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International audience; Benthic exchanges of oxygen and nutrient at the sediment-water interface were investigated under light and dark conditions at 5 selected sites in a sub-tropical atoll. Mean oxygen fluxes were - 1316.5 ± 242.0 μmol m−2 h−1 and mean effluxes of oxygen under light conditions were 2231.7 ± 626.4 μmol m−2 h−1, presumably due to microphytobenthos present at the sediment-water interface. The consequences of this high related productivity was a systematic consumption of nutrients (DIN, PO4 and Si(OH)4) during almost all light incubations, contrasting with the effluxes of nutrients during dark incubations. Our results suggest that the sediments were net autotrophic and the oxygen balance in favor of microbenthic production when compared to community demand. Diurnal rates of gross benthic primary productivity were high (3423 ± 1192 μmol m−2 h−1) which emphasize the role of microphytobenthos in maintaining the oxygen reservoir in tropical lagoons.

A biogeochemical model (ECO3M-Atoll) was configured to simulate the lower food web in Ahe Atoll lagoon where phytoplankton is mostly nitrogen limited. Understanding the dynamics of phytoplankton – the main food source for oysters – is crucial for the management and the allocation of new pearl farming sites. After parametrizing the model with in situ observations, we tested different hypotheses about nitrogen cycling (benthic remineralization, atmospheric N fixation, etc.) and compared the results to a large observational dataset. Model results show that simulated (pico- and nano-) phytoplankton biomass and nitrogen concentrations are close to in situ data. The simulated biogeochemical processes (uptake and primary production) are also very similar to the observed values. In the model, primary production ranged from 1.00 to 2.00 mg C m−3 h−1 for pico- and 0.40 to 1.00 mg C m−3 h−1 for nanophytoplankton; mean N uptake was 2.02 μmol N m−3 h−1 for pico- and 1.25 μmol N m−3 h−1 for nanophytoplankton.

The sediment in Laguna de Términos, the largest and shallowest system in the Southwest portion of the Gulf of Mexico features a broad range of ecological and hydrobiological characteristics driven by annual weather cycles (dry and wet seasons), causing large salinity gradients during the wet season due to large river discharges. Four sampling campaigns were carried out during the wet and the dry seasons in 2009 and 2010 on a selection of 13 out of 35 stations. Measurements of Sediment Oxygen Demand (SOD) and nutrient fluxes at the sediment-water interface were performed using lab incubations with 15 cm diameter sediment cores. SOD fluctuated between 1327 ± 161 and 2248 ± 359 μmol m−2 h−1 for dry and wet seasons respectively. Silicate effluxes were also significantly higher during the wet seasons (89.4 ± 15.9 μmol m−2 h−1) than during the dry season (46.5 ± 11.4 μmol m−2 h−1). PO4 fluxes were low all over the study period without seasonal trend. No significant difference was measured for DIN fluxes but there was a tendency for DIN uptake during the wet season (−2.9 ± 18.8 μmol m−2 h−1) and conversely an efflux during the dry season (24.3 ± 7.3 μmol m−2 h−1). SOD correlated to organic matter and chloropigment content of the sediments while silicate fluxes responded to enhanced chloropigments in the sediments. During both seasons, total benthic nutrient fluxes overwhelmed largely riverine inputs and benthic carbon mineralization rates approximated a significant proportion of the pelagic organic carbon production. We conclude that benthic processes in Laguna de Términos are largely driven by weather variability and that they contribute substantially to carbon and nutrient budgets in this shallow sub-tropical system.

International audience; Chronic hypoxia and anoxia have strong impacts on coastal aquatic ecosystems worldwide. Such situations are essentially driven by high benthic oxygen (O2) demand resulting from organic matter mineralization in surface sediment and amplified by a low mixing of the water column. However, the benthic O2 demand may greatly vary according to the O2 availability, sediment biogeochemical properties, and bioturbation by macrobenthic fauna. Here we examined how the sediment O2 demand vary in response to seasonal and long-term (pluri-decadal) hypoxia in the Berre lagoon, a coastal ecosystem impacted by chronic hypoxia events since ca 60 years. Oxygen penetration depth, diffusive and total O2 fluxes were measured in situ using a microelectrode autonomous profiler and benthic chamber deployments at three sites impacted by quite-permanent (PA), seasonal (PI) and occasional (PO) hypoxia in August 2016. Those measurements were seasonally repeated at site PI between August 2015 and August 2016. Additional physical and chemical characteristics were also measured in surface sediment. Sediment profile images analysis combined to measures of benthic macrofaunal communities characteristics were determined in order to estimate the quality of the benthic ecosystem. The highest benthic O2 demand was observed after seasonal anoxia in relation to the important accumulation of reduced chemical species in surface sediment. Interestingly, both pluri-decadal hypoxia and normoxia produced relatively high benthic O2 demand related to a higher accumulation of organic matter and to the presence of reduced chemical species at site dominated by hypoxia, and to the presence of fresher organic matter and active bioturbating macrofaunal communities in normoxic site. The low benthic O2 demand at site seasonally impacted by hypoxia was explained by the degraded state of the macrofaunal community and by the lower accumulation of reduced chemical species. The occurrence of hypoxia and anoxia situations in the Berre lagoon was predicted from the competition between kinetics of benthic O2 demand and water column reoxygenation events induced by strong wind. The good correspondence between the measured and predicted hypoxia/anoxia occurrence clearly indicates that the chronic deoxygenation events in the Berre lagoon, and the associated degraded state of the benthic ecosystem are driven jointly by the benthic O2 demand and the intensity and duration of the water column stratification.

International audience; Editorial on the Research Topic Zooplankton and Nekton: Gatekeepers of the Biological Pump Zooplankton and nekton organisms create and destroy particles in manifold ways. They feed on the diverse components of the plankton community and on detrital matter. They disaggregate these components, but also repackage them into fecal pellets. Zooplankton and nekton thereby contributes to the attenuation, but also to the export of vertically settling particles. Many zooplankton and nekton organisms also ascend to the surface layer of the ocean at dusk to feed during the dark hours, and return to midwater at the break of dawn. This diurnal vertical migration (DVM) shuttles organic matter from the surface ocean to deeper layers, where it is metabolized and excreted. This active flux (as opposed to the passive flux of sinking particles) can contribute substantially to the biological pump, the downward export of carbon and nutrients into the oceans interior. Due to their multiple roles in oceanic particle dynamics, zooplankton and nekton organisms can actually be considered the gatekeepers of the biological pump. Several articles in this Research Topic deal with the contribution of zooplankton and nekton-mediated active flux to the total export of organic matter. Using biomass and enzyme transport system (ETS) assessments of respiratory flux for both mesozooplankton and micronekton communities, Hernández-León et al. estimated the total active transport of carbon (respiration, excretion, mortality, and egestion) along a transect in the Atlantic from the Canary Islands to Brazil. They found that active flux by these communities ranged from 25 to 80% of the total particulate organic carbon export at 150 m depth and that the importance of active flux increased with increasing surface productivity. Kwong et al. compared biomass, diel vertical migration, and active flux of mesozooplankton and micronekton across a range of mesoscale eddy structures along the east-coast of Australia during winter and spring. They found that although all eddy regimes had similar integrated biomass of mesozooplakton and micronekton, the organisms in the individual eddies had different migratory behavior, which resulted in contrasting importance of active flux. Kiko et al. assessed the impact of mesozooplankton DVM on elemental cycling at three stations in the Eastern Tropical North Atlantic. They found that approximately 31 to 41% of the total nitrogen loss from the upper 200 m of the water column was attributable to DVM mediated fluxes. They also suggest that gut flux-the flux created by migrators when they evacuate their gut at DVM-depth-and migrator mortality at DVM-depth contribute to an Intermediate Particle Maximum. In their study conducted in the Peruvian upwelling system (which features a severe midwater oxygen minimum zone), Kiko and Hauss concluded that the metabolic suppression

