Modelling eutrophication events in a coastal ecosystem. Sensitivity analysis

To study the eutrophication of Vilaine bay, we developed a three-dimensional hydrodynamic model and a biological model simulating phosphorus and nitrogen cycles and dissolved oxygen. To couple these models, advective currents, calculated by the hydrodynamic model, were averaged over time and space. The resulting ecological box model simulates seasonal evolution of nutrients, phytoplankton and oxygen reasonably well and reproduces the bayʹs spatial heterogeneity. Following the calibration step, the modelʹs behaviour was analysed by means of various sensitivity studies, which led to several conclusions about eutrophication of 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 bay can be considered as a closed system where the river Vilaine is the primary source of nutrients. Oxygen depletion is mostly due to consumption in the water column (plankton community respiration and remineralization). Reduction of nitrogen inputs from the river are more effective in reducing phytoplankton biomass and hypoxia than reduction in phosphorus loadings. Better knowledge of the benthic stocks of nutrients and processes controlling them is required to confirm the previous results. Spatial and temporal scales used in the model allow the simulation of seasonal trends but cannot reproduce episodic events.