This paper discusses the process of the formation of solitary waves when a column of the granular material drops into the water. The article combines the controlled experiments and numerical modeling to gain an insight into major physical processes involved. A rectangular glass flume is used in the laboratory. The experimental setup incorporates a release mechanism that allows precise adjustment of grain size, water depth, and initial column height. The development of the free surface is followed in fine detail, frame by frame, by means of high-speed shadowgraphy. Results contain distinct trends. The depth of water, size of the grain, and topography of the bed are especially critical in determining the amplitude and shape of waves, particularly when a step is involved. The effect of the grains on the water causes the transfer of momentum which pushes one steep wave. Its conduct is the same as that of a single wave (i.e. stable, self-sustaining) over time. These observations were supplemented by numerical simulations with the simulating waves till shore (SWASH) model. The model replicates the primary characteristics observed during the experiments: strength, velocity of propagation, and spreading of the wave. Simulated data agree with measured data, which substantiates the fact that the model is accurate in describing nonlinear shallow-water dynamics. Use of statistical descriptors, including skewness and kurtosis, also describe the change in the asymmetry and sharpness of the wave as it travels. The combination of these methods provides a coherent description of wave generation through the collapsing of the granular. They demonstrate that a localized mass movement can cause solitary-like waves of nonlinear nature. In addition to physical understanding, this work provides a solid structure through which the generation of a tsunami as a result of landslides can be studied as a phenomenon that is not easily forecast yet its calculation is valuable when measuring the hazards before the coastal area.

The annual MOOSE-GE cruise series is the backbone of the MOOSE regional ocean observing system providing a unique dataset to observe and understand large-scale physical, biogeochemical and biological processes in the northwestern Mediterranean basin, a key region that is responding to climate change faster than many other parts of the world. These cruises address major scientific challenges, such as monitoring the variability and impact of deep and intermediate convection, which plays a crucial role in deep-water ventilation, coastal-open ocean exchanges, carbon sequestration and the evolution of phytoplankton production in this highly dynamic system. They also allow the assessment of climate change effects on ocean physics, marine biodiversity, biological resources, and seawater chemistry, including oxygen, nutrients, and dissolved carbon. Sustained observations are required to track rapid trends such as increasing temperature and salinity in intermediate and deep waters, declining oxygen concentrations, nutrients and carbonate system inter-annual variabilities, expected increased stratification of the water column and variations in heat and salt contents. These long-term datasets are indispensable both for climate model validation and process studies, as well as for assessing the environmental status of the Mediterranean Sea.

The objective of the TONGA oceanographic expedition was to study the control of productivity and carbon sequestration by micronutrients of shallow hydrothermal origin in the Western Tropical South Pacific (WTSP) Ocean. The 37-day oceanographic survey took place on board the R/V L’Atalante in 2019 between Oct. 31 to Dec. 6 (Nouméa-Nouméa). Over a large area of the WTSP the team acquired numerous results on both the entire water column (up to the sediment) and the atmosphere. Specific task are represented on figure 1: (task 1) to characterize chemically and optically shallow hydrothermal fluids and to compare the source from below (shallow hydrothermal fluids) with the source from above (atmospheric deposition); (task 2) to quantify the dynamical dispersion of the fluids at small and regional scale; (task 3) to investigate the impact of the shallow hydrothermal sources on the biological activity and diversity, and the feedback to the atmosphere via the oceanic emissions of primary and secondary aerosols. (Task 4) to communicate about the campaign (see for example our Tweeter account (https://twitter.com/tongaproject) and the movie (26’) both in French (https://www.youtube.com/watch?v=e5kAd0i6Dck) and English (https://www.youtube.com/watch?v=UeABf-cVR-k). A long west to east (up to the blue waters of the gyre) transect allowed to characterize the different biogeochemical provinces crossed and a focus in the region of the Lau Basin allowed to investigate the impact of shallow hydrothermal sources. A series of short and long stations allowed to fully characterize the stocks and the fluxes in the different provinces. Short-term (up to 10 days) processes studies have been conducted (drifting moorings and minicosms experiments). Part of these results will feed into important modeling work. A fixed mooring line launched at the end of the campaign and recovered in Nov. 2020 as well as the 7 ARGO floats and 20 drifting buoys that were dropped during the campaign provide a broader temporal context of the acquisitions done during the campaign. An important focus of the campaign was the trace metal characterization of the entire water column. For this, TONGA has been labeled by the international program GEOTRACES (https://www.geotraces.org/). The impact on biological communities of fluids is supported by the international IMBER program (https://imber.info/). The TONGA project is also part of the LEFE program (funding by LEFE-CYBER and LEFE-GMMC), the ANR (Appel à projets génériques) and the Fondation A-MIDeX of the Aix-Marseille Université.

Abstract Iron (Fe) is an essential micronutrient for diazotrophs, which are abundant in the Western Tropical South Pacific Ocean (WTSP). Their success depends on the numerous trace metals, particularly Fe, released from shallow hydrothermal vents along the Tonga Arc. This study aimed to explore the spatio‐temporal impact of hydrothermal fluids on particulate trace metal concentrations and biological activity. To identify the composition of sinking particles across a wide area of the WTSP, we deployed sediment traps at various depths, both close and further west of the Tonga Arc. Seafloor sediments were cored at these deployment sites, including at a remote location in the South Pacific Gyre. The sinking particles were composed of a large amount of biological material (up to 88 mg d −1 ), indicative of the high productivity of the region. A significant portion of this material (∼21 ± 12 wt.%) was lithogenic of hydrothermal origin, as revealed through Al‐Fe‐Mn tracing. The sinking material showed similar patterns between lithogenic and biogenic fractions, indicating that hydrothermal input within the photic layer triggered surface production. A hydrothermal fingerprint was suggested in the sediments due to the high sedimentation rates (>47 cm kyr −1 ) and the presence of large, heterogeneous, metal‐rich particles. The presence of nearby active deep hydrothermal sources was suspected near the Lau Ridge due to the large particle size (1–976 μm) and the significant excess of Fe and Mn (2–20 wt.%). Overall, this study revealed that hydrothermal sources have a significant influence on the biogeochemical signature of particles in the region.

Iron is an essential nutrient that regulates productivity in ~30% of the ocean. Compared with deep (>2000 meter) hydrothermal activity at mid-ocean ridges that provide iron to the ocean’s interior, shallow (<500 meter) hydrothermal fluids are likely to influence the surface’s ecosystem. However, their effect is unknown. In this work, we show that fluids emitted along the Tonga volcanic arc (South Pacific) have a substantial impact on iron concentrations in the photic layer through vertical diffusion. This enrichment stimulates biological activity, resulting in an extensive patch of chlorophyll (360,000 square kilometers). Diazotroph activity is two to eight times higher and carbon export fluxes are two to three times higher in iron-enriched waters than in adjacent unfertilized waters. Such findings reveal a previously undescribed mechanism of natural iron fertilization in the ocean that fuels regional hotspot sinks for atmospheric CO 2 .

