Water fluxes and renewal rates at Pertuis d'Antioche/Marennes-Oléron Bay, France

Water renewal is crucial for removal of pollutants (e.g. oil), organic loadings, and oxygen supply and within shellfish farms. This study presents a 3-D hydrodynamic model with local resolution of 500 m used to quantify water renewal and water fluxes at Pertuis d'Antioche/Marennes-Oléron Bay, France. Open boundary conditions were based on MyOcean products (http://www.myocean.eu/) downscaled using four nested domains with increasing resolution from the large to the local scale. Results from a six-month model run were compared with data giving a good agreement. The spatial variation of water renewal was characterized by dividing the coastal waters into boxes which grouped the main shellfish areas. A Lagrangian model was coupled to the hydrodynamic model to track the history of water renewal. The boxes were filled with tracers and the pattern of particles passing through them was quantified. Model results were integrated over the volume of the boxes to calculate the water fluxes across box boundaries. The results of the integration provide horizontal and vertical fluxes of water which were used to assess the renewal of surface and bottom waters in the bay. Depending on tide and freshwater conditions, the water renewal ranges between 2 and 18 days. Main current patterns were investigated by drawing streamlines in the residual velocity field. It was found that eddies can slow down renewal rates, and either favour transport of tracers or locally trap them inside the bay. An assessment of typical trajectories of water particles released at different locations and depths provided insights into the potential dispersion of passive tracers, including spilled oil, and larvae.