Frontiers in Marine Science launched the Marine Ecosystems Ecology (FMARS-MEE) section in 2014, with a paper that identified eight grand challenges for the discipline (Borja, 2014). Since then, this section has published a total of 370 papers, including 336 addressing aspects of those challenges. As editors of the journal, with a wide range of marine ecology expertise, we felt it was timely to evaluate research advances related to those challenges; and to update the scope of the section to reflect the grand challenges we envision for the next 10 years. This output will match with the United Nations (UN) Decade on Oceans Science for Sustainable Development (DOSSD; Claudet et al., 2020), UN Decade of Ecosystems Restoration (DER; Young and Schwartz, 2019), and the UN Sustainable Development Goals (SDGs; Visbeck et al., 2014). info:eu-repo/semantics/publishedVersion

International audience; The 2009-2010 El Niño was accompanied by a severe drought strongly impacting Mexico as well as Central America, the Caribbean, and the southern USA. The present work aims at assessing how such a major climatic event impacted the hydrological typology of transitional waters in Terminos Lagoon, one of the largest shallow tropical lagoons fringing the Gulf of Mexico. Spatiotemporal inter-comparison of hydrological conditions was conducted by pairing a reference multiparametric dataset (14 hydrological parameters versus 34 sampling stations) averaged over the October 2008 to July 2010 period with each sampling occurrence dataset and running Principal Component Analyses (PCA), setting the reference-survey dataset as active variables and each sampling occurrence dataset as non-active (supplementary) variables. It revealed that the exceptional deficit in freshwater supply to the lagoon during the 2009-2010 El Niño drastically reduced hydrological diversity and lowered the trophic status of the lagoon. Short-term shifts in environmental status are common in transitional waters and responsible for temporary shifts in community structure but climate change projections show a significant long-term decrease in the freshwater discharge at the regional scale that will impact Terminos Lagoon as well as other coastal lagoons of Mexico and Central America. When combined with sea level rise, such a decrease will result in a long-term shift in hydrological conditions with a subsequent increase in salinity and a decrease in the diversity of environmental conditions affecting trophic status, will have a long-term impact on the biota.

Marine sediments are an important carbonate reservoir whose partial dissolution could buffer seawater pH decreases in the water column as a consequence of anthropogenic CO 2 uptake by the ocean. This study investigates the impact of ocean acidification on the carbonate chemistry at the sediment-water interface (SWI) of shallow-water carbonate sediments . Twelve sediment cores were sampled at one station in the Bay of Villefranche (NW Mediterranean Sea). Four sediment cores were immediately analyzed in order to determine the initial distribution (T 0 ) of dissolved inorganic carbon (DIC), total alkalinity (TA), pH and dissolved oxygen (O 2 ) in the porewaters and to quantify sediment-water fluxes. Four other cores were kept submerged in the laboratory for 25 days with ambient seawater (pH T = 8.12) and the remaining four cores were incubated with acidified seawater (average pH offset of −0.68). This acidification experiment was carried out in an open-flow system, in the dark and at in-situ temperature (15 °C). Every three days, sediment-water fluxes (DIC, TA, pH, O2 and nutrients) were determined using a whole core 12-h incubation technique. Additionally, vertical O2 and pH microprofiles were regularly recorded in the first 2 cm of the sediment during the entire experiment. At the end of the experiment, TA, DIC and Ca2+ concentrations were analyzed in the porewaters and the abundance and taxonomic composition of meiofaunal organisms were assessed. The saturation states of the porewaters with respect to calcite and aragonite were over-saturated but under-saturated with respect to 12 mol% Mg-calcite, in both acidified and non-acidified treatments. The sediment-water fluxes of TA and DIC increased in the acidified treatment, likely as a consequence of enhanced carbonate dissolution. In contrast, the acidification of the overlying water did not significantly affect the O 2 and nutrients fluxes at the SWI. Meiofaunal abundance decreased in both treatments over the duration of the experiment, but the organisms seemed unaffected by the acidification. Our results demonstrate that carbonate dissolution increased under acidified conditions but other parameters, such as microbial redox processes, were apparently not affected by the pH decrease, at least during the duration of our experiment. The dissolution of sedimentary carbonates and the associated release of TA may potentially buffer bottom water, depending on the intensity of the TA flux, the TA/DIC ratio, vertical mixing and, therefore, the residence time of bottom water. Under certain conditions, this process may mitigate the effect of ocean acidification on benthic ecosystems .

The 2009–2010 period was marked by an episode of intense drought known as the El Niño Modoki event. Sampling of the Términos Lagoon (Mexico) was carried out in November 2009 in order to understand the influence of these particular environmental conditions on organic matter fluxes within the lagoon's pelagic ecosystem and, more specifically, on the relationship between phyto- and bacterioplankton communities. The measurements presented here concern biogeochemical parameters (nutrients, dissolved and particulate organic matter [POM], and dissolved polycyclic aromatic hydrocarbons [PAHs]), phytoplankton (biomass and photosynthesis), and bacteria (diversity and abundance, including PAH degradation bacteria and ectoenzymatic activities). During the studied period, the water column of the Términos Lagoon functioned globally as a sink and, more precisely, as a nitrogen assimilator. This was due to the high production of particulate and dissolved organic matter (DOM), even though exportation of autochthonous matter to the Gulf of Mexico was weak. We found that bottom-up control accounted for a large portion of the variability of phytoplankton productivity. Nitrogen and phosphorus stoichiometry mostly accounted for the heterogeneity in phytoplankton and free-living prokaryote distribution in the lagoon. In the eastern part, we found a clear decoupling between areas enriched in dissolved inorganic nitrogen near the Puerto Real coastal inlet and areas enriched in phosphate (PO4) near the Candelaria estuary. Such a decoupling limited the potential for primary production, resulting in an accumulation of dissolved organic carbon and nitrogen (DOC and DON, respectively) near the river mouths. In the western part of the lagoon, maximal phytoplankton development resulted from bacterial activity transforming particulate organic phosphorus (PP) and dissolved organic phosphorus (DOP) to available PO4 and the coupling between Palizada River inputs of nitrate (NO3) and PP. The Chumpan River contributed only marginally to PO4 inputs due to its very low contribution to overall river inputs. The highest dissolved total PAH concentrations were measured in the El Carmen Inlet, suggesting that the anthropogenic pollution of the zone is probably related to the oil-platform exploitation activities in the shallow waters of the southern of the Gulf of Mexico. We also found that a complex array of biogeochemical and phytoplanktonic parameters were the driving force behind the geographical distribution of bacterial community structure and activities. Finally, we showed that nutrients brought by the Palizada River supported an abundant bacterial community of PAH degraders, which are of significance in this important oil-production zone.

