MODELING EUTROPHICATION EVENTS IN A COASTAL ECOSYSTEM - SENSITIVITY ANALYSIS

To study the eutrophication of Vilaine bay, we developed a three-dimen sional hydrodynamic model and a biological model simulating phosphorus and nitrogen cycles and dissolved oxygen. To couple these models, adv ective currents, calculated by the hydrodynamic model, were averaged o ver time and space. The resulting ecological box model simulates seaso nal evolution of nutrients, phytoplankton and oxygen reasonably well a nd reproduces the bay's spatial heterogeneity. Following the calibrati on step, the model's behaviour was analysed by means of various sensit ivity studies, which led to several conclusions about eutrophication o f the bay. In winter and spring, the Vilaine bay behaves like an open system enriched by the Loire plume. During the rest of the year, the b ay can be considered as a closed system where the river Vilaine is the primary source of nutrients. Oxygen depletion is mostly due to consum ption in the water column (plankton community respiration and reminera lization). Reduction of nitrogen inputs from the river are more effect ive in reducing phytoplankton biomass and hypoxia than reduction in ph osphorus loadings. Better knowledge of the benthic stocks of nutrients and processes controlling them is required to confirm the previous re sults. Spatial and temporal scales used in the model allow the simulat ion of seasonal trends but cannot reproduce episodic events.