In the Western Tropical South Pacific (WTSP) Ocean, a hotspot of dinitrogen fixation has been identified. The survival of diazotrophs depends, among others, on the availability of dissolved iron (DFe) largely originating, as recently revealed, from shallow hydrothermal sources located along the Tonga-Kermadec arc that fertilize the Lau Basin with this element. On the opposite, these fluids, released directly close to the photic layer, can introduce numerous trace metals at concentrations that can be toxic to surface communities. Here, we performed an innovative 9-day experiment in 300 L reactors onboard the TONGA expedition, to examine the effects of hydrothermal fluids on natural plankton communities in the WTSP Ocean. Different volumes of fluids were mixed with non-hydrothermally influenced surface waters (mixing ratio from 0 to 14.5%) and the response of the communities was studied by monitoring numerous stocks and fluxes (phytoplankton biomass, community composition, net community production, N2 fixation, thiol production, organic carbon and metal concentrations in exported material). Despite an initial toxic effect of hydrothermal fluids on phytoplankton communities, these inputs led to higher net community production and N2 fixation rates, as well as elevated export of organic matter relative to control. This fertilizing effect was achieved through detoxification of the environment, rich in potentially toxic elements (e.g., Cu, Cd, Hg), likely by resistant Synechococcus ecotypes able to produce strong binding ligands, especially thiols (thioacetamide-like and glutathione-like compounds). The striking increase of thiols quickly after fluid addition likely detoxified the environment, rendering it more favorable for phytoplankton growth. Indeed, phytoplankton groups stressed by the addition of fluids were then able to recover important growth rates, probably favored by the supply of numerous fertilizing trace metals (notably Fe) from hydrothermal fluids and new nitrogen provided by N2 fixation. These experimental results are in good agreement with in-situ observations, proving the causal link between the supply of hydrothermal fluids emitted at shallow depth into the surface layer and the intense biological productivity largely supported by diazotrophs in the WTSP Ocean. This study highlights the importance of considering shallow hydrothermal systems for a better understanding of the biological carbon pump.

Mass fluxes and trace metal (TM) fluxes were measured from samples collected in 2003 to 2005 from sediment traps deployed at 1000 m depth at the DYFAMED (DYnamique des Flux Atmosphériques en MEDiterranée) time-series station (central Ligurian Sea, 2350 m depth). A highly significant correlation is observed between all TM fluxes, whatever the nature and emission source of the TM (e.g., crustal such as Al, Fe, Co, or anthropogenic such as Zn, Cd, Pb) and the mass flux. Because these TMs originate from different emission sources, and, therefore, their atmospheric deposition to the sea surface varies with different seasonal patterns, it is suggested that fluxes of particulate organic carbon determine fluxes of TMs, and not the contrary. The seasonal sequence of the transfer of TMs to sea floor (winter convection, spring bloom and nutrient depletion of surface waters in summer and autumn) is briefly examined to highlight the concomitant temporal variability of mass and TM fluxes. This suggests that the TM downward transfer is totally controlled by the seasonal variability of biogenic carbon production, itself depending upon the intensity of winter convection. This may be a peculiarity of marine regions such as the Ligurian Sea, where hydrodynamical features (and, therefore, spring blooms) are strongly constrained by climatic and meteorological conditions (winter temperature, wind events, rain events).

Abstract. Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built on laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near-surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminum (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. The Fe/Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al/Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (∼ 100–1000 m), while dFe input from dust was only transient in the surface mixed layer. The rapid transfer of dust to depth, the Fe-binding ligand pool in excess to dFe in subsurface (while nearly saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (< 0.5 nmol kg−1) cause us to question the residence time for this dust-derived subsurface reservoir and hence its role as a supply mechanism for the surface ocean, stressing the need for further studies. Finally, these contrasting responses indicate that dAl is a poor tracer of dFe input in the Mediterranean Sea.

Lithogenic elements such as aluminum (Al), iron (Fe), rare earth elements (REEs), thorium (232Th and 230Th, given as Th) and protactinium (Pa) are often assumed to be insoluble. In this study, their dissolution from Saharan dust reaching Mediterranean seawater was studied through tank experiments over 3 to 4 d under controlled conditions including controls without dust addition as well as dust seeding under present and future climate conditions (+3 °C and −0.3 pH). Unfiltered surface seawater from three oligotrophic regions (Tyrrhenian Sea, Ionian Sea and Algerian Basin) were used. The maximum dissolution was low for all seeding experiments: less than 0.3 % for Fe, 1 % for 232Th and Al, about 2 %–5 % for REEs and less than 6 % for Pa. Different behaviors were observed: dissolved Al increased until the end of the experiments, Fe did not dissolve significantly, and Th and light REEs were scavenged back on particles after a fast initial release. The constant 230Th/232Th ratio during the scavenging phase suggests that there is little or no further dissolution after the initial Th release. Quite unexpectedly, comparison of present and future conditions indicates that changes in temperature and/or pH influence the release of Th and REEs in seawater, leading to lower Th release and a higher light REE release under increased greenhouse conditions.

Abstract. Although atmospheric dust fluxes from arid as well as human-impacted areas represent a significant source of nutrients to surface waters of the Mediterranean Sea, studies focusing on the evolution of the metabolic balance of the plankton community following a dust deposition event are scarce, and none were conducted in the context of projected future levels of temperature and pH. Moreover, most of the experiments took place in coastal areas. In the framework of the PEACETIME project, three dust-addition perturbation experiments were conducted in 300 L tanks filled with surface seawater collected in the Tyrrhenian Sea (TYR), Ionian Sea (ION) and Algerian basin (FAST) on board the R/V Pourquoi Pas? in late spring 2017. For each experiment, six tanks were used to follow the evolution of chemical and biological stocks, biological activity and particle export. The impacts of a dust deposition event simulated at their surface were followed under present environmental conditions and under a realistic climate change scenario for 2100 (ca. +3 ∘C and −0.3 pH units). The tested waters were all typical of stratified oligotrophic conditions encountered in the open Mediterranean Sea at this period of the year, with low rates of primary production and a metabolic balance towards net heterotrophy. The release of nutrients after dust seeding had very contrasting impacts on the metabolism of the communities, depending on the station investigated. At TYR, the release of new nutrients was followed by a negative impact on both particulate and dissolved 14C-based production rates, while heterotrophic bacterial production strongly increased, driving the community to an even more heterotrophic state. At ION and FAST, the efficiency of organic matter export due to mineral/organic aggregation processes was lower than at TYR and likely related to a lower quantity/age of dissolved organic matter present at the time of the seeding and a smaller production of DOM following dust addition. This was also reflected by lower initial concentrations in transparent exopolymer particles (TEPs) and a lower increase in TEP concentrations following the dust addition, as compared to TYR. At ION and FAST, both the autotrophic and heterotrophic community benefited from dust addition, with a stronger relative increase in autotrophic processes observed at FAST. Our study showed that the potential positive impact of dust deposition on primary production depends on the initial composition and metabolic state of the investigated community. This impact is constrained by the quantity of nutrients added in order to sustain both the fast response of heterotrophic prokaryotes and the delayed one of primary producers. Finally, under future environmental conditions, heterotrophic metabolism was overall more impacted than primary production, with the consequence that all integrated net community production rates decreased with no detectable impact on carbon export, therefore reducing the capacity of surface waters to sequester anthropogenic CO2.

Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built onto laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminium (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes.Dissolved Al (dAl) removal was dominant over dAl released from dust. Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (~100–1000 m), while dFe input from dust was only transient in the surface mixed-layer. The rapid transfer of dust to depth (up to ~180 m d⁻¹), the Fe-binding ligand pool in excess to dFe in subsurface (while nearly-saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment.At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (< 0.5 nmol kg⁻¹) questions the residence time for this dust-derived subsurface reservoir, and hence its role as a supply mechanism for the surface ocean, stressing the need for further studies. Finally, these contrasting responses indicate that dAl is a poor tracer of dFe input in the Mediterranean Sea.

Nous présentons des méthodes de régularisation évanescente qui permettent de trouver, parmi toutes les solutions de l'équation d'équilibre, la solution du problème de complétion de données qui s'approche au mieux des données de type Cauchy ou de champs partiels. Nous utilisons ici la méthode des solutions fondamentales en tant que méthode numérique pour discrétiser l'espace des solutions de l'équation d'équilibre. Nous nous intéressons, dans un premier temps, à la résolution de problèmes de Cauchy associés à l'équation d'Helmholtz. Ensuite, nous présentons l'analyse d'un essai de flexion trois points. Les mesures de champs partielles obtenues par corrélation d'images numériques sont complétées et débruitées, ce qui met en évidence les performances de la méthode.

La généralisation de la méthode de régularisation évanescente aux problèmes de complétion de données à partir de mesures de champs partielles est appliquée à des données expérimentales afin de compléter et débruiter des champs de déplacements issus de corrélation d’images numériques (CIN) et d’identifier les conditions aux limites. La méthode inverse est implémentée en utilisant la méthode des éléments finis et la méthode des solutions fondamentales. No us présentons des résultats obtenus à partir de mesures réalisées lors d’un essai de flexion 3 points.

The fate of diazotroph (N 2 fixers) derived carbon (C) and nitrogen (N) and their contribution to vertical export of C and N in the western tropical South Pacific Ocean was studied during OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment). Our specific objective during OUTPACE was to determine whether autocatalytic programmed cell death (PCD), occurring in some diazotrophs, is an important mechanism affecting diazotroph mortality and a factor regulating the vertical flux of organic matter and, thus, the fate of the blooms. We sampled at three long duration (LD) stations of 5 days each (LDA, LDB and LDC) where drifting sediment traps were deployed at 150, 325 and 500 m depth. LDA and LDB were characterized by high chlorophyll a (Chl a) concentrations (0.2–0.6 µg L −1) and dominated by dense biomass of the filamentous cyanobac-terium Trichodesmium as well as UCYN-B and diatom– diazotroph associations (Rhizosolenia with Richelia-detected by microscopy and het-1 nifH copies). Station LDC was located at an ultra-oligotrophic area of the South Pacific gyre with extremely low Chl a concentration (∼ 0.02 µg L −1) with limited biomass of diazotrophs predominantly the unicellu-lar UCYN-B. Our measurements of biomass from LDA and LDB yielded high activities of caspase-like and metacas-pase proteases that are indicative of PCD in Trichodesmium and other phytoplankton. Metacaspase activity, reported here for the first time from oceanic populations, was highest at the surface of both LDA and LDB, where we also obtained high concentrations of transparent exopolymeric particles (TEP). TEP were negatively correlated with dissolved inorganic phosphorus and positively coupled to both the dissolved and particulate organic carbon pools. Our results reflect the increase in TEP production under nutrient stress and its role as a source of sticky carbon facilitating aggregation and rapid vertical sinking. Evidence for bloom decline was observed at both LDA and LDB. However, the physiological status and rates of decline of the blooms differed between the stations, influencing the amount of accumulated diazotrophic organic matter and mass flux observed in the traps during our experimental time frame. At LDA sediment traps contained the greatest export of particulate matter and significant numbers of both intact and decaying Trichodesmium, UCYN-B and het-1 compared to LDB where the bloom decline began only 2 days prior to leaving the station and to LDC where no evidence for bloom or bloom decline was seen. Substantiating previous findings from laboratory cultures linking PCD to carbon export in Trichodesmium, our results from OUT-Published by Copernicus Publications on behalf of the European Geosciences Union. 3894 D. Spungin et al.: Programmed cell death in diazotrophs and the fate of organic matter PACE indicate that nutrient limitation may induce PCD in high biomass blooms such as displayed by Trichodesmium or diatom–diazotroph associations. Furthermore, PCD combined with high TEP production will tend to facilitate cellular aggregation and bloom termination and will expedite vertical flux to depth.

Surface waters (0–200 m) of the western tropical South Pacific (WTSP) were sampled along a longitudinal 4000 km transect (OUTPACE cruise, DOI: 10.17600/15000900) during the austral summer (stratified) period (18 February to 3 April 2015) between the Melanesian Archipelago (MA) and the western part of the SP gyre (WGY). Two distinct areas were considered for the MA, the western MA (WMA), and the eastern MA (EMA). The main carbon (C), nitrogen (N), and phosphorus (P) pools and fluxes provide a basis for the characterization of the expected trend from oligotrophy to ultra-oligotrophy, and the building of first-order budgets at the daily and seasonal timescales (using climatology). Sea surface chlorophyll a well reflected the expected oligotrophic gradient with higher values obtained at WMA, lower values at WGY, and intermediate values at EMA. As expected, the euphotic zone depth, the deep chlorophyll maximum, and nitracline depth deepen from west to east. Nevertheless, phosphaclines and nitraclines did not match. The decoupling between phosphacline and nitracline depths in the MA allows for excess P to be locally provided in the upper water by winter mixing. We found a significant biological "soft tissue" carbon pump in the MA sustained almost exclusively by dinitrogen (N₂) fixation and essentially controlled by phosphate availability in this iron-rich environment. The MA appears to be a net sink for atmospheric CO₂, while the WGY is in quasi-steady state. We suggest that the necessary excess P, allowing the success of nitrogen fixers and subsequent carbon production and export, is mainly brought to the upper surface by local deep winter convection at an annual timescale rather than by surface circulation. While the origin of the decoupling between phosphacline and nitracline remains uncertain, the direct link between local P upper water enrichment, N₂ fixation, and organic carbon production and export, offers a possible shorter timescale than previously thought between N input by N₂ fixation and carbon export. The low iron availability in the SP gyre and P availability in the MA during the stratified period may appear as the ultimate control of N input by N₂ fixation. Because of the huge volume of water to consider, and because the SP Ocean is the place of intense denitrification in the east (N sink) and N₂ fixation in the west (N source), precise seasonal C, N, P, and iron (Fe) budgets would be of prime interest to understand the efficiency, at the present time and in the future, of the oceanic biological carbon pump.

Constraining the rates and spatial distribution of dinitrogen (N 2) fixation fluxes to the ocean informs our understanding of the environmental sensitivities of N 2 fixa-tion as well as the timescale over which the fluxes of nitrogen (N) to and from the ocean may respond to each other. Here we quantify rates of N 2 fixation as well as its contribution to export production along a zonal transect in the western tropical South Pacific (WTSP) Ocean using N isotope (" δ 15 N ") budgets. Comparing measurements of water column nitrate + nitrite δ 15 N with the δ 15 N of sinking particulate N at a western, central, and eastern station, these δ 15 N budgets indicate high, modest, and low rates of N 2 fixation at the respective stations. The results also imply that N 2 fixation supports exceptionally high, i.e. ≥ 50 %, of export production at the western and central stations, which are also prox-imal to the largest iron sources. These geochemically based rates of N 2 fixation are equal to or greater than those previously reported in the tropical North Atlantic, indicating that the WTSP Ocean has the capacity to support globally significant rates of N 2 fixation, which may compensate for N removal in the oxygen-deficient zones of the eastern tropical Pacific.

