Bioavailability of sediment-bound Cd, Cr and Zn to the green mussel Perna viridis and the Manila clam Ruditapes philippinarum

We assessed the degree to which Cd, Cr and Zn bound with sediment were assimilated by the green mussel Perna viridis and the Manila clam Ruditapes philippinarum. The influences of the metal concentration in the sediment, the presence of phytoplankton, and the oxidation condition of the sediment on metal assimilation were examined. No major difference was found for metal assimilation efficiency (AE) in sediment with different metal concentrations, except for Cd in the green mussels, in which the AE increased by 1.7x when the Cd concentration in sediment was elevated to 15x the natural background level. The higher assimilation of Cd with increasing Cd load in ingested sediment may be due to the higher desorption of Cd in the acidic gut of the bivalves. Both mussels and clams assimilated metals at a higher efficiency from the diatom diet (Thalassiosira pseudonana) than from inorganic sediment particles. The presence of algal particles had little influence on metal assimilation from ingested sediment, and conversely, the presence of sedimentary particles had little effect on metal assimilation from ingested diatom (except for Cd in the mussels). In the mussels, AEs were higher from oxic sediment than from anoxic sediment by 3.1x for Cd, 2.0x for Cr, and 1.4x for Zn, and in the clams AEs were higher from oxic sediment by 2.8x for Cd, 2.0x for Cr, and 2.0x for Zn. Our study suggested that metals associated with anoxic sediment can be potentially available to marine bivalves, and that metal AEs determined for a single diet were probably not affected by the presence of other food particles.