Human pressures on ecosystems have increased signif icantly over the last decades, and especially Mediterranean coasts are strongly impacted by the d ev lopment of big cities and industrialized area, s uch as the Bay of Marseille. Prior to any investigation s on ecological or contamination impacts, it was necessary to understand sediment dynamics and its r esponse to natural extreme events such as storm or heavy rainfall events. In situ observations were de dicated to understand the behavior of bed sediments , a d their spatial distribution. A benthic station was d eployed for three months to observe the impact of e xtreme events on hydrodynamics and sediment dynamics in a critical zone of the study area. These measurements were used to calibrate and validate the RHOMA MARS3 D/WWIII configuration, both in terms of hydrodynamics, waves and sediment dynamics. Numeric al simulations were then analyzed in order to examine the influence of extreme meteorological eve nts on the Marseille coastal physical ecosystem.

Human pressures on ecosystems have increased significantly over the last decades, and especially Mediterranean coasts are strongly impacted by the development of big cities and industrialized area, such as the Bay of Marseille. Prior to any investigations on ecological or contamination impacts, it was necessary to understand sediment dynamics and its response to natural extreme events such as storm or heavy rainfall events. In situ observations were dedicated to understand the behavior of bed sediments, and their spatial distribution. A benthic station was deployed for three months to observe the impact of extreme events on hydrodynamics and sediment dynamics in a critical zone of the study area. These measurementswere used to calibrate and validate the RHOMA MARS3D/WWIII configuration, both in terms of hydrodynamics, waves and sediment dynamics. Numerical simulations were then analyzed in order to examine the influence of extreme meteorological events on the Marseille coastal physical ecosystem.

The present contribution aims to provide an insight into the sediment dynamics of the Bay of Marseille (BoM), France, an area characterised by a mostly rocky, steep-sloped, and protected shoreline. In terms of sediment composition, the northern part of the study area is dominated by fines, whereas sands of mean grain size 0.2 < d(50) < 1 mm occur at most other locations. Posidonia oceanica meadows occupy many nearshore areas. Critical bed-shear stress was determined through tests in a unidirectional flume using 15 natural cores. Measured values ranged from 0.04 N/m(2) to 0.46 N/m2 and were expressed as a function of the logarithm of d50. Mud content had no observable effect on sediment cohesion, whereas a weak positive trend between critical shear stress and clay content was discerned. Autonomous benthic stations (Acoustic Doppler Current Profiler/Conductivity, Temperature, Depth/Turbidity/ALTUS Data Collection System) were deployed for periods of 2-3 months, and several suspension and erosion events were identified for significant wave heights exceeding 1.5 m. Grain-size trends showed that the protected BoM appeared to favour accumulation of sediments advected from the surrounding areas. The southern sector of the BoM was found to be the area with the highest sediment agitation and erosion rates but with weak transport trends. The northern sector, exposed to dominant SW waves, was shown to be the second most active area in terms of sediment dynamics, whereas the several Posidonia oceanica meadows and sheltered locations along the study area appeared to be possible sediment sinks. The latter included locations close to port installations and the western part. of the Cortiou area, known to have water quality issues, which, according to the present findings, are linked to low sediment mobility and/or dispersion of pollutants.

Results of a large number of erosion tests on artificially generated and relatively dense sand–mud mixtures are presented. Soil sample compositions are varied concerning clay–silt and sand–silt ratio, and clay mineralogy. The experimental set-up consists of a re-circulating small-scale rectangular erosion flume with unidirectional flow conditions. The erosion threshold and erosion rate are studied through step by step increasing the flow rate during a test. Results clearly indicate time-decreasing erosion during which individual flocs are randomly eroded, and time-independent (steady) erosion during which both sand and mud particles are continuously and uniformly eroded. These two erosion types appear to be floc and surface erosion, respectively (Winterwerp and Van Kesteren, 2004). Floc erosion relates to the stochastic character of both the flow conditions and (surficial) sediment strength, whereas surface erosion relates to the plasticity index, which is a bulk soil mechanical parameter characterizing cohesiveness. The surface erosion threshold is discussed following a geotechnical approach, which argues that surface erosion is a drained process. This implies that cohesiveness rather than packing density is important for the erosion threshold, which is confirmed by the experimental data. Simultaneously with the erosion tests, also the undrained shear strength of the applied soil samples was determined. A model is proposed and validated to predict the undrained shear strength as function of the granular porosity in combination with the plasticity index. The comparison of the undrained shear strength with the surface erosion threshold further confirms the applicability of a geotechnical approach to understand the erosion of mixed sediments. Finally, the study provides a valuable data set that can be used as a reference for future research on erosion behavior of (natural) sediment mixtures.

