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 mu g 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 C-13 and delta N-15, 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 delta C-13 and delta N-15 signatures were region dependent. Oysters from the most marine influenced sites contained gut and tissue cadmium residues of 30.4 +/- 3.08 (S.E.) mu g g(-1) and 6.0 0.6 mu g 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 mu g g(-1) and 10.2 +/- 0.68 mu g 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. (C) 2009 Elsevier Ltd. All rights reserved.