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Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

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 (238Pu and 239,240Pu) 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 238Pu and 239,240Pu 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 m 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 238Pu and 239,240Pu concentrations reached 1. 26 and 5. 97 Bq kg−1 respectively and were found within the layer 280–290 cm. The 238Pu/239,240Pu 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 238Pu/239,240Pu 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 238Pu and from 0. 33 to 1. 72 Bq kg−1 for 239,240Pu. The 238Pu/239,240Pu 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 Pu 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. [Copyright &y& Elsevier]

Abstract: 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 50mgl−1. Resuspension fluxes of sediment along the path of the trawls range from 190gm−2 s−1, for the coarsest sediment (clayey silt) to 800gm−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 1mm 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. [Copyright &y& Elsevier]

Sediment properties were studied experimentally using a laboratory flume and sediments sampled from both fresh and saltwater systems, i.e. the Rhône River and the adjacent continental shelf (Golfe du Lion, Mediterranean Sea). An experimental protocol is proposed which includes the development of an analytical method based on a multi-class model in order to determine the erosion and deposition fluxes of these sediments. This approach highlights the presence of a “fluff layer” characterized by a very low critical erosion shear stress and a second layer with a significantly larger critical erosion shear stress. If a global model is used, i.e. only one class of particles is considered, this distinction between the two layers cannot be made as sediment parameters are identified for the whole eroded particles including the “fluff layer”.Critical shear stresses of erosion were 0.05, 0.04 and 0.025 N m-2, respectively, for the first class of particles eroded at Rhône 1, Rhône 2 and SOFI stations. The second class of particles showed critical shear stress 10–20 times higher, around 0.20 N m-2. The use of the multi-class model allowed estimation of the erosion rates in the order of 2.5×10-6, 6.2×10-5 and 2×10-6 kg m-2 s-1 for Rhône 1, Rhône 2 and SOFI. Settling velocities were in the order of 0.012–0.045 mm s-1 for the first class and between 0.05 and 0.09 mm s-1 for the second class.For the river experiments, it is shown that the “fluff layer”, corresponding to the first class of particles being eroded, is mainly representative of recent deposits of suspended particles probably trapped in the overlying water column during sediment sampling. This highlights the difficulties in sampling sediments and transporting these from field to lab and more generally into flumes, and the limits of applying a global model to flume experiments when studying sediment properties. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]