Modélisation du devenir à moyen terme dans l'eau et le sédiment des éléments majeurs (N, P, Si) rejetés par la Seine en Baie de Seine

The Bay of Seine is a coastal ecosystem submitted to a large river input (Seine river), which brings important quantities of nutrients. This nutrient enrichment leads to frequent eutrophication events and, from the beginning of the eighties, regular toxic phytoplanktonic (Dynophysis) blooms are observed. In order to study the nutrient cycles (nitrogen, phosphorus, silica) and their contribution to phytoplanktonic stock regulation, a three dimensional model has been developed, based on the coupling between an hydrodynamic part taking into account sediment dynamics (SAM-3D) and an ecological part, which accounts for the main biological compartments and processes. This model is able to reproduce the main physical and biological characteristics of the ecosystem. The study of the phytoplanktonic growth limiting factors has shown the influence of light limitation in the Seine plume due to high concentrations of suspended matter but also quantified the limitation by each of the three nutrients. In spite of nitrogen loading increase during the last twenty years, no phytoplanktonic stock increase has been observed. This may be explained by the phosphorus loading reduction during the same period : the model shows the reversing influence of nitrogen and phosphorus limitations. Furthermore, according to the model results, it seems that flagellate occurrence in the bay (which some species should be toxic) is highly correlated with low Si/N and Si/P ratio in the Seine river inputs. Finally, thanks to the model, nitrogen and phosphorus loading reductions have been tested to estimate their impact on phytoplanktonic production.