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Local and global anthropogenic pressures due to climate change and to local uses and activities are exerting significant cumulative impacts to greater extents of the oceans and seas. Coastal ecosystems are particularly threatened by the intensity and coexistence of several marine uses and pressures, including sewage and urban constructions, tourism, ship traffic, fisheries and aquaculture. Assessment of pressures and the identification of mitigation measures are key urgent actions, as already highlighted by the EU Marine Strategy Framework Directive and the United Nations Sustainable Development Goal 14. The aim of this work, developed within the Interreg-Med project SHAREMED, is to systematize existing knowledge on threats and pollution, including those of transboundary origin, for long term strategies and common action marine spatial planning, jointly developed with stakeholders. The quest is to assess coexisting environmental threats, and their propagation in space and time, at proper spatial and temporal scales, according to the type and action of each stressor (i.e. global vs. local). Cumulative pressures are tackled within a dedicated Atlas comprising three sub-basinsins of the Mediterranean Sea: the North Adriatic Sea, the Sicilian Channel and the North-Western region. The Atlas integrates information generated at the best available resolutions by 1) in-situ sampling, 2) remote observations, 3) numerical models, and 4) focusing on target ecosystems and habitat forming species. These sub-basins are subjected to multiple local and larger scale (e.g. climate) pressures that propagate in space and time, and across political boundaries, that need to be addressed through coordinated actions, based on evidence-rooted common understanding. Interactions with relevant Stakeholders, solicited through an online survey, and meetings, were used to select target ecosystems and to identify the key relevant pressures. The Atlas is based on open-access databases and portals, literature reviews and from ad-hoc model simulations concerning marine heatwaves, ship traffic, oil pollution, marine litter and fishing efforts. We will present the main preliminary results and needs and gaps in observations related to marine ecosystems threats.

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.

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

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 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.

The dispersion and fate of the Rhone River inputs to the Gulf of Lions (Northwestern Mediterranean Sea) have been studied through the spatial and temporal distributions of plutonium isotopes in continental shelf sediments. Plutonium isotopes (Pu-238 and Pu-239,Pu-240) are appropriate tracers to follow the dispersion of particulate matter due both to their high affinity for particles and their long half-lives. In the Rhone River valley, plutonium isotopes originate from both the weathering of the catchment basin contaminated by global atmospheric fallout, and the liquid effluents released from the Marcoule reprocessing plant since 1961.;This work presents a first detailed study on Pu-238 and Pu-239,Pu-240 distributions in sediments from the Rhone prodelta to the adjacent continental shelf, since the decommissioning of Marcoule in 1997. The vertical distribution of Pu isotopes has been analysed in a 4.75 in long core sampled in 2001 at the Rhone mouth. Despite this length, plutonium is found at the last 10 cm, manifesting the high sedimentation rate of the prodeltaic area and its ability for trapping fine-grained sediments and associated contaminants. The highest Pu-238 and Pu-239,Pu-240 concentrations reached 1.26 and 5.97 Bq kg(-1) respectively and were found within the layer 280-290 cm. The Pu-238/Pu-239,Pu-240 activity ratios (AR) demonstrated an efficient and huge trapping of the Pu isotopes derived from Marcoule. The fresh sediments, located on the top of the core, show lower plutonium activity concentrations and lower Pu-238/Pu-239,Pu-240 ratios. This decrease is in close relation with the shut down of the Marcoule reprocessing plant in 1997.;In 2001, plutonium isotopes were also analysed in 21 surface sediments located offshore and concentrations ranged from 0.03 to 0.17 Bq kg(-1) for Pu-238 and from 0.33 to 1.72 Bq kg(-1) for Pu-239,24(0). The Pu-238/Pu-239,Pu-240 AR ranged from 0.24 close to the river mouth to 0.06 southwards, indicating the decreasing influence of the Marcoule releases (global fallout AR 0.03-0.05 and Marcoule AR 0.30). This is in good agreement with the main direction spread of the Rhone River plume and the bottom current. This dataset has been compared to those obtained in the same area in 1984 and 1990 in order to follow the time trend in Pit concentrations. This comparison highlights the decrease with time in plutonium concentrations close to the Rhone River mouth, but further away this reduction is not so evident. (c) 2007 Elsevier B.V. All rights reserved.

International audience

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.

Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists. Our understanding of sediment biogeochemistry and exchange fluxes was revolutionized by the introduction of benthic chambers and in situ micro-electrode profilers that allow for the direct measurement of chemical fluxes between sediment and water at the sea floor and for porewater composition. Since then, rapid progress in the technology of in situ sensors and benthic chambers (such as the introduction of gel probes, voltammetric electrodes or one- and two-dimensional optodes) have yielded major breakthroughs in the scientific understanding of benthic biogeochemistry. This paper is a synthesis of discussions held during the workshop on sediment biogeochemistry at the "Benthic Dynamics: in situ surveillance of the sediment-water interface" international conference (Aberdeen, UK--March 25-29, 2002). We present a review of existing in situ technologies for the study of benthic biogeochemistry dynamics and related scientific applications. Limitations and possible improvement (e.g., technology coupling) of these technologies and future development of new sensors are discussed. There are countless important scientific and technical issues that lend themselves to investigation using in situ benthic biogeochemical assessment. While the increasing availability of these tools will lead research in yet unanticipated directions, a few emerging issues include greater insight into the controls on organic matter (OM) mineralization, better models for the understanding of benthic fluxes to reconcile microelectrode and larger-scale chamber measurements, insight into the impacts of redox changes on trace metal behavior, new insights into geochemical reaction pathways in surface sediments, and a better understanding of contaminant fate in nearshore sediments.

