Bioavailability of sedimentary metals from a contaminated bay

Sediments are considered the sink for metals in aquatic environments because of their strong metal-binding capacity, but they are a potential source for metal ingestion by marine benthic animals as well. Sediment contamination is now a major environmental problem in many countries, including China. This study examines the bioavailability to a marine suspension-feeder (the clam Mactra veneriformis) and a deposit-feeder (the peanut worm Sipunculus nudus) of sediment-bound Cd, Cr and Zn along a gradient of metal contamination. To this end, metal assimilation efficiency and metal extraction by gut juice were quantified and compared. Metal concentrations in sediments collected from 4 sites were 5.40 to 43.7 mgCd kg–1, 32.8 to 71.2 mgCr kg–1, and 278 to 5 380 mgZn kg–1. Metal assimilation by the clams was independent of the metal concentration in the sediment. In the deposit-feeding sipunculid, the bioavailability of sediment-bound Cd increased with increasing contamination of the sediment, as a result of the increasing partitioning of Cd into the easily exchangeable phase, and decreasing partitioning into the reducible phase. Cd assimilation also increased with increasing total organic carbon content in the sediments.The Cd assimilation efficiency was directly correlated to Cd extraction by gut juices collected in vitro from the peanut worm. No such relationship was found for Cr and Zn. Geochemical and biological controls on metal bioavailability therefore differ between the metals and with the extent of sediment contamination. The limitation of the radiotracer technique to quantify metal assimilation efficiency by marine benthic invertebrates from sediments should be recognized when it is used to predict metal concentrations in target animals.