Biogeochemical mechanisms of the cadmium contamination of Crassostrea gigas in the Marennes Oleron bay

The Marennes-Oleron Bay, hosting the largest oyster production in France, is influenced by the historic polymetallic pollution of the Gironde Estuary, with cadmium levels in oysters close to the consumption limit level (5 μg.g^-1 dw, EC No.466/2001). The aim of this pluridisciplinary work was to characterize the behaviour of trace metals in the coastal zone and the mechanisms of Cd contamination in oysters in the Marennes Oleron Bay. Seven oceanographic cruises were conducted during contrasting season to characterize trace metals behaviour and speciation in the Gironde and Charente estuaries and the coastal zone. Then, a spatial and temporal study of trace metals in the surface sediments of the Marennes-Oleron Bay showed punctual Cd-enriched sediments in the southern part, reflecting the connection with the Gironde waters and the particulate Cd inputs via the Maumusson inlet. Thus, this area was chosen to study Cd bioaccumulation in oysters over a three months transplantation. The regional hydrodynamic, observed by satellite images, played an important role on Cd speciation and the temporal variability of dissolved and particulate Cd concentrations. Cadmium bioaccumulation in organs of oysters reared on tables at 60 cm height was more important than in oysters reared near the sediment, suggesting the absence of Cd released during tidal suspension from sediment and micro-phyto-benthos. Furthermore, as the immersion time was closed between the two rearing conditions, we suggested Cd bioaccumulation via the direct pathway and also via trophic pathway of contamination by pelagic plankton ingestion. This trophic pathway of Cd contamination was validated during laboratory experiments using a simultaneous tracing of Cd direct and trophic pathways in oysters by stable isotope spikes at concentrations 10-fold higher than the Gironde Estuary and at realistic concentrations observed in the Marennes-Oleron Bay. (author)