A biogeochemical box model describing the south-west lagoon of New-Caledonia was developed in order to simulate the seasonal cycle of carbon and nitrogen. We used fluxes generated by a 3D hydrodynamic model to simulate horizontal exchanges between boxes and added freshwater influxes as nitrogen sources from the land. Average residence time proved to be less than 11 days for the lagoon as a whole. Standard simulations showed baseline values of chlorophyll a between 0.2 and 0.4 μg*L-1. Influences of freshwater influxes proved to be significant (increases up to 1 μg*L-1) only in shallow areas protected from wind exposure and during short periods of heavy rainfall (tropical depressions). Tropical climatic events have reduced impact in space and time and long-term simulations over decades with increased nutrient inputs did not show any significant process of eutrophication. Hydrodynamics seemed to be one of the major control factors with respect to organic matter cycling 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.

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.

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.

Pigment concentrations (chlorophylla, chlorophyllidea and pheopigmentsa) were measured by HPLC and spectrophotometry with acidification on 57 samples collected in different marine coastal sediments, containing autochthonous microphytes, and with various organic matter contents (plant detritus, biodeposits or hydrocarbons). Statistical analysis shows that the spectrophotometry with acidification, as compared to HPLC, gives reliable values for chlorophylla. Chlorophyllidea concentrations may be considered as negligible. Though spectrophotometric methods are sometimes questioned when applied to sediments they appear to give easy, quick and good estimates of Chla contents in benthic microphytes for hydrobiological studies in coastal areas.

In marine biogeochemical modeling, the sediment is usually represented by diagenetic models, but in shallow ecosystems these models are incomplete, because they do not take into account the benthic primary production. While microphytobenthos (MPB) is known to strongly impact mineralization pathways and nutrient fluxes, MPB is rarely integrated as an explicit variable. To investigate the impact of microphytobenthos on early diagenesis in sediment, we built a fine-scale dynamic model, based on the diagenetic model OMEXDIA and including MPB and associated processes. The model outputs were similar to a data set of MPB-colonized sediment sampled in Florida Bay, suggesting that the model can recreate a realistic situation. The model showed that MPB activities induced a strong diurnal rhythm on concentration profiles, fluxes, and mineralization processes. When MPB was present at the sediment surface, the total mineralization was strongly enhanced thanks to the supply of labile organic matter. In contrast, coupled nitrification-denitrification was inhibited by a factor of 3.8. This inhibition can be explained by the competition for nitrate and ammonium between MPB and bacteria. Nitrogen uptake of MPB represented 96% of the daily supply of dissolved inorganic nitrogen. This was more than 50 times greater than N consumption by denitrification. With MPB, sediment nitrogen flux to the water column was reduced by a factor of 70, suggesting that sediment colonized by MPB represents a minor source of nutrients for phytoplankton and bacterioplankton. Results showed that current diagenetic models are not well-suited for shallow ecosystems with significant MPB primary production. (C) 2010 Elsevier B.V. All rights reserved.

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.

ln order to investigate trophic relationships in a simplified food chain including mussel biodeposition, in situ experiments were performed in an estuarine mussel-rope cultivation (Gulf of Fos). Cylindrical enclosures were placed at a depth of 2 m for one month during winter (sea temperature 10°C). Shrimps, Palaemon serratus, were kept in the cylinders, with and without mussel biodeposits as food supply. A control group was fed on mussel flesh. A comparison between the different biochemical components in both food and feeders was undertaken at three trophic levels; phytoplankton, mussels and shrimps. Under oligotrophic water conditions, shrimps could feed on mussel biodeposits. There were no noticeable differences between shrimps fed on mussel tissue or shrimps fed on mussel biodeposits, either in lipid and protein content, or in free amino acid, fatty acid or carotenoid pigment content. On the other hand, fasted shrimps exhibited significant modifications. The ecological significance of such anenergy flow pathway in intensive shellfish cultivation zones is discussed.

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.

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.

In order to estimate the qualitative variation of sedimented mussel deposits, biochemical and microbial measurements were undertaken after a sediment enrichment with fresh faeces and pseudo-faeces collected in a mussel farming area (Carteau, Gulf of Fos).

Upward nutrient fluxes at the sediment-water interface were studied in a mussel farming zone (Carteau, Gulf of Fos, France) in order to estimate the impact of organic matter input from biodeposition. Nitrate, nitrite, ammonia, silicate, phosphate and oxygen were measured. Fluxes were estimated by means of polyacrylate benthic chambers placed at sites located under (UM) and outside (OM) the rope hanging structures. Transformation of biodeposited organic matter increases phosphate, silicate and ammonia fluxes. No variation in nitrite fluxes could be detected and only minor differences were observed in nitrate and the oxygen production/consumption equilibrium at the two stations. Phosphate and silicate fluxes, which were always higher at the UM than at the OM site, decreased from spring to winter. Ammonia fluxes were very high under mussel cultures in May and September and lower in November. The fact that ammonia flux was always higher at the UM than at the OM sites might be explained by degradation of mussel biodeposit, as well as by benthic macrafauna excretion. Discrepancies between fluxes of the nutrients studied at the UM and OM sites increased as organic particulate matter in the water column decreased. Variations of oxygen flux followed a different pattern, since they were correlated with presence and abundance of photosynthetic microphytes on the bottom and in the water. Bottom respiration exceeded production of oxygen only in May 1988 at the UM station. As it now stands, biodeposit input into the sediment under mussel ropes does not affect the ecosystem, although the flow of nutrients towards the water column is higher than in other areas.

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.