Three-dimensional (3D) ecological modelling of the Bay of Seine (English Channel, France)

A three-dimensional model, based on the coupling between a hydrodynamic part taking into account sediment dynamics (SiAM-3D) and an ecological part which accounts for the main biological compartments and processes, has been developed to study nutrient cycles (nitrogen, phosphorus, silica) and their contribution to phytoplankton stock regulation in the Bay of Seine (French coast of the English Channel). The role of each nutrient in phytoplankton production has been identified. Spring phosphorus and summer silicate limitation of diatom growth is observed and confirmed by the model. The simulations show a shift from spring nitrogen to spring phosphorus limitation of diatom growth between the 1970s and the 1990s which is related to an increase in Seine river nitrogen loading and a decrease in phosphorus. Several scenarios of nutrient input reductions were investigated and show a greater sensitivity of flagellates to these reductions. The use of an original numerical method to track the nutrient coming from different sources shows the non-negligible impact of offshore inputs in annual plankton production. In some cases, 50% of the annual diatom production could be related to open sea nitrogen loading. The annual production of flagellates supported by the open sea loading did not exceed 36%.