Modélisation sur vingt ans (1976-1995) de la production phytoplanctonique en baie de Seine

This paper presents an ecological modelling of the Bay of Seine (Eastern Channel) over the last twenty years, chosen as a typical case of eutrophication in a river plume. In the physical sub-model, the Bay is divided into 42 boxes and water fluxes between them are calculated automatically using Ifremer's "Elise" software. A two-layer, vertical thermohaline model is then linked with the horizontal circulation scheme in order to take vertical stratification into account. The biological submodel deals with two chemical elements, nitrogen and silicon, and splits phytoplankton into diatoms and flagellates. Results from this ecological model point out the spatial concordance of highest phytoplanktonic concentrations with the river plume spreading in the bay. Contrary to diatoms, flagellate production appears to be mainly confined to the eastern bay, due to the vertical haline stratification in front of the river mouth. As far as the whole bay is concerned, the interannual fluctuations of diatom production are related to the level of spring insolation, whereas silicon inputs regulate diatom production in the river plume. The flagellate summer production in the plume is enhanced by high water temperature and high N/Si ratios, which appear during dry years with low discharge regimes. Finally, interannual increase of flagellate production could be related to gradual increase of nitrogen loadings, contrasting with silicon loadings, which remained stable for twenty years.