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In order to improve bacterial water quality of shellfish farming areas, studies were conducted in the English Channel (Morlaix) and on the Mediterranean coast (Toulon). These two areas were chosen in order to compare behaviour of fecal bacteria in two different ecosystems. In the estuary of Morlaix sediments are polluted by way of settlement, but most of the bacteria are mixed with turbid waters and are able to survive a very long time (T90 are in a range of several hours to several days). By measuring the increase in salt tolerance of the strains grown in natural organic matter, it was demonstrated that Salmonella can tolerate coastal water salinities. Moreover, because light penetration is prevented by suspended matter, the solar bactericidal effect is very low. On the contrary, through lack of nutrients and very high sunlight intensity, die-off rates in Mediterranean waters are very high (at the surface T90 are less than 2 hours, and several hours in deep waters). A close relationship was found between the light intensity received by bacteria and the T90. Predicted T90 must be found using these two parameters(including turbidity and deep effect on light intensity). The authors suggest that precautions must be taken to carry out impact studies depending on water quality of the area, especially in turbid areas. The knowledge of these mechanisms is very important to evaluate waste water impact on the quality of shellfish farming areas, and to improve elimination of fccal bacteria in sewage treatment plants.

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A phytoplankton bloom was observed in late winter 2000, on the continental shelf offshore of southern Brittany, in northwestern Bay of Biscay. This bloom appeared initially along the 120-m isobath, in stratified and clear waters, at the interface between the oceanic water and the plumes of southern Brittany rivers (mainly the Loire and Vilaine). The development of the bloom was triggered by favourable meteorological conditions, characterised by solar irradiance reaching the maximum level expected for that period of the year. Outside of the bloom area, the phytoplankton photosynthesis was irradiance limited: inshore, because of the stronger attenuation of the light; offshore, because of the weak stratification. The hydrological conditions at the onset of the bloom were observed in the field, during the oceanographic cruise MODYCOT. However, without SeaWiFS, the only observations related to this major event in the primary production would have been those of the coastal phytoplankton network (REPHY (REseau PHYtoplankton)). Observed initially offshore by SeaWiFS, as early as March 5, 2001, the phytoplankton bloom extended onshore and was observed at REPHY stations by mid-March. On, March 15, fishermen reported that they were handicapped in trawling, the presence of mucilage in their nets carried clogging by very heavy slime. This mucilage has been shown to be produced by the diatom Coscinodiscus and, especially, C. wailesii. These observations were made as the main bloom was declining. A simple calculation, based upon the SeaWiFS chlorophyll concentration maps observed from March 5-16, shows that phosphate was probably totally depleted offshore, by March 16. The former bloom area appeared very poor in chlorophyll on the SeaWiFS image of April 7, which was the first image available after 3 weeks of an overcast sky. A second cruise undertaken at the end of April confirmed the phosphate depletion and the low chlorophyll concentration in that area.

As part of the French National Programme for Coastal Oceanography, this paper focuses on improvement of biogeochemical modelling in the Bay of Seine (Eastern Channel), by introducing phosphorus in nutrient cycles. The Bay of Seine receives the Seine river, which exhibits very high nutrient concentrations, and this coastal zone constitutes a typical case of eutrophication in a river plume area. In terms of analyses, sequential extraction and analysis of sedimentary phosphorus were used in order to measure various forms of particulate phosphorus in suspended matter and sediment (calcium-bound phosphate, Fe/Al-bound phosphate, exchangeable phosphate and organic phosphorus). In the modelling approach, the Bay is divided into 42 boxes and a two-layer, vertical thermohaline model is linked with the horizontal circulation scheme to take vertical stratification into account. The previous biological sub-model, with only nitrogen and silicon as nutrients, was improved by taking account of (a) various forms of bioavailable phosphorus (dissolved phosphate, exchangeable phosphate and organic phosphorus) and (b) experimental parameters which govern the adsorption/desorption of phosphate at the solid-water interface. The ecological model accurately reproduces dissolved nutrient behaviour in the river plume, and concentrations of diatoms, flagellates and zooplankton are consistent with observed values. Simulated exchangeable phosphate and organic phosphorus in suspended matter are in the range of measured concentrations, whereas these particulate variables are simulated with less accuracy in the sediment. After evaluating the respective roles of nitrogen, phosphorus and silicon as limiting factors for phytoplanktonic growth, the model was used to estimate consequences of reduced phosphorus or nitrogen input for phytoplanktonic production in the Bay of Seine.