BIOAVAILABILITY OF SEDIMENT-BOUND METHYL AND INORGANIC MERCURY TO A MARINE BIVALVE

The extent to which ingested suspended sediment can be a source of mer cury for suspension feeders is largely unknown, yet this information i s required to evaluate the biological significance of sediment contami nation. We used radiotracer methodology to experimentally compare the bioavailability of dissolved and sediment-bound mercury for a marine b ivalve. We examined the relative importance of specific sedimentary co mponents that may exert control on the uptake of Hg(ll) and CH3Hg(Il) from sediments in the mussel Mytilus edulis, which is widely used in n ational monitoring programs as a bioindicator organism for detecting c oastal contamination. Iron and manganese oxides, montmorillonite clay, and silica, with and without organic coatings, as well as natural sed iment particles were radio-labeled with either Hg-203(ll) Or (CH3Hg)-H g-203 using nanomolar levels of Hg. Fulvic acid coatings enhanced the sorption of both Hg(ll) and CH3Hg(II) onto every type of particle, and partition coefficients were as high as 10(5). The assimilation effici encies (AE) in mussels were very low for Hg(ll) (19%) but were typical ly >30% for CH3Hg(II) and as high as 87%. The organic coatings increas ed AE for CH3Hg(II) bound to all types of particles, but had no impact on Hg(II). Unlike Cd, Co, and Ag, there was little (<10%) desorption of Hg-203 from particles at pH 5 (to simulate the behavior of food-bou nd metals in the acidic gut of bivalves), and it did not influence Hg assimilation in mussels. The absorption of CH3Hg(II) by mussels is rap id and efficient even from uncoated inorganic particles that were rapi dly egested out of the animal's gut due to their low nutritional value . The concentration factor (10(3)) of dissolved Hg(ll) in mussels was half that of dissolved CH3Hg(ll). These results suggest that contamina ted sediments can be a significant source of Hg, especially organic me rcury, to marine suspension feeders and that CH3Hg(II) is assimilable from all major sedimentary components.