The wide range of ecological goods and services provided by tropical coastal lagoons and wetlands are under considerable pressure due to the synergistic effects of local anthropogenic impact and global climate change. In transitional waters, salinity is a key driver of ecological processes mostly depending on the balance between marine and river inputs, a balance that can be significantly modified by climate change and by anthropogenic alteration of the watershed. Mesoamerica being considered as a climate change hot-spot and as an ecoregion strongly vulnerable to global change, our study aimed at analyzing the relationship between salinity, river runoff, and rainfall variability in a tropical coastal lagoon and to assess the respective influence of climate change and watershed management. The study focusing on the large and shallow coastal lagoon of Laguna de Terminos in south eastern Mexico established: (1) the variability in salinity distribution along the yearly cycle and the occurrence of a high salinity anomaly period during the wet season of 2009; (2) the relationship between lagoon waters salinity and river inputs further underlying the anomalous situation encountered in 2009; (3) a long term increase in river discharge during the past 60 years, indicating potential salinity decrease in the lagoon during that same period; (4) an absence of any change in rainfall linking the increase in runoff to watershed management rather than long term trend in climate change. Additionally, the specific context of the 2009–2010 Central-Equatorial Pacific El Nino is underlined and the potential relationship between river discharge and ENSO is discussed. Those results should be of significant practical value to decision-makers who are often keen to point the finger at global climate change when local environmental management is also and sometime most significantly responsible.

The influence of riverine inputs on biogeochemical cycling and organic matter recycling in sediments on the continental shelf off the Rhone River mouth (NW Mediterranean Sea) was investigated by measuring sediment oxygen uptake rates using a combination of in situ and laboratory techniques. Four stations were investigated during two cruises in June 2001 and June 2002, with depths ranging from 9 to 192 m and over a distance to the Rhone River mouth ranging from 4 to 36 km. Diffusive oxygen uptake (DOU) rates were determined using an in situ sediment microprofiler and total oxygen uptake (TOU) rates were measured using sediment core incubations. There was good agreement between these two techniques which indicates that the non-diffusive fraction of the oxygen flux was minimal at the investigated stations. DOU rates ranged from 3.7 +/- 0.4 mmol O-2 m(-2) d(-1) at the continental shelf break to 19.3 +/- 0.5 mmol O-2 m(-2) d(-1) in front of the Rhone River mouth. Sediment oxygen uptake rates mostly decreased with increasing depth and with distance from the Rhone mouth. The highest oxygen uptake rate was observed at 63 m on the Rhone prodelta, corresponding to intense remineralization of organic matter. This oxygen uptake rate was much larger than expected for the increasing bathymetry, which indicates that biogeochemical cycles and benthic deposition are largely influenced by the Rhone River inputs. This functioning was also supported by the detailed spatial distribution of total organic carbon (TOC), total nitrogen (TN) and C/N atomic ratio in surficial sediments. Sediments of the Rhone prodelta are enriched in organic carbon (2-2.2%) relative to the continental shelf sediments (< 1%) and showed C/N ratios exceeding Redfield stoichiometry for fresh marine organic matter. A positive exponential correlation was found between DOU and TOC contents (r(2) = 0.98, n = 4). South-westward of the Rhone River mouth, sediments contained highly degraded organic matter of both terrestrial and marine origin, due to direct inputs from the Rhone River, sedimentation of marine organic matter and organic material redeposition after resuspension events. (c) 2008 Elsevier Ltd. All rights reserved.

Regions of Restricted Exchange (RREs) are an important feature of the European coastline. They are historically preferred sites for human settlement and aquaculture and their ecosystems, and consequent human use, may be at risk from eutrophication. The OAERRE project (EVK3-CT1999-0002) concerns 'Oceanographic Applications to Eutrophication in Regions of Restricted Exchange'. It began in July 2000, and studies six sites. Four of these sites are fjords: Kongsfjorden (west coast of Spitzbergen); Gullmaren (Skagerrak coast of Sweden); Himmerfjarden (Baltic coast of Sweden); and the Firth of Clyde (west coast of Scotland). Two are bays sheltered by sand bars: Golfe de Fos (French Mediterranean); and Ria Formosa (Portuguese Algarve). Together they exemplify a range of hydrographic and enrichment conditions. The project aims to understand the physical, biogeochemical and biological processes, and their interactions, that determine the trophic status of these coastal marine RRE through the development of simple screening models to define, predict and assess eutrophication. This paper introduces the sites and describes the component parts of a basic screening model and its application to each site using historical data. The model forms the starting point for the OAERRE project and views an RRE as a well-mixed box, exchanging with the sea at a daily rate E determined by physical processes, and converting nutrient to phytoplankton chlorophyll at a fixed yield q. It thus uses nutrient levels to estimate maximum biomass; these preliminary results are discussed in relation to objective criteria used to assess trophic status. The influence of factors such as grazing and vertical mixing on key parameters in the screening model are further studied using simulations of a complex 'research' model for the Firth of Clyde. The future development of screening models in general and within OAERRE in particular is discussed. In addition, the paper looks ahead with a broad discussion of progress in the scientific understanding of eutrophication and the legal and socioeconomic issues that need to be taken into account in managing the trophic status of RREs.

Sediment Oxygen Consumption (SOC) was investigated during a winter (Southern Hemisphere) cruise in the southwest lagoon of New Caledonia. Oxygen fluxes were measured at 11 sampling stations distributed along two coast to reef transects. Three different methods of flux measurements were used: diver-operated benthic chambers, laboratory incubation of sediment cores and oxygen microprofiles determinations. SOC values varied between 450 and 2250 μmol O2 m−2 h−1. The level of agreement between the three techniques strongly varied as a function of sediment type. Most of the SOC values from the grey sand zone in the middle part of the lagoon and the muddy bottoms of the bays did not show significant differences. A central station presenting a dense seagrass bed gave lower SOC determined by oxygen microprofiles compared to the two other methods. In coarse carbonated sands from the back reef area, SOC measured by in situ benthic chambers were higher than SOC measured by incubation techniques. This discrepancy could be explained by physical disturbance of the sediments, macroscale variability in benthic communities or technical efficiency of the sediment sampling device and probably by a combination of all three processes. Nevertheless, for the other sediment types that represented 85% of the lagoon bottoms, the results from the three techniques used for SOC determination were strongly convergent. Based on this assumption, it could be stated that the oxygen fluxes were essentially driven by microbial activity compared to biologically mediated vertical transport of solutes. The SOC values determined during this study were in agreement with budgets previously calculated for the lagoon. Regardless of the back reef area, spatial variability in SOC can be explained by the organic matter content of sediments which clearly showed a coast to reef gradient with higher organic carbon and nitrogen contents in the coastal sediments. The C/N ratios demonstrate the higher rate of freshly deposited organic matter near the coast compared to more central stations in the lagoon.

