The uptake and release of material by oysters (Crassostrea gigas) and mussels (Mytilus edulis) in Marennes-Oléron Bay (SW France) were measured in situ using benthic ecosystem tunnels (BEST). There was a large variation in seston concentrations, owing to high sedimentation/erosion dynamics. Organic matter was diluted by resuspended sediment, but had a low C/N ratio, i.e. a high quality, owing to resuspension of microphytobenthos. In many cases a significant decrease of chlorophyll concentrations was observed in tunnels with oysters and mussels. Average clearance rates per tidal cycle ranged from 4 to 7, and from 0.9 to 2.7 1.g−1AFDW.h−1 throughout the season, for oysters and mussels respectively. Calculation of the filtration pressure (the fraction of the primary production filtered per day), indicated that bivalves in the bay were able to filter much more than the daily pelagic primary production. It seemed obvious that the bivalve stocks depended not only on phytoplankton but also on microphytobenthos. Significant regeneration of nitrogen could be demonstrated from musselbeds
on the sediment, at higher rates than direct excretion of ammonium. No significant release of nutrients was observed for the oysters or mussels, when they were put on tables, as practised by the oyster farmers. lt seemed likely that biodeposits were flushed away from the tunnels in this case, and mineralization occurred elsewhere. It was concluded that the carrying capacity of the bay for herbivores is extensively exploited by the shellfish. Mineralization of bivalve biodeposits represent a large potential for nitrogen regeneration.