The uptake and release of material by oysters (Crassostrea gigas) in the intertidal zone were estimated in situ in the Bay of Marennes-Oléron (France) in spring and autumn. 10 m long plexiglass tunnels were placed over small tables with bags containing oysters or empty oyster shells (control). Although in the very turbid conditions of the water column sedimentation of suspended particulate material could be observed in control experiments, higher fluxes (uptake) of seston, POC and PON and especially chlorophyll occurred in the presence of live animals. Release of dissolved nutrients was not evident from the tunnel with oysters, which is in contrast with studies in other areas with bivalves. This is ascribed to limited mineralization of biodeposits as an effect of resuspension and consequent loss of a significant fraction of biodeposits from the tunnel.