The fate of the organic matter (OM) produced by marine life controls the major biogeochemical cycles of the Earth's system. The OM produced through photosynthe-sis is either preserved, exported towards sediments or degraded through remineralisation in the water column. The productive eastern boundary upwelling systems (EBUSs) associated with oxygen minimum zones (OMZs) would be expected to foster OM preservation due to low O 2 conditions. But their intense and diverse microbial activity should enhance OM degradation. To investigate this contradiction, sediment traps were deployed near the oxycline and in the OMZ core on an instrumented moored line off Peru. Data provided high-temporal-resolution O 2 series charac-terising two seasonal steady states at the upper trap: sub-oxic ([O 2 ] < 25 µmol kg −1) and hypoxic-oxic (15 < [O 2 ] < 160 µmol kg −1) in austral summer and winter-spring, respectively. The OMZ vertical transfer efficiency of particulate organic carbon (POC) between traps (T eff) can be classi-Published by Copernicus Publications on behalf of the European Geosciences Union. 5094 M. Bretagnon et al.: Modulation of the vertical particle transfer efficiency fied into three main ranges (high, intermediate, low). These different T eff ranges suggest that both predominant preservation (high T eff > 50 %) and remineralisation (intermediate T eff 20 < 50 % or low T eff < 6 %) configurations can occur. An efficient OMZ vertical transfer (T eff > 50 %) has been reported in summer and winter associated with extreme limitation in O 2 concentrations or OM quantity for OM degradation. However, higher levels of O 2 or OM, or less refractory OM, at the oxycline, even in a co-limitation context, can decrease the OMZ transfer efficiency to below 50 %. This is especially true in summer during intraseasonal wind-driven oxygenation events. In late winter and early spring, high oxy-genation conditions together with high fluxes of sinking particles trigger a shutdown of the OMZ transfer (T eff < 6 %). Transfer efficiency of chemical elements composing the majority of the flux (nitrogen, phosphorus, silica, calcium car-bonate) follows the same trend as for carbon, with the lowest transfer level being in late winter and early spring. Regarding particulate isotopes, vertical transfer of δ 15 N suggests a complex pattern of 15 N impoverishment or enrichment according to T eff modulation. This sensitivity of OM to O 2 fluctuations and particle concentration calls for further investigation into OM and O 2-driven remineralisation processes. This should include consideration of the intermittent behaviour of OMZ towards OM demonstrated in past studies and climate projections .

We performed nitrogen (N) budgets in the photic layer of three contrasting stations representing different trophic conditions in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (FebruaryMarch 2015). Using a Lagrangian strategy, we sampled the same water mass for the entire duration of each long-duration (5 days) station, allowing us to consider only vertical exchanges for the budgets. We quantified all major vertical N fluxes both entering (N-2 fixation, nitrate turbulent diffusion, atmospheric deposition) and leaving the photic layer (particulate N export). The three stations were characterized by a strong nitracline and contrasted deep chlorophyll maximum depths, which were lower in the oligotrophic Melanesian archipelago (MA, stations LD A and LD B) than in the ultra-oligotrophic waters of the South Pacific Gyre (SPG, station LD C). N-2 fixation rates were extremely high at both LD A (593 +/- 51 mu mol N m(-2) d(-1)) and LD B (706 +/- 302 mu mol N m(-2)d(-1)), and the diazotroph community was dominated by Trichodesmium. N-2 fixation rates were lower (59 +/- 16 mu mol N m(-2) d(-1)) at LD C, and the diazotroph community was dominated by unicellular N-2-fixing cyanobacteria (UCYN). At all stations, N-2 fixation was the major source of new N (> 90 %) before atmospheric deposition and upward nitrate fluxes induced by turbulence. N-2 fixation contributed circa 1318 % of primary production in the MA region and 3 in the SPG water and sustained nearly all new primary production at all stations. The e ratio (e ratio articulate carbon export / primary production) was maximum at LD A (9.7 ) and was higher than the e ratio in most studied oligotrophic regions (< 5), indicating a high efficiency of the WTSP to export carbon relative to primary production. The direct export of diazotrophs assessed by qPCR of the nifH gene in sediment traps represented up to 30.6 of the PC export at LD A, while their contribution was 5 and < 0.1 % at LD B and LD C, respectively. At the three studied stations, the sum of all N input to the photic layer exceeded the N output through organic matter export. This disequilibrium leading to N accumulation in the upper layer appears as a characteristic of the WTSP during the summer season.

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

Nous nous intéressons à un problème d’identification de conditions aux limites à partir de mesures de champs partielles. Le problème inverse est résolu en utilisant une méthode du type régularisation évanescente. Elle consiste à rechercher parmi les solutions de l’opérateur de Lamé celle qui s’approche au mieux des mesures de champs partielles. Les performances et la robustesse de la méthode sont mises en évidence en utilisant d’une part des cas tests numériques et d’autre part deux essais de compression où les mesures de champs partielles sont obtenues par corrélation d’images numériques.

Les limites de l'instrumentation expérimentale actuelle engendrent plusieurs difficultés comme par exemple le fait que les mesures, sont éventuellement entachées d'un bruit, ne se font que sur une partie de l'échantillon ou que les conditions aux limites sont mal connues. Ces difficultés donnent naissance à des problèmes d'identification qui peuvent être considérés comme des problèmes inverses. Une méthode inverse (méthode de régularisation évanescente) a été introduite précédemment pour résoudre des problèmes inverses d'identification de conditions aux limites inaccessibles à la mesure à partir de mesures surabondantes disponibles sur une partie de la frontière (problème de Cauchy associé à l'équation de Laplace [1][2] ou à l'équation de Lamé [3]). La méthode de régularisation évanescente repose sur l'idée de chercher parmi toutes les solutions d'équilibre celle qui s'approche au mieux des conditions aux limites disponibles sur une partie de la frontière. La résolution du problème inverse est ramenée à une suite de problèmes d'optimisation sous contraintes (algorithme de point fixe) faisant intervenir plusieurs termes. Le premier terme des fonctionnelles est un terme de relaxation qui représente l'écart entre les données et la solution optimale calculée. Le second terme agit sur tout le domaine et exprime la distance entre deux solutions optimales. Ce terme de régularisation tend vers zéro au fur et à mesure des itérations. La solution ainsi calculée ne dépend pas d'un coefficient de régularisation, vérifie l'équation d'équilibre et est stable vis à vis du bruit sur les données puisque celles-ci sont recalculées afin d'être compatibles. La méthode est, en particulier, capable de débruiter les données, est généralisable à tout opérateur elliptique et peut être implémentée par différentes méthodes numériques (méthode des éléments finis, méthode des éléments de frontière, méthode des solutions fondamentales,...). Nous présentons ici la généralisation de la méthode de régularisation évanescente aux problèmes d'identification à partir de mesures de champs partielles où le champ est uniquement mesurable sur une zone centrale du domaine étudié. Avec notre technique, nous reconstruisons le champ solution de l'équation d'équilibre sur l'ensemble du domaine et les conditions aux limites qui étaient inaccessibles à la mesure. La technique fait naturellement apparaître, dans la zone centrale où les mesures sont disponibles, un résidu comprenant le bruit de mesures et les éventuels écarts à l'équilibre. La méthode est implémentée, en thermique et en élasticité linéaire, en utilisant la méthode des éléments finis. Elle est validée avec des données synthétiques mais est aussi appliquée à des situations expérimentales en utilisant des mesures de température ou en utilisant des champs de déplacements obtenus par corrélations d'images. L'exploitation de ces mesures permet de mettre en évidence les performances et la robustesse, vis à vis de données bruitées, de la méthode. Références[1] A. Cimetière, F. Delvare, and F. Pons. Une méthode inverse à régularisation évanescente. Comptes Rendus de l'Académie des Sciences-Series IIB-Mechanics, 328(9) :639?644, 2000. [2] A. Cimetière, F. Delvare, M. Jaoua, and F. Pons. Solution of the Cauchy problem using iterated Tikhonov regularization. Inverse Problems, 17(3) :553?570, 2001. [3] F. Delvare, A. Cimetière, J.-L. Hanus, and P. Bailly. An iterative method for the Cauchy problem in linear elasticity with fading regularization effect. Computer Methods in Applied Mechanics and Engineering, 199(49) 3336?3344, 2010.