The present contribution aims to give insight into the sediment dynamics of the Bay of Marseille, under the general context of assessing the origin and fate of pollutants. Towards the objectives of the study, bed cover information was obtained from sediment samples and erodibility tests in flumes; while in-situ measurements (ADCP/ /Turbidity/bed level) were combined with satellite images and hydrodynamic models. The results showed that a) the estimated critical bed shear stress was in general lower than the one obtained following a theoretical Shields approach; b) sediment suspension was wave dominated and for conditions beyond the 90% annual exceedence values, bottom sediments can be active along most of the study area. Sinks and sources are scarce and fine suspended sediments from the Rhone River plume can episodically contribute to the sediment budget of the Bay of

Natural sediments are often mixtures of cohesive and non-cohesive sediment, and there are few data on the specific behaviour of these sediments, in particular, their erodibility. After a brief synthesis of recent works on the erosion threshold and the erosion rate of non-cohesive, cohesive and mixed sediments, the erodimetre, a new erosion device developed by IFREMER, is described. The instrument is portable and enables the separate quantification of the erosion of mud and sand fractions. Four data sets are presented, consisting of erosion tests on pure sands, laboratory mixtures with two types of mud and well-sorted sands, and natural mixed sediments. A clear relationship between the critical shear stress for erosion and the mud volume fraction (over the whole range) is shown. The correlation is not so good with the clay fraction. When the sand is fine (140 μm), the relationship is linear. When the sand size increases (280 μm), a sharp transition from non-cohesive to cohesive behaviour appears when the mud fraction exceeds 35–40%. It is suggested that the ratio between the grain sizes of the sand and the fine fractions, and more generally the whole size spectrum of the sediment, should be considered to characterise the sediment erodibility. This first interpretation extends the conceptual framework for the erosion behaviour described by Van Ledden et al. (2004).

Mont Saint Michel Bay is a 30 km wide bay located on the French side of the English Channel. The tidal range reaches 14 m during spring tides, which leads, along with the gentle slope of the bathymetry, to a tidal flat up to 11 km wide. The sedimentary coverage exhibits strong longshore and cross-shore gradients, with purely muddy environments to the west of the domain, and pure sands to the east, where the natural channels of three incoming rivers induce rapid morphological changes. The contributions of tides and waves to sedimentary processes are analysed by means of numerical modelling and results of field observations. Maximum tidal bottom shear stresses are shown to account for the sediment distribution throughout the bay, while the longshore gradient in wave intensity seems to drive the amount of suspended sediment concentration. The bay has been a priviledged ground for shellfish farming for over a century. Farming structures (oyster tables, mussel posts and wooden fences used as permanent fishing nets) have significantly hindered natural flow patterns, creating quiescent areas which significantly increase fine sediments deposits in farming areas. The paper focuses on introducing into a numerical model the effects of mussel farms on flow circulation and sediment dynamics.

The French Atlantic shelf is subjected to strong anthropic influences (urban, industrial and agricultural discharges) of two main rivers (Loire and Gironde).The extension and consequences of these continental loadings for the nutrient and chlorophyll distribution have never been studied before on the Bay of Biscay continental shelf as a whole.We present the first synoptic view of the nutrient distribution and evolution on the French Atlantic shelf.Nutrient concentrations of the surface layer were studied during four cruises in April, June, September 1999 and March 2000.Until June, the freshwater inputs induce a nitrate gradient from river mouths to offshore waters in the vicinity of the 100 m isobath.The Redfield's ratio study highlights the nitrate excess in river loadings.The early spring situation is characterised by high N:P ratios in front of the two estuaries and by a potential Si-limitation in the northern part.Nitrate removal continues in spite of the Plimitation and the increase in silicate concentrations during summer supposes high regeneration processes.At the end of summer, the water column is thermally stratified and the surface mixed layer is totally depleted in nitrate.

