The authors collaborate within the SO RECThau (Service of Observation and Research incoastal Environment of Thau) of the OSU OREME. They find themselves here on a projectof hydro-biogeochemical numerical modelling complementary of their activities ofobservations. Illustrated on the period of the RESTHAU campaign (November 2008), theaim is to evaluate the impact of storms and floods episodes on primary production.The coupled model Symphonie-ECO3M was implemented on a fairly original computinggrid encompassing the Thau lagoon, the canals, the foreshore and open sea to the offshorezone. The hydrodynamic model is coupled to the ECO3M model based on mechanisticformulations of biological processes. The model uses 35 state variables and reproduces acomplex planktonic ecosystem with several primary producers and several consumers. Thebiochemical cycles of the biogenic elements (C, N, P, Si) are managed by the model.Owing to field data, the November 2008 storm period was investigated. The first resultsshow no increase in primary production during and after the storm in spite of an enrichmentof the lagoon in nutrients which seems to indicate a limitation by light or temperature. Therole of predation by oysters (more than 10 000 tonnes in the lagoon) is also investigated bycomparing the modified model integrating this compartment with mesocosms results.