In a coastal lagoon with a shallow, 25 m water column off the southwest coast of New Caledonia, large-volume (similar to 50 m(3)) mesocosm experiments were undertaken to track the fate of newly fixed nitrogen (N). The mesocosms were intentionally fertilized with 0.8 mu M dissolved inorganic phosphorus to stimulate diazotrophy. N isotopic evidence indicates that the dominant source of N fueling export production shifted from subsurface nitrate (NO3-) assimilated prior to the start of the 23-day experiments to N-2 fixation by the end of the experiments. While the delta N-15 of the sinking particulate N (PNsink) flux changed during the experiments, the delta N-15 of the suspended PN (PNsusp) and dissolved organic N (DON) pools did not. This is consistent with previous observations that the delta N-15 of surface ocean N pools is less responsive than that of PNsink to changes in the dominant source of new N to surface waters. In spite of the absence of detectable NO3- in the mesocosms, the delta N-15 of PNsink indicated that NO3- continued to fuel a significant fraction of export production (20 to 60 %) throughout the 23-day experiments, with N-2 fixation dominating export after about 2 weeks. The low rates of organic N export during the first 14 days were largely supported by NO3-, and phytoplankton abundance data suggest that sinking material primarily comprised large diatoms. Concurrent molecular and taxonomic studies indicate that the diazotroph community was dominated by diatom-diazotroph assemblages (DDAs) at this time. However, these DDAs represented a minor fraction (<5 %) of the total diatom community and contributed very little new N via N-2 fixation; they were thus not important for driving export production, either directly or indirectly. The unicellular cyanobacterial diazotroph, a Cyanothece-like UCYN-C, proliferated during the last phase of the experiments when N-2 fixation, primary production, and the flux of PNsink increased significantly, and delta N-15 budgets reflected a predominantly diazotrophic source of N fueling export. At this time, the export flux itself was likely dominated by the non-diazotrophic diatom, Cylindrotheca closterium, along with lesser contributions from other eukaryotic phytoplankton and aggregated UCYN-C cells, as well as fecal pellets from zooplankton. Despite comprising a small fraction of the total biomass, UCYN-C was largely responsible for driving export production during the last similar to 10 days of the experiments both directly (similar to 5 to 22% of PNsink) and through the rapid transfer of its newly fixed N to other phytoplankton; we infer that this newly fixed N was transferred rapidly through the dissolved N (including DON) and PNsusp pools. This inference reconciles previous observations of invariant oligotrophic surface ocean DON concentrations and delta N-15 with incubation studies showing that diazotrophs can release a significant fraction of their newly fixed N as some form of DON.

A sediment trap moored in the naturally iron-fertilized Kerguelen Plateau in the Southern Ocean provided an annual record of particulate organic carbon and nitrogen fluxes at 289 m. At the trap deployment depth, current speeds were typically low (~ 10 cm s−1) and primarily tidal-driven (M2 tidal component). Although advection was weak, the sediment trap may have been subject to hydrodynamical and biological (swimmer feeding on trap funnel) biases. Particulate organic carbon (POC) flux was generally low (< 0.5 mmol m−2 d−1), although two episodic export events (< 14 days) of 1.5 mmol m−2 d−1 were recorded. These increases in flux occurred with a 1-month time lag from peaks in surface chlorophyll and together accounted for approximately 40% of the annual flux budget. The annual POC flux of 98.2 ± 4.4 mmol m−2 yr−1 was low considering the shallow deployment depth but comparable to independent estimates made at similar depths (~ 300 m) over the plateau, and to deep-ocean (> 2 km) fluxes measured from similarly productive iron-fertilized blooms. Although undertrapping cannot be excluded in shallow moored sediment trap deployment, we hypothesize that grazing pressure, including mesozooplankton and mesopelagic fishes, may be responsible for the low POC flux beneath the base of the winter mixed layer. The importance of plankton community structure in controlling the temporal variability of export fluxes is addressed in a companion paper.

In the oligotrophic ocean characterized by nitrate (NO − 3) depletion in surface waters, dinitrogen (N 2) fixation and dissolved organic nitrogen (DON) can represent significant nitrogen (N) sources for the ecosystem. In this study, we deployed large in situ mesocosms in New Caledonia in order to investigate (1) the contribution of N 2 fixation and DON use to primary production (PP) and particle export and (2) the fate of the freshly produced particulate organic N (PON), i.e., whether it is preferentially accumulated and recycled in the water column or exported out of the system. The mesocosms were fertilized with phosphate (PO 3− 4) in order to prevent phosphorus (P) limitation and promote N 2 fixation. The diazotrophic community was dominated by diatom–diazotroph associations (DDAs) during the first part of the experiment for 10 days (P1) followed by the unicel-lular N 2-fixing cyanobacteria UCYN-C for the last 9 days (P2) of the experiment. N 2 fixation rates averaged 9.8 ± 4.0 and 27.7 ± 8.6 nmol L −1 d −1 during P1 and P2, respectively. NO − 3 concentrations (< 0.04 µmol L −1) in the mesocosms were a negligible source of N, indicating that N 2 fixation was the main driver of new production throughout the experiment. The contribution of N 2 fixation to PP was not significantly different (p > 0.05) during P1 (9.0 ± 3.3 %) and P2 (12.6 ± 6.1 %). However, the e ratio that quantifies the efficiency of a system to export particulate organic carbon (POC export) compared to PP (e ratio = POC export / PP) was significantly higher (p < 0.05) during P2 (39.7 ± 24.9 %) than during P1 (23.9 ± 20.2 %), indicating that the production sustained by UCYN-C was more efficient at promoting C export than the production sustained by DDAs. During P1, PON was stable and the total amount of N provided by N 2 fixation (0.10 ± 0.02 µmol L −1) was not significantly different (p > 0.05) from the total amount of PON exported (0.10 ± 0.04 µmol L −1), suggesting a rapid and probably direct export of the recently fixed N 2 by the DDAs. During P2, both PON concentrations and PON export increased in the mesocosms by a factor 1.5–2. Unlike in P1, this PON production was not totally explained by the new N provided by N 2 fixation. The use of DON, whose concentrations decreased significantly (p < 0.05) from 5.3 ± 0.5 µmol L −1 to 4.4 ± 0.5 µmol L −1 , appeared to be the missing N source. The DON consumption (∼ 0.9 µmol L −1) during P2 is higher Published by Copernicus Publications on behalf of the European Geosciences Union. 4100 H. Berthelot et al.: Dinitrogen fixation and dissolved organic nitrogen fueled primary production than the total amount of new N brought by N 2 fixation (∼ 0.25 µmol L −1) during the same period. These results suggest that while DDAs mainly rely on N 2 fixation for their N requirements, both N 2 fixation and DON can be significant N sources for primary production and particulate export following UCYN-C blooms in the New Caledonia lagoon and by extension in the N-limited oceans where similar events are likely to occur.

