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

As part of a French National Program aiming at better understanding and modelling the coastal eutrophication process, the Bay of Seine (Eastern Channel) has been chosen as a typical case of phytoplanktonic eutrophication in a river plume. Flow rates and nutrient concentrations in the river Seine show a continuous increase of N loadings during the last decade, whereas Si loadings are stationary and P loadings decrease, due to a reduction of industrial discharges. Seasonal cruises made in 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 horizontal advection, vertical stratification, meteorologically-induced versus man-induced variations of nutrient loadings in the global eutrophication process, the mathematical modelling approach was applied. Based 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 built thanks to IFREMER's ''ELISE'' software. Using 1978 data, a simple model of N and Si biogeochemical cycles first revealed the main importance of vertical haline stratification in the phytoplanktonic proliferation process. Then, simulations combining rainy (1988) or dry (1990) discharge regimes with increasing nitrogen mean levels showed that the interannual 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.