As part of the Programme National d'Oceanographie Cotiere, the nutrient dynamics of the Bay of Seine were studied between 1992 and 1994 in order to complement work on ecological modelling. Firstly, the River Seine's nutrient fluxes were established: 80 000-130 000 t a(-1) of dissolved inorganic nitrogen, 6 400-8 400 t a(-1) of dissolved phosphorus and 20 000-77 000 t a(-1) of dissolved silicium. Estuarine processes were taken into account. Consequences of nutrient loading for the bay were then evaluated at the pelagic level (nutrient and chlorophyll enrichments) and the benthic level (distribution of various phosphorus fractions in superficial sediments). The large continental inputs always induce concentration gradients in the water from the mouth of the river to the northwest of the bay. The northward spreading of fine particulate matter controls the distribution of adsorbed phosphate and iron-aluminium bound phosphate in sediments. In contrast, calcium bound phosphate, the main fraction in sediments, is not affected by river inputs. Organic phosphorus in sediments is related to phytoplankton blooms, with increasing concentrations during productive periods; afterwards the fast recycling prevents sedimentary accumulation. The nutrient depletions observed beyond the turbid plume during spring 1992 enabled the calculation of nutrient uptake rates, and the comparison of these rates with phytoplankton biomass (chlorophyll a + phaeopigments). Ratios of nutrient consumption to pigment concentration were estimated at 1 mu mol mu g(-1) for nitrogen, 0.05 for phosphorus and 0.5 for silicate. These values, as N/P and Si/N uptake ratios (respectively 17.5 and 0.4), were similar to usual values. (C) Elsevier, Paris.

Afin d'analyser la réponse de l'écosystème « Rade de Brest » à l'enrichissement du milieu, un suivi du cycle des concentrations en sels nutritifs et en chlorophylle a été réalisé en 1993 et ces résultats ont été comparés à des études antérieures. Bien que les apports de nitrates aient doublé en 20 ans, occasionnant une diminution des ratios Si/N, les stocks phytoplanctoniques n'ont pas augmenté durant cette période. L'analyse des données ainsi que la modélisation écologique ont permis de mettre en évidence plusieurs facteurs de régulation du milieu, contribuant à expliquer la relative stabilité de cet écosystème malgré l'augmentation des apports de nitrate.

(Revise le 19 mars 1998, accept6 le 27 mars 1998) Abstract - Nutrients and phytoplanktonic growth in the Bay of Seine, France. As part of the Programme National d'oceanographie C&i&e, the nutrient dynamics of the Bay of Seine were studied between 1992 and 1994 in order to com- plement work on ecological modelling. Firstly, the River Seine's nutrient fluxes were established: 80 000-130 000 t a-' of dissolved inorganic nitrogen, 6 400-X 400 t a-' of dissolved phosphorus and 20 000-77 000 t 6' of dissolved silicium. Estuarine processes were taken into account. Consequences of nutrient loading for the bay were then evaluated at the pelagic level (nutrient and chlorophyll enrichments) and the benthic level (distribution of various phosphorus fractions in superficial sediments). The large continental inputs always induce concentration gradients in the water from the mouth of the river to the northwest of the bay. The northward spreading of fine particulate matter controls the distribution of adsorbed phosphate and iron-aluminium bound phosphate in sediments. In contrast, calcium bound phosphate, the main fraction in sediments, is not affected by river inputs. Organic phosphorus in sediments is related to phytoplankton blooms. with increasing concentrations during productive periods; afterwards the fast recycling prevents sedimentary accumulation. The nutrient depletions observed beyond the turbid plume during spring 1992 enabled the calculation of nutrient uptake rates, and the comparison of these rates with phytoplankton biomass (chlorophyll N + phaeopigments). Ratios of nutrient consumption to pigment concentration were estimated at 1 umol pg-' for nitrogen, 0.05 for phosphorus and 0.5 for silicate. These values, as N/P and Si/N uptake ratios (respectively 17.5 and 0.4), were similar to usual values, 0 Elsevier. Paris