Measurements of oxygen fluxes at the water-sediment interface (transparent and opaque enclosures) were made on bare sediments inhabited by microphytes on the one hand, and on sediments diversely colonized by macrophytes (macroalgae and seagrasses) on the other hand. Five stations, typical of different biotopes of a Mediterranean shallow lagoon were studied from May to July 1993 in order to observe an anoxic event which usually occurs at that time of the year. Average diurnal respiration of benthic communities ranged from 40 mg (or 1.25 mmol) O-2 m(-2) h(-1) in bare sediments (31 % of the lagoon area) to 76 mg (or 2.37 mmol) O-2 m(-2) h(-1) in sediments with a medium coverage of macroalgae (37 % of the total area) and, finally, to 100 mg (or 3.12 mmol) O-2 m(-2) h(-1) in the denser macrophytic area (32 % of the total area). The highest diurnal gross production was observed in the zone colonized by macroalgae and seagrasses, and especially in corridors between shellfish-cultivation tables (300 mg O-2 m(-2) h(-1), or 9.37 mmol O-2 m(-2) h(-1), equivalent to 113 mg C m(-2) h(-1)). Overall, during this period, net bottom oxygen production was close to nil in ca. 30 %, and positive in 70 % of the lagoon area. The average net oxygen production for the whole lagoon in summer is thus in the order of 100 mg O-2 m(-2) h(-1). In 1993, at the end of July, an anoxic event was avoided due to a period of strong wind., Les flux d'oxygène ont été mesurés à l'interface eau-sédiment (enceintes transparentes et opaques) d'une part, sur des sédiments apparemment nus (mais peuplés de microphytes), et d'autre part sur des sédiments plus ou moins couverts de macrophytes (macroalgues et phanérogames). Cinq stations, caractéristiques des différents biotopes d'une lagune méditerranéenne peu profonde, ont été étudiées de mai à juillet 1993, dans le but de saisir un des épisodes d'anoxie habituels à cette époque. La respiration diurne d'oxygène des communautés benthiques (cloches noires) augmente en moyenne de 40 mg (ou 1,25 mmol) O2 m−2 h−1 sur les sédiments nus, représentant 31 % de la surface totale de la lagune, à 76 mg (ou 2,37 mmol) O2 m−2 h−1 vers les sédiments moyennement peuplés en macroalgues (37 % de la surface totale) et jusqu'à 100 mg (ou 3,12 mmol) O2 m−2 h−1 dans les zones densément peuplées de macrophytes (32 % de la surface totale). La production brute diurne est maximale dans les zones à macrophytes avec phanérogames et en particulier dans les corridors entre les tables ostréicoles (300 mg O2 m−2 h−1 ou 9,37 mmol O2 m−2 h−1, ou encore équivalente à 113 mg C m−2 h−1). À cette période pré-estivale et estivale de l'année, la production nette d'oxygène à la surface des sédiments est nulle pour environ 30 % de la surface de la lagune et positive pour environ 70 % de cette surface. En moyenne pour l'ensemble de la lagune, la production nette due aux végétaux benthiques est d'environ 100 mg O2 m−2 h−1. À la fin de juillet 1993, une crise anoxique a été évitée grâce à un fort coup de vent.

International audience; Since 2005, wind-driven 'sporadic' coastal upwelling events have been identified off the southwestern reef of New Caledonia. Several studies have described the main physical processes and induced surface patterns using 1D and 3D modelling, as well as in situ measurements. Previous models were applied at the mesoscale without taking into account the lagoon. Using a recently developed 3D coupled physical-biogeochemical model that considers the complex ocean-lagoon interface, we aim to understand better the impact of the upwelling on the lagoon. The model was found to be in good agreement with measured data reported in previous publications about two upwelling events. However, in general, levels of surface chlorophyll-a were overestimated by the model in the upwelling area when compared to ocean colour data and several hypotheses have been proposed to explain the discrepancy. We then tracked rich upwelled water using a forward Lagrangian transport analysis. Upwelled waters from the upper nutricline were found to be able to reach the South West lagoon. An anti-cyclonic eddy was detected near the upwelling area, potentially responsible, in part, for the rich water intrusions into the lagoon.

We present an empirical model that describes the ratio of phytoplankton chlorophyll a to carbon, Chl : C, as a function of temperature, daily irradiance, and nutrient-limited growth rate. Our model is based on 219 published measurements of algal cultures exposed to light-limited or nutrient-limited growth conditions. We illustrate an approach for using this estimator of Chl : C to calculate phytoplankton population growth rate from measured primary productivity. This adaptive Chl : C model gives rise to interactive light-nutrient effects in which growth efficiency increases with nutrient availability under low-light conditions. One implication of this interaction is the enhancement of phytoplankton growth efficiency, in addition to enhancement of biomass yield, as a response to eutrophication.

Effects of an experimental pollution by neogenous and fossil organic matter on microphyto- and macrozoobenthic communities were studied in situ over a 1 yr period (July 1989 to May 1990) in a shallow microtidal bay (Gulf of Fos, south coast of France). Three experimental enclosures of 1 m2 non-defaunated sediments were covered with 1 cm of polluted defaunated sediments. The sediment in one enclosure contained natural biodeposits with a high organic matter content (BD), that in a second enclosure contained Arabian light crude oil (BAL), and the sediment in the third enclosure contained diesel oil-based cuttings (CUT). Pollution by contaminants did not prevent microphytobenthos from colouizing sediments. Population changes over time were quite similar in all enclosures, except in CUT, where a four times higher chlorophyll a content appeared to be related to a decreased number of grazers and consequently lower grazing rates of animals. Toxicity to the fauna was immediate in the case of BAL and occurred within 3 mo at CUT. Opportunistic species settled in all contaminated sediments; this occurred quite rapidly in BD and BAL which recovered within 3 mo to levels comparable with control sediments. In CUT, natural populations had not recovered after 1 yr, whereas a quasi-monospecific population of Capitella capitata was still present. During the first 3 mo, the oxygen demand of the sediment was higher in oil-contaminated sediments than in controls. On the whole, the changes in fluxes and organism assemblages in our weakly tidal area appear to be consistent with other findings in macrotidal seas.

International audience; Considering the growing concern about the impact of anthropogenic inputs on coral reefs and coral reef lagoons, surprisingly little attention has been given to the relationship between those inputs and the trophic status of lagoon waters. The present paper describes the distribution of biogeochemical parameters in the coral reef lagoon of New Caledonia where environmental conditions allegedly range from pristine oligotrophic to anthropogenically influenced. The study objectives were to: (i) identify terrigeneous and anthropogenic inputs and propose a typology of lagoon waters, (ii) determine temporal variability of water biogeochemical parameters at time-scales ranging from hours to seasons. Combined ACP-cluster analyses revealed that over the 2000 km2 lagoon area around the city of Nouméa, "natural" terrigeneous versus oceanic influences affecting all stations only accounted for less than 20% of the spatial variability whereas 60% of that spatial variability could be attributed to significant eutrophication of a limited number of inshore stations. ACP analysis allowed to unambiguously discriminating between the natural trophic enrichment along the offshore-inshore gradient and anthropogenically induced eutrophication. High temporal variability in dissolved inorganic nutrients concentrations strongly hindered their use as indicators of environmental status. Due to longer turn over time, particulate organic material and more specifically chlorophyll a appeared as more reliable nonconservative tracer of trophic status. Results further provided evidence that ENSO occurrences might temporarily lower the trophic status of the New Caledonia lagoon. It is concluded that, due to such high frequency temporal variability, the use of biogeochemical parameters in environmental surveys require adapted sampling strategies, data management and environmental alert methods.