We report here the elemental composition of sinking particles in sediment traps and in the water column following four artificial dust seeding experiments (each representing a flux of 10 g m−2). Dry or wet dust deposition were simulated during two large mesocosms field campaigns that took place in the coastal water of Corsica (NW Mediterranean Sea) representative of oligotrophic conditions. The dust additions were carried out with fresh or artificially aged dust (i.e., enriched in nitrate and sulfate by mimicking cloud processing) for various biogeochemical conditions, enabling us to test the effect of these parameters on the chemical composition and settling of dust after deposition. The rates and mechanisms of total mass, particulate organic carbon (POC) and chemical elements (Al, Ba, Ca, Co, Cu, Fe, K, Li, Mg, Mn, Mo, N, Nd, P, S, Sr and Ti) transfer from the mesocosm surface to the sediment traps installed at the base of the mesocosms after dust deposition show that (1) 15% of the initial dust mass was dissolved in the water column in the first 24 h after seeding. Except for Ca, S and N, the elemental composition of dust particles was constant during their settling, showing the relevance of using interelemental ratios, such as Ti/Al as proxy of lithogenic fluxes. (2) Whatever the type of seeding (using fresh dust to simulate dry deposition or artificially aged dust to simulate wet deposition), the particulate phase both in the water column and in the sediment traps was dominated by dust particles. (3) Due to the high Ba content in dust, Ba/Al cannot be used as productivity proxy in the case of high dust input in the sediment traps. Instead, our data suggests that the ratio Co/Al could be a good productivity proxy in this case. (4) After 7 days, between 30 and 68% of added dust was still in suspension in the mesocosms. This difference in the dust settling was directly associated with a difference in POC export, since POC fluxes were highly correlated to dust lithogenic fluxes signifying a ballast effect of dust. The highest fraction of remaining dust in suspension in the mesocosm at the end of the experiment was found inversely correlated to Chl a increase. This suggests that the fertilizing effect of dust on autotrophs organisms, the ballast effect, and POC fluxes are strongly correlated. (5) Our data emphasize a typical mass ratio Lithogenic/POC fluxes around 30 which could be used as reference to estimate the POC export triggered by wet dust deposition event.

Lithogenic particles, such as desert dust, have been postulated to influence particulate organic carbon (POC) export to the deep ocean by acting as mineral ballasts. However, an accurate understanding and quantification of the POC-dust association that occurs within the upper ocean is required in order to affine the "ballast hypothesis". In the framework of the DUNE project, two artificial seedings were performed seven days apart within large mesocosms. A suite of optical and biogeochemical measurements were used to quantify surface POC export following simulated dust events within a low-nutrient low-chlorophyll ecosystem. The two successive seedings led to a 2.3-6.7 fold higher POC flux as compared to the POC flux observed in controlled mesocosms. A simple linear regression analysis revealed that the lithogenic fluxes explained more than 85% of the variance in POC fluxes. At the scale of a dust deposition event, we estimated that 42-50% of POC fluxes were strictly associated with lithogenic particles through an aggregation process. Lithogenic ballasting also likely impacted the remaining POC fraction which resulted from the fertilization effect. The observations support the "ballast hypothesis" and provide a quantitative estimation of the surface POC export abiotically triggered by dust deposition. In this work, we demonstrate that the strength of such a "lithogenic carbon pump" depends on the biogeochemical conditions of the water column at the time of deposition. Based on these observations, we suggest that this "lithogenic carbon pump" could represent a major component of the biological pump in oceanic areas subjected to intense atmospheric forcing.

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

Soil dust deposition is recognized as a major source of iron to the open ocean at global and regional scales. However, the processes that control the speciation and cycle of iron in the surface ocean after dust deposition are poorly documented mainly due to the logistical difficulties to investigate in-situ, natural dust events. The development of clean mesocosms in the frame of the DUNE project (a DUst experiment in a low Nutrient low chlorophyll Ecosystem) was a unique opportunity to investigate these processes at the unexplored scale of one dust deposition event. During the DUNE-1-P mesocosm seeding experiment, iron stocks (dissolved and particulate concentrations in the water column) and fluxes (export of particulate iron in sediment traps) were followed during 8 days after an artificial dust seeding mimicking a wet deposition of 10 g m −2. The addition of dust at the surface of the mesocosms was immediately followed by a decrease of dissolved iron [dFe] concentration in the 0-10 m water column. This decrease was likely due to dFe scavenging on settling dust particles and mineral organic aggregates. The scavenging ratio of dissolved iron on dust particles averaged 0.37 ± 0.12 nmol mg −1. Batch dissolution experiments conducted in parallel to the mesocosm experiment showed a increase (up to 600 %) in dust iron dissolution capacity in dust-fertilized waters compared to control conditions. This study gives evidences of complex and unexpected effects of dust deposition on surface ocean biogeo-chemistry: (1) large dust deposition events may be a sink for surface ocean dissolved iron and (2) successive dust deposi-tion events may induce different biogeochemical responses in the surface ocean.

Abstract. Simultaneous measurements of atmospheric deposition and of sinking particles at 200 and 1000 m depth, were performed in the Ligurian Sea (North-Western Mediterranean) between 2003 and 2007, along with phytoplanktonic activity derived from satellite images. Atmospheric deposition of Saharan dust particles was very irregular and confirmed the importance of sporadic high magnitude events over the annual average (11.4 g m−2 yr−1 for the 4 years). The average marine total mass flux was 31 g m−2 yr−1, the larger fraction being the lithogenic one (~37%). The marine total mass flux displayed a seasonal pattern with a maximum in winter, occurring before the onset of the spring bloom. The highest POC fluxes did not occur during the spring bloom nor could they be directly related to any noticeable increase in the surface phytoplanktonic biomass. Over the 4 years of the study, the strongest POC fluxes were concomitant with large increases of the lithogenic marine flux, which had originated from either recent Saharan fallout events (February 2004 and August 2005), from "old" Saharan dust "stored" in the upper water column layer (March 2003 and February 2005), or alternatively from lithogenic material originating from Ligurian riverine flooding (December 2003, Arno, Roya and Var rivers). Those associated export fluxes defined as "lithogenic events", are believed to result from a combination of forcing (winter mixing or Saharan events, in particular extreme ones), biological (zooplankton) activity, and also organic-mineral aggregation inducing a ballast effect. By fertilising the surface layer, mixed Saharan dust events were shown to be able to induce "lithogenic events" during the stratification period. These events would be more efficient in transferring POC to the deeper layers than the spring bloom itself. The extreme Saharan event of February 2004 exported ~45% of the total annual POC, compared to an average of ~25% for the bloom period. This emphasises the role played by these "lithogenic events", and in particular those that are induced by the more extreme Saharan events, in the carbon export efficiency in the North-western Mediterranean Sea.

