MODELING THE EUTROPHICATION PROCESS IN A RIVER PLUME - THE SEINE CASE-STUDY (FRANCE)

As part of a French National Program aiming at better understanding an d modelling the coastal eutrophication process, the Bay of Seine (East ern Channel) has been chosen as a typical case of phytoplanktonic eutr ophication in a river plume. Flow rates and nutrient concentrations in the river Seine show a continuous increase of N loadings during the l ast decade, whereas Si loadings are stationary and P loadings decrease , due to a reduction of industrial discharges. Seasonal cruises made i n 1978 and 1992 showed the nutrient enrichment of the eastern part of the Bay and the high chlorophyll content of the plume area, especially in the surface layer. In order to quantify the respective roles of ho rizontal advection, vertical stratification, meteorologically-induced versus man-induced variations of nutrient loadings in the global eutro phication process, the mathematical modelling approach was applied. Ba sed on a one nautical mile mesh size map of computed lagrangian tidal residual currents, a box-model of the whole Bay of Seine has been buil t thanks to IFREMER's ''ELISE'' software. Using 1978 data, a simple mo del of N and Si biogeochemical cycles first revealed the main importan ce of vertical haline stratification in the phytoplanktonic proliferat ion process. Then, simulations combining rainy (1988) or dry (1990) di scharge regimes with increasing nitrogen mean levels showed that the i nterannual fluctuation of the global marine phytoplanktonic production in the bay is mainly conditioned, through run-off, by the interannual meteorologic variations, which blur the relatively weaker effect due to anthropogenic gradual increase of nitrogen loadings.