The Vilaine bay (South Brittany) has been showing for many years eutrophication troubles (red tides, anoxia, toxic blooms...). Facing these biological and economical problems, scientific studies have been performed and the results are joined in an ecological box model of the bay. It simulates the primary production through nitrogen cycle, dissolved oxygen, and salinity, used to calibrate the advection-dispersion fluxes of the physical submodel. The model is running several years to bring out the seasonal characteristics of this site. A special attention is reported on the oxygen balance sheet. Simulations try to quantify the oxygen demand and to recognize the major cause of anoxia. These results are applied to reproduce the July 1982 anoxia, which caused drastic fish mortalities.