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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.

International audience; The ubiquity of microplastics (MPs) has now been demonstrated throughout Earth’s geographic areas in both terrestrial and marine environments. Marine ecosystems are the end point of the plastic waste cycle, and marine sediments are increasingly considered to be a sink for plastic debris, with possible deleterious effects on seafloor ecosystems. However, the low concentration of MPs in a complex matrix such as marine sediment makes their reliable analysis difficult. MP concentration in marine sediments is usually determined by various extraction procedures followed by optical, spectroscopy or mass spectrometry techniques, and are therefore hard to compare. Therefore, reliable determination of MPs in sediment is a challenging task. Here we present a short review on studies dealing with analytical determination protocols and MP detection in marine sediments and discuss the advantages of the different techniques used. This analysis of the literature reveals that most of the 70 studies were carried out in European and Asian coastal environments. The use of NaCl saturated solution, hydrogen peroxide (H2O2) and Fourier transform infrared spectroscopy (FTIR) are by far the most-used techniques for the different steps of separation, digestion and identification, respectively. Based on this body of literature, we present recommendations and suggestions for future research in order to increase the reliability of results and to facilitate inter-comparison. The use of NaCl saturated and H2O2 solution is strongly recommended for the separation and digestion steps, respectively, whereas during the filtration step it is necessary to choose filter type based on the identification technique. More thorough investigation is needed to establish a systematic protocol for MP identification. A combination of techniques would permit to identify all items, and personal bias could be avoided if automatic identification was implemented. Nanoplastics (NP) occurrence in marine sediments is also discussed, although no data are available to date.

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

International audience; RESUMEN Se estudian, mediante el análisis de observaciones y simulaciones numéricas, las inundaciones causadas por frentes fríos intensos y cómo éstas podrían cambiar en diferentes escenarios de aumento del nivel del mar en la Laguna de Términos, México. El modelo numérico de circulación oceánica utilizado es el Finite-Volume Community Ocean Model (el forzamiento meteorológico proviene de simulaciones de alta resolución con el modelo Weather Research and Forecasting (WRF). Las simulaciones oceánicas se llevaron a cabo utilizando mallas no estructuradas y configuraciones con la posibilidad de inundación y secado en las celdas. Los resultados de las simulaciones numéricas se compararon con observaciones. Teniendo en cuenta el nivel del mar actual, un evento intenso de Norte inunda un área de 940 km 2 , que actualmente está cubierta por manglares. Esto indica que los Nortes son importantes en la irrigación de extensas áreas de manglar. Para los escenarios de aumento del nivel del mar de 0.25, 0.50 y 0.75 m, las áreas inundadas se incrementan en 297, 1062 y 2152 km 2 , respectivamente, lo cual implica un aumento considerable en las áreas inundadas en Isla del Carmen. Se sugiere considerar estos resultados en la planificación a largo plazo. ABSTRACT Floods caused by intense cold fronts and how they could change under different scenarios of sea level rise in the Terminos Lagoon, Mexico, are studied through the analysis of observations and numerical simulations. The numerical ocean model used is the Finite-Volume Community Ocean Model (FVCOM) of the Marine Ecosystem Dynamics Modeling Laboratory of the University of Massachusetts-Dartmouth, and the meteorological forcing comes from high-resolution simulations with the Weather Research and Forecasting model (WRF). The ocean simulations were carried out using unstructured meshes with the possibility of flooding and drying in their cells. Results of the numerical simulations were compared against observations. Considering the current mean sea level, an intense northern cold front event (or Norte) floods an area of 940 km 2 that is currently covered by mangroves. This indicates that Nortes are important in the irrigation of extensive mangrove areas. Under scenarios of sea level rise of 0.25, 0.50 and 0.75 m the flooded areas increase by 297, 1062 and 2152 km 2 , representing a considerable increase in the areas flooded at Isla del Carmen. It is suggested to consider this situation in long-term planning.

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 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.

International audience

The French archipelago of New Caledonia hosts one of the three most extensive reef systems in the world and was partly added to the UNESCO World Heritage List in 2008. In the very specific context of rapidly growing land-based mining extraction and ore processing activities, industrial managers, political decision makers, lagoon users and citizens have been expressing an increasing demand for the implementation of efficient environmental monitoring programs dedicated to the effects related to mining activities. As a result, New Caledonia lagoons were thoroughly studied particularly during the last two decades through national research projects. This chapter aims at reviewing the state-of-the-art of knowledge about the general features of New Caledonia geography and climatology, lagoon environmental drivers including hydrodynamics, pelagic and benthic processes, and the potential stress of terrestrial and anthropogenic inputs on marine communities. Insights addressing economic, ecological and political issues related to the complex status of self-governed New Caledonia are discussed with respect to living resources and environmental management.

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.

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.

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.

A recent companion paper reported an experimental protocol used to analyze sediment properties. This protocol identified for both freshwater and marine sediments a surface layer with specific dynamic properties (critical erosion shear stresses in the range 0.025-0.05Nm-2) and a second layer with critical erosion shear stresses about ten times larger. The present study compares these former results with recent work which extended the applicability domain of the Shields diagram to very fine particles. The surface layer is shown to consist in fine and unconsolidated sediments that behave like noncohesive material whereas the second layer is characterized as being cohesive. The surface layer is mainly representative of recent deposits of suspended particles. This points out the existence of a fluffy layer of fine sized particles resting near the bed, with specific erosion characteristics, which has to be considered separately when studying sediment properties. © 2007 ASCE.

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; no abstract

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.

A study coupling core incubation and a large number of in situ microelectrode measurements (n=18) was performed at a single station in the Golfe de Fos, a semi-enclosed estuarine system in the South of France on the Mediterranean Sea. Total and diffusive oxygen fluxes were determined for two different seasons (October 2000 and May 2001). The average total and diffusive oxygen fluxes were 21.5 and 12.7 mmol m−2 day−1, respectively, in October and 18.5 and 16.5 mmol m−2 day−1, respectively, in May. The average total oxygen demand does not vary significantly between the two periods (21.5±5 to 18.5±4 mmol m−2 day−1). The spread of the values obtained by microelectrode measurements reveals the variability at the decimeter scale indicating a control by small-scale processes. Little difference was found in May between the total and diffusive oxygen fluxes when the number of microelectrode profiles (n=18) and core incubation (n=9) was sufficient to catch larger fluxes linked to "organic hot spots". Although present at a population density of 5700 individuals for 0.1 m2, macrofauna is mostly composed of surface deposit feeders. The little difference found between total and diffusive oxygen demand points towards not only the population density but also the faunal assemblage as a determining factor for sediment irrigation

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

Critical shear stress of erosion and erosion rate of particulate inorganic and organic matter were measured in a flume at three muddy stations. Critical shear stress ranged between 0.022 and 0.038 N m −2 . At the deepest site, annual erosion of particulate organic nitrogen and phosphorus could exceed net deposition fluxes, showing the importance of erosion processes. Erosion may modify total system mineralisation rates by introducing sedimentary particulate organic matter into the water column and thus this process must be taken into account in studies of biogeochemical cycles. To cite this article: E. Schaaff et al., C. R. Geoscience 334 (2002) 1071–1077.

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