The objective of this paper was to explore the potentialities of sequential statistical estimation methods to assimilate ocean color observations in a primary production model coupled to a 3D hydrodynamic model. The study site was the gulf of Fos—Rhone delta region on the French Mediterranean coast. The high rate of primary production generally observed in this area is mainly due to strong nutrient inputs of the Rhone River. The assimilation method is derived from the singular evolutive extended Kalman filter (SEEK), which uses an error subspace represented by multivariate empirical orthogonal functions (EOF). SeaWiFS chlorophyll data were assimilated by the ecosystem model during a simulation performed under realistic meteorological conditions for the year 2001. An ‘adaptive’ computing method of the EOF was applied in order to lower the instabilities of the filter. Data assimilation system permitted to reduce the mean absolute error between model and data from 1.51 to 0.77 mg m � 3 thanks to the SEEK filter, showing a substantial 49% gain. Efficiency of the SEEK filter was then investigated considering several areas of interest inside the modelled domain. Finally, impact of the assimilation scheme on non-observed variables was illustrated and discussed. Throughout this experimentation the data assimilation system showed its potential regarding operational systems. & 2010 Elsevier Ltd. All rights reserved. Massive industrialisation during the last century has induced major change in the biogeochemical dynamics of marine coastal areas. The Gulf of Fos in the north-west of the Mediterranean Sea is an interesting case study of the influence of anthropogenic pressure on marine ecosystem behaviour. This semi-enclosed estuary regularly receives strong nutrient inputs from the Rhone River. The Rhone River nutrient discharge is directly influenced by upstream agricultural activities. Enrichment of nutrients in such areas can lead to eutrophication processes with undesirable consequences on water quality (Smith et al., 1999). Three-dimensional coupled physical–biogeochemical modelling is a promising tool for understanding and predicting such phenomena. The forecasting potential of this kind of model has to be ascertained in order to fit real-time observations. In that sense data assimilation allows driving constantly models on a realistic trajectory by correcting the system state according to

International audience; résumé

In order to quantify the spatial and seasonal variations of sediment oxygen consumption and nutrient fluxes, we performed a spatial survey in the south west lagoon of New Caledonia during the two major seasons (dry and wet) based on a network of 11 sampling stations. Stations were selected along two barrier reef to land transects representing most types of sediments encountered in the lagoon. Fluxes were measured using ex-situ sediment incubations and compared to sediment characteristics. Sediment oxygen consumption (SOC) varied between 500 and 2000 mu mol m(-2). depending on season and stations. Nutrient effluxes from sediment were highly variable with highest fluxes measured in muddy sediments near the coast. Inter-sample variability was as high as seasonal differences so that no seasonally driven temperature effect could be observed on benthic nutrient fluxes in our temperature range. Nutrient fluxes, generally directed from the sediment to the water column, varied between -5.0 and 70.0 mu mol m(-2) h(-1) for ammonia and between 2.5 and +12.5 mu mol m(-2) h(-1) for PO4 and NO2+3. SOC and nutrient fluxes were compared to pelagic primary production rates in order to highlight the tight coupling existing between the benthic and pelagic compartments in this shallow tropical lagoon. Under specific occasions of low pelagic productivity, oxygen sediment consumption and related carbon and nutrient fluxes could balance nearly all net primary production in the lagoon. These biogeochemical estimates point to the functional importance of sediment biogeochemistry in the lagoon of New Caledonia. (C) 2010 Elsevier Ltd. All rights reserved.

The fate of three radionuclides (3H, 137Cs, 106Ru) has been studied in the spreading area of the Rhone river and the nearby Gulf of Fos. The role of filter-feeders such as Mytilus sp., in this fate was investigated by periodically sampling the water column, molluscs and underlying sediments during 1986 and 1987. Results show that radionuclides may be a valuable tracer for Rhone river effluents entering the coastal area either in sediments or in the flesh of filter feeders. For example, tritium levels in the organic matter of superficial sediment decreases from the river mouth (3113 Bq l−13H of combustion water) to more offshore areas (219 Bq l−13H). This phenomenon is related to the settling characteristics of suspended matter in such areas. In areas with high biological activity, the role of filter feeders seems to dominate the transfer of radionuclides from the water column to the bottom, due to concentration of these elements in biodeposits. Deposition rates ranged from 13–50 Bq m−2 d−1 137Cs for September and May respectively. This transfer undergoes temporal fluctuations correlated with seasonal variations of the main hydrobiological parameters.

International audience; In order to predict eutrophication events in coastal areas we tested an assimilation scheme based on sequential data assimilation of SeaWiFS chlorophyll data into a coupled 3D physical–biogeochemical model. The area investigated is a semi-enclosed estuarine system (Gulf of Fos–North-western Mediterranean Sea) closely linked to the Rhone River delta. This system is subjected to episodic eutrophication caused by certain hydrodynamic conditions and intermittent nutrient inputs. The 3D hydrodynamic model Symphonie was coupled to the biogeochemical modelling platform Eco3M. Surface chlorophyll concentrations were derived from SeaWiFS data using the OC5 algorithm and were sequentially assimilated using a singular evolutive extended Kalman filter. Assimilation efficiency was evaluated through an independent in situ data set collected during a field survey that took place in May 2001 (ModelFos cruise). An original approach was used in constructing the state vector and the observation vector. By assimilating pseudo-salinity extracted from the model biogeochemical dynamics in both open sea and plume region were respected. We proved that substantial improvements were made in short-term forecasts by integrating such satellite-estimated chlorophyll maps. We showed that missing freshwater inputs could be corrected to a certain extent by the assimilation process. Simulated concentrations of surface chlorophyll and other basic components of the pelagic ecosystem such as nitrates were improved by assimilating surface chlorophyll maps. Finally we showed the coherent spatial behaviour of the filter over the whole modelled domain.

Biogeochemical processes in sediments under the influence of the Rhone River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC (delta C-13(OC)) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhone delta system. Large spatial variations were observed in both the total O-2 uptake (5.2 to 29.3 mmol m(-2) d(-1)) and NH4+ release (-0.1 to -3.5 mmol m(-2) d(-1)) rates at the sediment-water interface. The highest fluxes were measured near the Rhone River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of delta C-13(OC) ranged from -26.83 parts per thousand to -23.88 parts per thousand with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhone mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhone POC input is either buried (similar to 52%) or remineralized (similar to 8%), mostly on the prodelta area. The remaining fraction (similar to 40%) is either mineralized in the water or exported outside the Rhone delta system in dissolved or particulate forms.

Fluxes of the heavy metals chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), lead (Pb), cadmium (Cd) and zinc (Zn) delivered by rivers to the Gulf of Lion (NW Mediterranean Sea) were estimated over a three year study of the River Rhone and its smaller tributaries. Most of the particulate metal fluxes (80-90%) delivered by these rivers occurred within a very short period of time (less than 12%), a typical trend for the Mediterranean environment, where highly contrasting hydrological regimes were observed over the year. Temporal and spatial variations in the fluxes of these particulate metals were driven by the fluxes in both water discharge and suspended particulate matter load. On the shelf, these particulate metal fluxes, largely arising from the Rhone watershed, were two to ten times more important than those resulting from atmospheric deposition. Co, Cr and Ni in the rivers and on the shelf surface sediments were mainly natural and associated with the finest particles. Cd and Phosphorus appeared to be associated with the silt fraction and to be enriched in the prodelta areas. Pb, Zn and Cu were more closely associated with the organic matter content and also showed enrichment in the organic rich prodeltaic sediments. Anthropogenic influences diminished offshore, except for Pb and Zn which could be supplied from the atmosphere by man-made aerosols. Although most of the metals tended to be enriched in the prodelta areas, these did not constitute a permanent sink due to resuspension processes affecting these shallow depths. A resuspension experiment conducted on sediment cores from the Rhone prodelta demonstrated that metal deposited on the surface layer, especially those associated with the organic matter, may be resuspended; this should be taken into account for a complete understanding of the biogeochemical cycle of these metals. (c) 2007 Elsevier Ltd. All rights reserved.