Short-term changes in the flux of particulate matter were determined in the central north western Mediter-ranean Sea (near DYFAMED site) using drifting sediment traps at 200 m depth in the course of the DYNAPROC 2 cruise (14 September-17 October 2004). In this period of marked water column stratification, POC fluxes varied by an order of magnitude, in the range of 0.03-0.29 mgC m −2 h −1 over the month and showed very rapid and high variations. Particulate carbon export represented less than 5% of integrated primary production, suggesting that phytoplank-ton production was essentially sustained by internal recycling of organic matter and retained within the photic zone. While PON and POP fluxes paralleled one another, the elemental ratios POC/PON and POC/POP, varied widely over short-term periods. Values of these ratios generally higher than the conventional Redfield ratio, together with the very low chlorophyll a flux recorded in the traps (mean 0.017 µg m −2 h −1), and the high phaeopigment and acyl lipid hydrolysis metabolite concentrations of the settling material , indicated that the organic matter reaching 200 m depth was reworked (by grazing, fecal pellets production, degradation) and that algal sinking, dominated by nano-and pi-coplankton, made a small contribution to the downward flux. Over time, the relative abundance of individual lipid classes in organic matter (OM) changed from glycolipids-dominated to neutral (wax esters, triacylglycerols) and phospholipids-dominated, suggesting ecosystem maturation as well as rapid Correspondence to: J. C. Marty () and continual exchanges between dissolved, suspended and sinking pools. Our most striking result was documenting the rapid change in fluxes of the various measured parameters. In the situation encountered here, with dominant regenerated production, a decrease of fluxes was noticed during windy periods (possibly through reduction of grazing). But fluxes increased as soon as calm conditions settle.

[ 1] The biogenic (BSi) and lithogenic (LSi) silica export fluxes were investigated in the northeast Atlantic (38 degrees - 45 degrees N, 16 degrees - 22 degrees W) as part of the Programme Ocean Multidisciplinaire Meso Echelle (POMME) program in 2001 - 2002. They were measured at four stations located on both sides of a frontal zone (40 degrees - 42 degrees N) by means of permanent moorings of sediment traps deployed at 400 and 1000 m depth. Averaged over the area, the annual BSi fluxes ( corrected from Th-230 trapping efficiencies) ranged between 0.240 mmol m(-2) d(-1) at 400 m to 0.316 mmol m(-2) d(-1) at 1000 m. The bulk annual BSi fluxes are comparable to bulk BSi export fluxes recorded for oligotrophic areas. The annual export flux of LSi ( range 0.029 mmol m(-2) d(-1) at 400 m to 0.054 mmol m(-2) d(-1) at 1000 m) was lower than BSi and accounted for 10% of the total silica export flux. Results show a strong coupling between the two siliceous particulate fractions, which is interpreted as reflecting LSi scavenging by BSi and limitation of BSi production in surface water by lithogenic ( trace metals) inputs. BSi export was maximum at the beginning of the productive season during the spring bloom. However, annual BSi export fluxes in 2001 were quite higher at 400 m in the southern area ( e. g., 0.249 - 0.288 at the southeast station versus 0.211 mmol BSi m(-2) d(-1) at the northeast station) contradictory to the classical south-north surface production increase. We suggest an advective lateral transport within the upper 400 m of siliceous particles from the northern, more productive area to the southern region.

The concentrations and fluxes of total mass, particulate organic carbon (POC), chlorophyll-a equivalent and lipid biomarker classes were measured in large sinking particulate matter collected in winter 1997/98 at two water depths (100 and 300 m) in the Almeria-Oran frontal zone (SW Mediterranean). Three water masses, comprising the Frontal Atlantic jet, the anticyclonic Atlantic gyre and the surrounding Mediterranean waters, were investigated over two days, using free-drifting sediment traps that collected at intervals of 7-9 h to evaluate night-day variations. Fluxes of mass, POC, chlorophyll-a and lipids out of the euphotic zone varied spatially, ranging from 136 to 1430 mg m(-2) d(-1) for mass, 21 to 105 Mg m(-2) d(-1) for POC, 0.1 to I mg m(-2) d(-1) for equivalents of chlorophyll-a, and 1.2 to 6 Mg m(-2) d(-1) for lipids, with the highest export rates measured in the Atlantic gyre downstream and the lowest in the jet core. All fluxes showed a decrease with increasing water column depth, but the flux difference between the sediment traps at 100 and 300 m depths was most pronounced in the gyre downstream, where the higher fluxes in the shallow trap were a consequence of the accumulation of relatively fresh phytoplanktonic material from the frontal zone. At the Mediterranean site, the relatively low mass fluxes coincided with relatively high organic carbon and lipid concentrations. Lipids comprised 20% of POC in particles leaving the euphotic zone of the Mediterranean site and only 6 to 10% in the frontal zone. In contrast, POC in particles transiting through the mesopelagic zone (at 300 m depth) comprised 6% of lipids at the Mediterranean site and up to 19% in the jet core of the Atlantic-modified water mass. These results demonstrate a strong decoupling between the shallow and deep traps in the frontal zone, denoting the presence of a transfrontal secondary circulation. In contrast, an important vertical export of the particulate organic matter with no influence of lateral advections was clearly observed in the Mediterranean waters. No substantial variation in flux and lipid composition of large particles was observed between day and night-time collection in the frontal and Mediterranean areas, indicating that the export flux resulted mainly from passive sedimentation of phytoplanktonic material. (c) 2005 Elsevier B.V. All rights reserved.

During 48 hour stations during the three Programme Océan Multidisciplinaire Méso Echelle (POMME) cruises in 2001 (late winter, spring, and late summer) at different locations within the region studied (38°-45°N, 15°-21°W), drifting sediment traps were deployed at 200 m and 400 m. Fluxes increased from late winter (POMME 1) to spring (POMME 2), with highest values in the North Atlantic gyre (109.1, 20.1, and 3.5 mg m À2 d À1 for mass, C, and N, respectively) and decreased during POMME 3 to reach threshold values (19.1 ± 6.0, 4.4 ± 1.1, and 0.7 ± 0.2 mg m À2 d À1 , respectively). Lipid class tracers and their fatty acid composition analyzed by gaseous chromatography were used to assess the quality and quantity of organic matter fluxes. Wide seasonal variability was observed in biogenic lipid fluxes (0.42 ± 0.19 and 0.39 ± 0.13 mg m À2 d À1 , 1.78 ± 1.08 and 0.69 ± 0.56 mg m À2 d À1 , and 0.71 ± 0.14 and 0.45 mg m À2 d À1 on average at 200 m and 400 m during late winter, spring, and late summer, respectively) in relation with the development of the spring phytoplankton bloom. In a northern persistent anticyclonic eddy a major export of algal matter occurred through zooplankton activity. In contrast with this pattern, the southernmost anticyclonic eddy exhibited the lowest particle fluxes in relation to the low productivity and the high bacterial carbon demand prevailing in the surface waters. In the main cyclonic structure (C4) and the saddle zone (during POMME 2) the pattern of lipid biotracers reflected the permanence of a zooplankton community and likely advective transfer of matter between 43.5°N and 42°N through subsurface water circulation.

