low than at high latitudes, so a greater proportion of production can be exported in polar than in tropical regions, Because bacterial production is also a function of temperature, it should be possible to compute euphotic zone heterotrophic respiration at Large scales using remotely sensed information.

A new approach is described to identify the dominant process (physical versus biological) in a pelagic marine ecosystem, from simple biological oceanographic field variables. The approach is based on quantification of the matching (M) between phytoplankton production (P) and losses, from held estimates of chlorophyll a (Chl) and P. Coefficient Ail is estimated for a wide range of oceanic and coastal environments and of trophic characteristics, using data from the literature. Results show that M characterizes the dominance of physical versus biological processes in pelagic systems. The coefficient may be especially useful as a means for extracting process information on pelagic marine ecosystems from large data sets of Chi and P, e.g. recorded by moored instruments or provided by satellite images of ocean colour.

A steady-state model of the planktonic food web of the lagoon of Takapoto Atoll (Tuamotu Archipelago, French Polynesia) was developed to assess the characteristics of that ecosystem. The planktonic food web was divided into 7 compartments, whose carbon biomasses and rates of exchange were determined from field data combined with inverse analysis, the latter being used to estimate missing rates. Results indicate that the system was characterized by high phytoplankton production and, in agreement with previous results, low bacterial production. Due to their high metabolism, metazoan zooplankton played a dominant role in the cycling of carbon. In contrast, the contribution of protozoa was small. The non-living particulate organic carbon compartment also played a key role, qualitatively and quantitatively, because detritus was directly consumed by all heterotrophic compartments.

A global assessment of carbon flux in the world ocean is one of the major undertakings of the Joint Global Ocean Flux Study (JGOFS). This has to be undertaken using historical in situ data of primary productivity. As required by the temporal and spatial scales involved in a global study, it can be conveniently done by combining, through appropriate models, remotely sensed information (chlorophyll a, temperature) with basic information about the parameters related to the carbon uptake by phytoplanktonic algae. This requires a better understanding as well as a more extended knowledge of these parameters which govern the radiative energy absorption and utilization by algae in photosynthesis. The measurement of the photosynthetic response of algae [the photosynthesis (P) versus irradiance (E) curves], besides being less shiptime consuming than in situ primary production experiments, allows the needed parameters to be derived and systematically studied as a function of the physical, chemical and ecological conditions. The aim of the present paper is to review the sig nificance of these parameters, especially in view of their introduction into models, to analyze the causes of their variations in the light of physiological considerations, and finally to provide methodological recommendations for meaningful determinations, and interpretation, of the data resulting from P versus E determinations. Of main concern are the available and usable irradiance, the chlorophyll a-specific absorption capabilities of the algae, the maximum light utilization coefficient (alpha), the maximum quantum yield (phi(m)), the maximum photosynthetic rate (P-m) and the light saturation index (E-k) The potential of other, non-intrusive, approaches, such as the stimulated variable fluorescence, or the sun-induced natural fluorescence techniques is also examined.

This paper describes an approach to determine, using a small number of food-web or hydrodynamic variables, the partitioning of phytoplankton production among 3 carbon fluxes, i.e, remineralization within the euphotic zone, food-web transfer, and sinking to depth of organic particles. In order to do so, the flows of biogenic carbon in the marine pelagic environment are reduced to 5 broad pathways, which are ordered along a continuum of decreasing export relative to primary production. At one end of the continuum, ungrazed phytoplankton is exported to depth and, at the other, biogenic carbon is remineralized within the bacteria-microzooplankton loop. In the present paper, export of biogenic carbon from the euphotic zone includes food-web transfer of primary production and downward flux of particulate organic carbon (POC) into deep waters. A simple model and data from the literature lead to the conclusions that: (1) the export characteristics of pelagic ecosystems are largely determined by the size structure of primary production and the matching (or, conversely, segregation) between primary production and grazing and (2) total export from the euphotic zone is a function of the delivery of mechanical energy to the upper water column whereas the partitioning of total export between food-web transfer and sinking of POC is controlled by temporal variations in depth of the surface mixed layer. Food-web transfer is significant for marine resources and sinking of POC may contribute to the regulation of climate change (sequestration at depth of biogenic carbon).

Biological oceanographers generally distinguish between two contrasting trophic pathways in the pelagic environment, i.e. the herbivorous and the microbial food webs. The former goes from large phytoplankton and zooplankton to fish, whereas the latter comprises small eukaryotic algae and cyanobacteria as well as heterotrophic bacteria and protozoa. The present paper describes a continuum of trophic pathways, between systems dominated by the herbivorous food web and those dominated by the microbial loop (i.e. almost closed system of heterotrophic bacteria and zooflagellate grazers, the latter releasing dissolved organic matter used as substrate by the bacteria). It is proposed that the continuum goes from the herbivorous web (or chain) to a `'multivorous food web'', to the microbial web, and finally the microbial loop. Characteristics of the various pathways may be summarized as a series of interconnected ratios. It is hypothesized that systems dominated by the herbivorous food web or the microbial loop are of transient nature and thus inherently unstable, whereas the multivorous and microbial food webs have higher stability and are thus longer lasting. This view is supported by a review of properties of several systems, that include herbivorous webs of the spring phytoplankton bloom and in upwelling areas, the multivorous web in the `'high nutrient low chlorophyll'' region of the North Pacific Ocean, a microbial web at a retreating ice edge off Antarctica, and the microbial loop in oligotrophic waters where the biomass of bacteria significantly exceeds that of phytoplankton.