International audience; in coastal environments, resuspension is a key issue regarding contaminant remobilisation from the sediments, A first attempt to quantify both sedimentary bed erosion and related plutonium isotope remobilisation from eroded particles was carried out through flume experiments. Erosion fluxes under controlled hydrodynamic stresses and their resulting plutonium remobilisation were measured for cohesive sediments sampled at several locations (8 to 97 in depth) near the Rhone river mouth (North-western Mediterranean Sea). Surface sediments were characterised in terms of plutonium content (Pu-238 and Pu-239,Pu-240) and particle size distribution. Laboratory resuspension experiments were performed under realistic hydrodynamic conditions close to those prevailing in coastal zones. Critical shear stresses of erosion ranged between 0.068 and 0.087 N m(-2) whereas erosion rate varied within a factor of 3 (57-176 g m(-2) h(-1)). After 1 hour of resuspension, the plutonium activity increased in the seawater particulate phase by factors ranging from 2 to 14, depending on the site at which the sediment was sampled. Plutonium fluxes ranged between 2 and 56 mBq m(-2) h(-1) for Pu-239,Pu-240 depending on location. The highest fluxes were measured for sediments from the deepest site, where the highest Pu activities and mud content were found. During our resuspension experiments, no significant increase was found in dissolved Pu activities. These laboratory experiments have provided a unique opportunity to investigate the behaviour of plutonium at the sediment-water interface in a coastal environment. They emphasised the importance of sediment resuspension in plutonium remobilisation and its possible dispersion on continental shelves. (c) 2005 Elsevier B.V. All rights reserved.

A high horizontal resolution 3D hydrodynamic model was applied to a semi-enclosed bay (Gulf of Fos, western Mediterranean Sea) in order to describe the specific circulation patterns driven by winds and to infer the scales of residence times. Freshwater inputs to this bay come from the Rhône river and navigation channels. Idealised simulations under typical wind forcing conditions were performed and are described in this paper. They revealed several features of the exchanges of water masses in the Gulf of Fos, in particular the intrusion of the Rhône river plume generated by south winds. During northern wind conditions, surface waters flushed out of the system. To compensate for this outflow, bottom currents transporting marine waters took the opposite direction. Residence times in the different areas of the gulf were also investigated. A realistic simulation was performed under actual conditions of Rhône river discharges, meteorological forcing and impact of the surrounding general circulation. Model outputs, mainly salinity fields, were compared to 10-day observations acquired during a multidisciplinary cruise that took place in May 2001. Model results and observations were in good agreement and showed a massive inflow of the Rhône river plume waters into the Gulf of Fos during a south wind event. Sensitivity studies showed that the salinity field was strongly dependent on the wind direction and on the local freshwater discharges.

International audience; A dedicated trawling experiment was performed at three sites on the Gulf of Lion continental shelf, with the aim of assessing the resuspension of particulate and dissolved matter triggered by different types of trawls on muddy sediments. The different configurations were: (i) bottom trawl, with bobbin for ground rope (Rockhopper): (ii) bottom trawl, without bobbin (Medits); and (iii) pelagic trawl, towed at 1 and 10m above the seabed.;The plumes of resuspended sediment were measured using the acoustic backscattered intensity, from a towed ADCP. Concomitant profiles of particle size-distribution, light transmission and water samples were collected, outside and inside the plumes. The analysis of the data enabled derivation of the major physical and chemical characteristics of the plumes generated by the trawls; likewise, and to quantify the resuspension fluxes of sediment, particulate (PN, POC) and dissolved (nutrients) elements. The residence time and dispersal of the plumes were monitored and modelled, considering the settling velocity of the particulate matter and the near-bottom turbulence.;The results indicate that the bottom trawls produce significant resuspension, whilst the near-bottom and mid-water pelagic trawls have no impact upon the sediment. The sediment clouds at several hundreds metres astern of the bottom trawls are 3-6m high and 70-200m, wide; they were generated both by the otter doors and the net. The average suspended sediment concentrations measured in the plumes reach 50 mg l(-1). Resuspension fluxes of sediment along the path of the trawls range from 190 g m(-2) s(-1), for the coarsest sediment (clayey silt) to 800 g m(-2) s(-1) for the finest sediment (silty clay). Whilst the resuspended loads of dissolved elements (nutrients) within the plume segment suggest a release of porewater, present at least in the first few centimetres of sediment, the particulate matter load only resulted from the resuspension of less than 1 mm thickness of the sediment bed. This discrepancy shows that a very small fraction of the sediment ploughed by the trawl is effectively injected into the water column.;The monitoring of the settling of the plumes indicates a rapid decay of the sediment load, during the first hour after its generation. Some of the sediment (about 10-15% of the initial load) remains in suspension; this is due, probably, to the near-bottom turbulence that prevents the redeposition of the fine particles and aggregates. Lateral spreading of the plume is strongly dependent upon the variability of horizontal currents. (c) 2005 Elsevier Ltd. All rights reserved.

International audience; A coupled three-dimensional physical-biological model was developed in order to simulate the ecological functioning and potential impacts of land-derived inputs in the southwest lagoon of New Caledonia. This model considered pelagic biogeochemical cycling of organic matter, taking into account advection and diffusion processes driven mainly by local wind fields and freshwater discharges. Modeled phytoplankton dynamics were strongly correlated with both freshwater nutrient inputs and wind-driven hydrodynamic processes, the latter resulting in a large input of oceanic water from the southeast part of the lagoon under trade wind conditions. In situ data obtained during the summer (January 1998) under trade wind conditions supported predicted concentration gradients along several coast to reef transects and provided a validation of the coupled physical-biogeochemical model. An additional sensitivity analysis showed that the alteration of the biogeochemical parameters did not strongly affect the results of the model. Freshwater inputs of nutrients were simulated using a realistic scenario corresponding to the summer rainy season of 1997-1998 in New Caledonia. Despite occasional flooding events from the main rivers considered in these simulations, no significant meso-scale phytoplankton bloom was identified. Hydrodynamically driven dispersion and rapid uptake of nutrients by phytoplankton were sufficient to spatially constrain the impact of river inputs and maintain oligotrophic conditions. The fine spatial grid of our three-dimensional model demonstrated that eutrophication in the southwest lagoon of New Caledonia is confined to the most restricted coastal embayments, while most of the lagoon experiences sustained oligotrophic conditions.

