Three dimensional (3D) ecological modelling of the "baie de Seine" (English Channel, France).

Philippe CUGIER
IFREMER/Centre de Brest
DEL-EC-BB
B.P. 70
29280 Plouzané (France)
tel: (33) 02-98-22-49-19
fax: (33) 02-98-22-45-48
Email: pcugier@ifremer.fr

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 with chlorophyll a concentration reaching regularly 40 µg/l. A three dimensional (3D) ecological model is developed and applied to the Bay of Seine area. It consists of a 3D model for hydrodynamics and sediment transport (SiAM3D) coupled to an ecological model simulating nutrient cycles (nitrogen, phosphorus, silicon) and two phytoplanktonic classes (diatoms and flagellates). The model is validated by confronting simulations with nine year long series of measurements and then used to better understand the ecosystem functioning. It quantified the limitation of each of the three nutrients. In spite of nitrogen loading increase from the last twenty years, no phytoplanktonic stock increase has been observed. This may be explained by the phosphorus loading reduction during the same period and the model shows the reversing influence of nitrogen and phosphorus limitation. The model also allows to show the role of the proportion of silicon with regard to nitrogen and phosphorus in the summer loadings of the Seine river on the more or less strong presence of flagellates in the plume. Finally, the model was used to test various hypotheses of reduction of nitrogen and phosphorus loading of the Seine river and their consequences on the phytoplanktonic production in the plume.