An ecosystem model is presented for estimating the shellfish carrying capacity of Marennes-Oleron Bay (France). It incorporates physical and biological processes: horizontal transport of suspended matter, feeding and growth of the cultivated oyster Crassostrea gigas and primary production. The precision and consistency of the model are tested by comparing simulations to observed data. The model smoothes both spatial and temporal variability of suspended-matter concentrations but reproduces mean levels and seasonal cycles with some accuracy. Carrying capacity of the shellfish system is assessed by computing the sensitivity of oyster growth to oyster abundance. Model results clearly indicate a density dependence of oyster growth. When stock is adjusted from 20% to 200% of the present value, maximal dry weight of oysters shows a mean decrease of approximately 25%. The spatial heterogeneity of the growth response is a consequence of the low level of primary production within the shellfish area. The hydrodynamic regime of the bay strongly controls the carrying capacity of the shellfish system: flushing time and available light energy are found to be 2 determining factors which prevent the phytoplankton from thriving in the bay; tidal currents ensure a fast renewal of food. Nevertheless, phytoplanktonic production accounts for a non-negligible part of food filtered by oysters and is identified as an important food source when stock level is low. The validity of the model is limited mainly by the present description of the physical transport of suspended and deposited matter.