THE UPTAKE AND RELEASE OF SUSPENDED AND DISSOLVED MATERIAL BY OYSTERSAND MUSSELS IN MARENNES-OLERON BAY

The uptake and release of material by oysters (Crassostrea gigas) and mussels (Mytilus edulis) in Marennes-Oleron Bay (SW France) were measu red in situ using benthic ecosystem tunnels (BEST). There was a large variation in seston concentrations, owing to high sedimentation/erosio n dynamics. Organic matter was diluted by resuspended sediment, but ha d a low C/N ratio, i.e. a high quality, owing to resuspension of micro phytobenthos. In many cases a significant decrease of chlorophyll conc entrations was observed in tunnels with oysters and mussels. Average c learance rates per tidal cycle ranged from 4 to 7, and from 0.9 to 2.7 l.g(-1) AFDW.h(-1) throughout the season, for oysters and mussels res pectively. 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 produ ction. 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. It seemed likely th at biodeposits were flushed away from the tunnels in this case, and mi neralization occurred elsewhere. It was concluded that the carrying ca pacity of the bay for herbivores is extensively exploited by the shell fish. Mineralization of bivalve biodeposits represent a large potentia l for nitrogen regeneration.