In the framework of the Programme Océan Multidisciplinaire Méso Echelle (POMME) experiment, a 1.5 year record (February 2001–June 2002) of downward particle flux at 400 m and 1000 m was measured by sediment traps at four moorings located in the northeast Atlantic between 39°–43°N and 17°–19°W. Thorium-230 was used to estimate sediment trap efficiency, revealing values ranging from 18.5 to 55%. The lowest trapping efficiency was observed for the trap having experienced the highest currents. Significant interannual variability between 2001 and 2002 was clearly linked to the differences observed in the mixed layer depth. At some sites, particulate organic carbon (POC) export was higher (up to a factor of 1.6) during summer than during the spring event. This could be related to the occurrence of short wind events that deepened the thermocline along with the presence of anticyclonic eddies, yielding an input of new nutrients. The average percentage of POC exported compared to the primary production of organic carbon in the surface waters ranged between 1.3 and 5.0%, with higher export efficiency during the spring. Finally, although the area was shown to present a relatively high mesoscale activity that might impact the geochemistry, POC export was rather homogeneous over the POMME area: 4.9 ± 1.6 gC m−2 yr−1 were exported below 1000 m between February 2001 and February 2002. Therefore a large fraction of the new production may be exported through convection and mode water circulation rather than by particle settling.

The distribution of pigments and fatty acid biogeochemistry of suspended and sinking particles collected in a frontal zone in the western Mediterranean (Almeria-Oran Front) was investigated to determine the sources and fate of the organic matter in relation to the physical forcing of the geostrophic front. Three hydrodynamic structures, comprising the Atlantic jet, an anticyclonic gyre and the surrounding Mediterranean waters, were explored. Fatty acid and pigment distributions differed markedly between the two classes of particles. Whereas branched fatty acids and flagellate pigments were present at higher percentages in suspended particles, the polyunsaturated fatty acids (PUFA) and pigments indicative of degradation and diatoms, were dominant in sinking particles. Fatty acid distribution and compound specific isotope analysis of delta(13)C suggested that the acids were totally marine in origin. Bacterial biomass, indicated by odd/branched chain fatty acids, was minimal and mainly associated with the planktonic biomarkers. Diatom biomarkers predominated in the frontal zone and gyre downstream. Flagellate biomarkers were mainly associated with the suspended particles of the Mediterranean site and the gyre upstream. Prokaryote pigments were more abundant in Mediterranean waters. The relatively high levels of fucoxanthin, polyunsaturated fatty acids and degradation pigments (phaeophorbides) in all sediment traps demonstrated that both diatom aggregates and fecal pellets contributed substantially to the sedimentation processes. Based on biomarker indices, high bacterial abundance with relatively degraded phytoplankton material and an enhanced grazing or zooplankton biomass were observed at the gyre upstream. Phytoplankton, showing little evidence of degradation or grazing by zooplankton, predominated at the gyre downstream. The high accumulation of partially degraded plant material with relatively higher bacterial signatures and low zooplankton grazing occurred in the Mediterranean waters influenced by the proximity of the Atlantic jet. (C) 2004 Elsevier B.V. All rights reserved.

Sterol and fatty alcohol biomarkers were analyzed in suspended and sinking particles from the water column (20-300 m) of the Almeria-Oran frontal zone to characterize the biogenic sources and biogeochemical processes. Diatom- and haptophyte-related sterols were predominant at all sites and vertical distributions of sterol, and fatty alcohol biomarkers in sinking particles were markedly different from suspended particles. In contrast to the relatively fresh sinking particles with elevated concentrations of phyto- and zooplanktonic sterols, suspended particles were extensively degraded with increasing depth and exhibited a more terrestrial and zooplanktonic signature with depth. Sterol and alcohol biomarkers distributions and delta(13)C values from the jet core and the associated gyre of Atlantic waters showed a decoupling between the sinking particles of 100- and 300-m depth, demonstrating the influence of lateral advection in the frontal zone. In contrast, vertical transport of the particulate organic matter in Mediterranean waters was interpreted from the similar isotopic and molecular composition at both depths. The high abundance of phytosterols and phytol below the euphotic zone at 100 m signified that downwelling of biomass occurred on the downstream side of the gyre. The high concentrations of phytosterols and POC, in combination with the high phytosterols/phytol ratio, indicated the accumulation of detrital plant material in the oligotrophic Mediterranean waters near the frontal zone. A higher contribution of phytol in the sinking particles collected during the night at the surface of the jet and at the upstream side of the gyre provided evidence of diel vertical zooplankton migration and important grazing by herbivorous zooplankton. Carbon isotope ratios of sterols confirmed that the 24-ethylcholest-5-en-3beta-ol, commonly associated with terrestrial sources, was a substantial constituent of the phytoplankton in this area. However, the more delta(13)C depleted values obtained for this compound in suspended particles suggested that there was some terrestrial contribution that only becomes evident after degradation of the more labile marine organic matter. (C) 2003 Elsevier Science B.V. All rights reserved.

During the PROSOPE cruise (Sept. 1999) in the Mediterranean Sea, dissolved iron concentrations in seawater and iron and aluminium concentrations in aerosols collected on board were investigated. Concentrations in aerosols were about two times higher in the Tyrrhenian Sea than in the west (Alboran Sea). This was in good agreement with the observed increase in dissolved iron concentrations in the surface waters from West to East. Depth profiles were characterised by a maximum in the surface mixed layer. Using an in vitro experiment, iron released from Saharan dust during the season characterized by a stratified water column and a low primary productivity was estimated: it resulted in an accumulation of 0.5-0.8 nM dissolved iron, in good agreement with the observed iron enrichment in the surface water (0.8 nM). This study confirms the significance of atmospheric input of Saharan origin on the iron cycle in the Mediterranean Sea.

A 2-yr record of downward particle flux was obtained with moored sediment traps at several depths of the water column in two regions characterized by different primary production levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. Particle fluxes, of ∼71–78% biogenic origin (i.e. consisting of CaCO3, organic matter and opal) on average, decrease about six-fold from the mesotrophic site (highest fluxes in the North Atlantic) nearer the Mauritanian margin (18°30′N, 21°00′W) to the remote, open-ocean, oligotrophic site (21°00′N, 31°00′W). This decrease largely reflects the difference in total primary production between the two sites, from ∼260 to ∼110 g organic C m−2 yr−1. At both sites, temporal variability of the downward particle flux seems to be linked to westward surface currents, which are likely to transport seaward biomass-rich water masses from regions nearer the coast. The influence of coastal upwelling is marked at the mesotrophic site. The large differences between the 1991 and 1992 records at that site, where carbon export is large, underscore the interest of long-term studies for export budget estimates. The different productivity regimes at the two sites seem to induce contrasting downward modes of transport of the particulate matter, as shown in particular by the faster settling rates and the higher E ratio (particulate organic carbon export versus total primary production) estimated at the mesotrophic site.