International audience; The erosion of fine-grained cohesive sediments is a ubiquitous phenomenon in marine and estuarine environments. Critical shear stress and erosion rates are predominant variables in transport mechanics. This study presents a comparison between in situ and laboratory measurements of sediment erodibility. The main goal was to assess the reliability of critical shear stress of erosion determined in a laboratory flume. The laboratory experiment allowed us to define the erodibility of sediments sampled at a reference station in the Gulf of Fos. This site is frequently influenced by Rhone river inflows. Two different values of critical shear stress of erosion were determined, the lowest (T-c1 = 0.02 +/- 0.002 N m(-2)) related to the erosion of the fluff layer, and the highest (tau(c2) = 0.05 +/- 0.005 N m(-2)) to a more consolidated layer or to coarser particles. These values of critical shear stress were very close to commonly in situ based estimates of fluff critical shear stresses, which ranged between 0.02 and 0.05 N m(-2). The comparison with acoustic backscatter data recorded during a field cruise in autumn attested in a qualitative way to the reliability of the critical shear stress of erosion determined in our laboratory flume.;The erosion coefficients determined during this study, varying between 3.4 10(-6) and 7.5 10(-6) kg m(-2) s(-1), were used to calculate in situ erosion rates. Between 6 and 9 October, the mean erosion rate at the reference station was 20 g m(-2) h(-1) and maximum values reached 65 g m(-2) h(-1). The calculated erosion rates were used to predict in situ variations in suspended sediment concentration (SPM). The predicted increase in SPM was 14 mg l(-1), close to the measured increase in SPM (10 mg l(-1)). The results from this initial attempt to compare in situ and laboratory erosion measurements are encouraging but further investigation is needed. Potential organic matter fluxes determined in our flume were around 1.7 mmol PON m(-2) h(-1) and 115 mu mol POP m(-2) h(-1). An estimation based on these results showed that the organic material introduced into the water column by erosion amounted to between 12 and 200% of the daily pelagic primary production. This is considerable and should be considered in biogeochemical studies of coastal systems. (c) 2005 Elsevier B.V. All rights reserved.

The spatial variability of oxygen and dissolved nutrient exchanges across the sediment-water interface was studied on the continental shelf in the Gulf of Lions (NW Mediterranean Sea). Replicate sediment cores were sampled at nine stations (64-162 m depth) along two lines parallel to the coast during two cruises in March and June 1998. Sediment-water exchanges were measured using the core incubation technique. Surficial sediments, bottom water and interstitial water characteristics were also described. Fluxes of oxygen (3.72-8.83 mmol m(-2) d(-1)), nitrate (0.026-0.283 mmol m(-2) d(-1)), ammonium (-0.022 to 0.204 mmol m(-2) d(-1)), nitrite (-0.034 to 0.002 mmol m(-2) d(-1)), phosphate (-0.007 to 0.029 mmol m(-2) d(-1)) and silicate (0.504-1.656 mmol m(-2) d(-1)) were generally quite low. This has to be related to the oligotrophy of the Mediterranean Sea. Fluxes showed a weak spatial variability, and a significant correlation could be established between oxygen fluxes and the organic carbon content of surficial sediments. A general increase in ammonium, nitrate and phosphate release was also observed towards the coast and the mouth of the Rhone River. Nitrite uptake and silicate release showed high variability between cruises, and the change in silicate fluxes depended mainly on the location on the eastern or western part of the continental shelf. Over the whole continental shelf, calculated sediment mineralization rate represents 342 kt a(-1) of organic carbon. The annual release from the sediments approximates to 14.1 kt dissolved inorganic nitrogen, 2.9 kt P, and 165 kt dissolved silica, which represent, respectively, an amount close to 5%, 7% and 28% of the nutrient requirements for primary production. When compared to nutrient inputs from the Rhone River, sediments appear to play a significant role in the biogeochemical cycles of the Gulf of Lions system, mainly for inorganic phosphorus and dissolved silica., Cette étude décrit la variabilité spatiale des échanges dissous à l’interface eau-sédiment dans le golfe du Lion. Des carottes sédimentaires ont été échantillonnées à neuf stations (64–162 m de profondeur) sur deux lignes parallèles à la côte, au cours de deux campagnes (mars et juin 1998). Les flux à l’interface eau-sédiment ont été mesurés par incubation de carottes. Des analyses complémentaires permettent de décrire les caractéristiques des sédiments superficiels, de l’eau de fond et de l’eau interstitielle. Les flux d’oxygène (3,72–8,83 mmol m–2 d–1), de nitrate (0,026–0,283 mmol m–2 d–1), d’ammonium (–0,022 à 0,204 mmol m–2 d–1), de nitrite (–0,034 à 0,002 mmol m–2 d–1), de phosphate (–0,007 à 0,029 mmol m–2 d–1) et de silicium dissous (0,504 à 1,656 mmol m–2 d–1) sont généralement faibles, en raison de l’oligotrophie de la mer Méditerranée. Les échanges à l’interface eau-sédiment montrent une faible variabilité spatiale, et un lien a été établi entre les flux d’oxygène et le contenu en carbone organique des sédiments superficiels. Une augmentation générale du relargage d’ammonium, de nitrate et de phosphate est également mise en évidence vers l’embouchure du Rhône. D’importants changements des flux de nitrites vers les sédiments ainsi que du relargage de silicate ont été observés entre les deux campagnes, avec pour ce dernier des différences majeures entre les zones situées à l’est et à l’ouest du golfe. Pour l’ensemble du plateau continental, les bilans à l’échelle du golfe du Lion correspondent à la minéralisation de 342 kt a–1 de carbone organique, et le relargage annuel de 14.1 kt d’azote inorganique dissous, 2.9 kt P, et 165 kt de silice dissoute. Ces valeurs équivalent à 5% DIN, 7% P et 28% DSi de la consommation par la production primaire, et sont plus élevées que les apports rhodaniens en ce qui concerne le phosphate et le silicium dissous.

Shelf-slope exchanges and budgets of organic and inorganic nutrients are calculated for the Gulf of Lion continental margin in the northwestern Mediterranean. Computations are based on data from three seasonal marine surveys performed in March 1998, June 1998 and January 1999 in the framework of the French Programme National d'Environnement Cotier. A Land-Ocean Interactions in the Coastal Zone type box model is used to calculate the advective exchanges between the shelf and the adjacent open sea and to estimate the budgets of non-conservative elements (DIP, DIN, DOP, DON, DOC, POC, PON, POP). These budgets consider river discharges, urban sewage supply, atmospheric deposition, and fluxes at the water-sediment and shelf-slope interfaces. Uncertainties on the fluxes and budgets are estimated to assess the robustness of the results with respect to the spatial variability of the system. Results indicate that shelf-slope exchanges by mixing predominate with respect to the major input terms (river discharge and sediment release). Budgets for inorganic nutrients, that show a strong concentration gradient between the shelf and the slope waters, are significantly different from zero and indicate an excess of these elements in the shelf water. For all surveys, these surpluses suggest (i) that the whole shelf system is autotrophic, (1.6-4.3 x 10(3) mol C s(-1)) and acts as a sink of CO2, and (ii) that it is a site of net denitrification (119 mol N s(-1)) and acts as a sink of N-2. Average shelf-slope fluxes of dissolved and particulate organic elements generally indicate an export to the open sea. However, the large uncertainty on these fluxes yields budgets for the shelf not significantly different from zero., Les échanges entre le plateau et l’océan ouvert et les bilans de nutriments organiques et inorganiques ont été estimés pour la marge continentale du golfe du Lion en Méditerranée nord occidentale. Les calculs sont fondés sur les données de trois campagnes de mesure saisonnières réalisées en mars 1998, juin 1998 et janvier 1999 dans le cadre du Programme National d’Environnement Côtier. Un modèle en boîte de type LOICZ a été utilisé pour calculer les échanges advectifs entre le plateau et l’océan ouvert adjacent et pour estimer les bilans d’éléments non-conservatifs (PID, NID, POD, NOD, COD, COP, NOP, POP). Ces bilans prennent en compte les apports des rivières, les effluents urbains, les dépôts atmosphériques, et les flux aux interfaces eau-sédiment et plateau-pente. Les incertitudes sur les flux et les bilans, liée à la variabilité spatiale du système, ont été estimées pour tester la robustesse des résultats. Les résultats indiquent que les échanges plateau-pente par mélange dominent les principales sources (rivière et sédiment). Les bilans pour les nutriments inorganiques, qui présentent un important gradient de concentration entre les eaux du plateau et celles de la pente, sont significativement différents de zéro et indiquent un excès de ces éléments sur le plateau. Pour toutes les campagnes, ces excès suggèrent que l’ensemble du système (i) est autotrophique (1.6–4.3 × 103 mol C s–1) et agit comme un puits CO2, et (ii) présente un taux de dénitrification nette de 119 mol N s–1 et agit comme une source de N2. Les flux moyens des éléments organiques dissous et particulaires entre le plateau et la pente indiquent généralement une exportation vers l’océan ouvert. Cependant, les grandes incertitudes associées à ces estimations ne permettent pas de conclure sur la réalité de cette exportation.

