Sources of dietary cadmium to the Pacific oyster Crassostrea gigas

Oysters from the Pacific north-west coast contain high cadmium concentrations (greater than 13.5 microg g(-1) dry weight), which exceed consumer guidelines for international markets. Oysters are selective filter-feeders and attempts which have focused on suspended particulate matter (SPM) as a means to assess the importance of diet as a route of cadmium exposure have met with limited success. Here we use actual gut contents as an alternate to SPM to assess if this is a better predictor of cadmium exposure to the oyster via the diet. We also applied stable isotope analysis, delta(13)C and delta(15)N, to determine the origin of organic material (from terrestrial to oceanic) ingested by the oyster. Oyster gut and tissue cadmium concentrations and corresponding isotopic signatures were determined every 2-3 months for 22 months from 10 locations on the west coast of B.C. Gut and tissue cadmium concentrations were correlated (r(2)=0.40; p<0.05), suggesting that diet could account for at least 40% of observed cadmium oysters residues. Oyster gut and tissue cadmium concentrations and delta13C and delta15N signatures were region dependent. Oysters from the most marine influenced sites contained gut and tissue cadmium residues of 30.4+/-3.08(S.E.) microg g(-1) and 6.0+/-0.6 microg g(-1), respectively, and a stable isotopic signature typical of marine phytoplankton. In contrast, oysters sampled from regions influenced by coastal processes contained significantly greater concentrations of cadmium, 43.0+/-2.4 microg g(-1) and 10.2+/-0.68 microg g(-1) gut and tissue, respectively, with isotopic signatures representative of terrestrial organic matter. This indicates that diet is an important source of cadmium to oysters from the Pacific north-west, however its importance is region dependent and cannot be simply ascribed to one source of organic matter alone.