International audience

Sediment-water dissolved inorganic nitrogen fluxes were measured monthly between November 1997 and January 1999 at two sites on the continental shelf in the Gulf of Lions. Exchanges of ammonium, nitrate and nitrite were measured using whole core incubations performed in the laboratory at in situ temperature and in the dark;. Stations were located on the shelf and near the shelf break. Nitrate release represented the major part of total dissolved inorganic nitrogen fluxes (52-98%). At station Soft, nitrate flux doubled from winter-early spring to August (4 to 10 mu mol m(-2) h(-1)). At station R, a large increase (2 to 14 mu mol m(-2) h(-1)) was recorded between December 1997 and January 1998, after which flux tended to decrease. Ammonium flux was generally directed out of the sediment, and was lower at station Sofi than at station R. Nitrite flux was always less than 1 mu mol m(-1) h(-1). A large part of the temporal variability observed at station R could be attributed to the input of organic matter associated with the discharge of the Rhone River. On the contrary, the influence of the Rhone River is limited at station Sofi, and the spring phytoplankton bloom deposition was probably the major cause of temporal variability. Comparisons of total fluxes with diffusive fluxes suggested a high nitrification rate in the upper centimetre of the sediment. Annual dissolved inorganic nitrogen release from sediments, equivalent to 20-34% of the Rhone River inputs, may contribute 4-8% of the requirements for primary production in the Gulf of Lions., Afin de décrire la variabilité temporelle des flux d’azote inorganique dissous sur le plateau continental du golfe du Lion, un suivi mensuel (novembre 1997–janvier 1999) a été effectué à deux stations. Les flux de NH4+, NO3–, NO2– ont été mesurés par incubation de carotte. Le flux de nitrate hors du sédiment représente la majeure partie des échanges (52–98%). A la station Sofi, les flux de nitrate ont doublé de la période hiver–début de printemps jusqu’au mois d’août (4 à 10 μmol m–2 h–1). Pour la station R, une forte augmentation (2 à 14 μmol m–2 h–1) a été enregistrée entre décembre 1997 et janvier 1998, après laquelle les flux ont tendance à diminuer. Les flux d’ammonium sont généralement dirigés vers la colonne d’eau, plus faibles à la station Sofi qu’à la station R. Les flux de nitrite sont toujours inférieurs à 1 μmol m–2 h–1. Une large partie de la variabilité temporelle observée à la station R a pu être attribuée à l’apport de matière organique consécutif à des débits rhodaniens maximaux. Au contraire, l’influence du Rhône à la station Sofi est limitée et la sédimentation de l’efflorescence phytoplanctonique printanière est certainement la cause majeure de la variabilité temporelle. La comparaison des flux totaux avec les flux diffusifs suggère des taux de nitrification élevés dans le centimètre superficiel du sédiment. En se basant sur l’évolution temporelle de ces deux stations, le flux d’azote inorganique dissous en provenance des sédiments, équivalent à 20–34% des apports rhodaniens, pourrait fournir de 4 à 8% de la demande annuelle pour la production primaire sur l’ensemble de l’année dans le golfe du Lion.

We present results of an intensive sampling program designed to measure weekly changes in ecosystem respiration (oxygen consumption in the water column and sediments) around the 1996 spring bloom in South San Francisco Bay, California, USA. Measurements were made at a shallow site (2 m, where mean photic depth was 60% of the water column height) and a deep site (15 m, mean photic depth was only 20% of the water column). We also estimated phytoplankton primary production weekly at both sites to develop estimates of net oxygen flux as the sum of pelagic production (PP), pelagic respiration (PR) and benthic respiration (BR). Over the 14 wk period from February 5 to May 14, PP ranged from 2 to 210, PR from 9 to 289, and BR from 0.1 to 48 mmol O2 m-2 d-1, illustrating large variability of estuarine oxygen fluxes at the weekly time scale. Pelagic production exceeded total respiration at the shallow site, but not at the deep site, demonstrating that the shallow domains are net autotrophic but the deep domains are net heterotrophic, even during the period of the spring bloom. If we take into account the potential primary production by benthic microalgae, the estuary as a whole is net autotrophic during spring, net heterotrophic during the nonbloom seasons, and has a balanced net metabolism over a full annual period. The seasonal shift from net autotrophy to heterotrophy during the transition from spring to summer was accompanied by a large shift from dominance by pelagic respiration to dominance by benthic respiration. This suggests that changes in net ecosystem metabolism can reflect changes in the pathways of energy flow in shallow coastal ecosystems

Numerical modeling is more and more considered as a complementary but useful tool in marine ecosystem studies. Since Riley (1965) or Steele (1962), simulation models were developped for many different purposes: eutrophication studies (Jorgensen 1986), microbiotique ecology (Salomon and Pommepuy 1991), North Sea ecology (Fransz et al. 1991) or oyster culture (Bacher 1989). The aim of this study is to describe the application of a vertical microbiological processes model to the Gulf of Fos (France), an estuarine system under freshwater input influence, and to make some conclusions that has been drawn from the model regarding the sources and sinks of nitrogen and organic carbon in the Gulf.

Benthic oxygen uptake and nutrient releases of N, P and Si were measured weekly at 2 sites in South San Francisco Bay around the 1996 spring bloom. Exchanges across the sediment-water interface were estimated from whole core incubations performed in the laboratory at in situ temperature and in dark. Fluxes changed significantly on a weekly time scale. Over a period of 15 wk the fluxes of dissolved inorganic N, P and Si ranged from -40 to +200, 0 to 13 and from 30 to 400 µmol m-2 h-1 respectively. Sediment oxygen demand increased from 10 before to 64 mg O2 m-2 h-1 just after the bloom period. During the bloom, nutrient fluxes represented about 20, 16 and 9% of the Si, P and N requirements for primary production. Before and after the bloom period, Si fluxes contributed up to 30 and >100% of this requirement and P and N fluxes up to 15 and 50% respectively. Simple empirical models explain most of the spatial-temporal variability of benthic fluxes of Si, P and NH4 (but not NO3) from 3 predictor variables: sediment porosity, nutrient concentration in bottom waters and chlorophyll content of surficial sediments. These models show that algal blooms influence benthic-pelagic nutrient exchange through 2 processes: (1) depletion of nutrients from the water column (which enhances gradient-driven transports across the sediment-water interface) and (2) sedimentation of labile phytodetritus (which promotes remineralization in or on the surficial sediments). Rates and patterns of nutrient cycling were very different at the shallow and deep study sites, illustrating the challenge of extrapolating measurements of coupled algae-nutrient dynamics to whole ecosystems.