Interactions between eutrophication and contaminants. II. Mobilization and bioaccumulation of Hg and Cd from marine sediments

The availability of labile organic matter and oxygen from the bottom water are primary factors most likely to affect the accumulation of contaminants in benthic invertebrates. To test this hypothesis, a two-factor microcosm experiment was designed. Three species of sediment-dwelling organisms were kept in sediments spiked with 203Hg and 109Cd. Blue mussels were kept downstream in separate vessels. Different treatments were obtained by combination of two levels of organic carbon (enriched, not enriched) with two levels of oxygen (7–9 mg l−1 and 2–3 mg l−1) in the water flowing through the aquaria. After an experimental period of three months, gamma emission from the two isotopes were counted in samples of sediments, porewater and biota. The results showed initial retention of a labile fraction of Cd in the carbon-enriched sediments. Hg was retained to the same extent in all treatments and no significant loss from the sediment was observed during the experimental period. In the porewater, Hg was not detectable whereas the concentration of Cd was consistent with the levels predicted from low sulphide ion activities recorded on AgAg2S electrodes and the solubility of CdS(s). Two-way ANOVA analyses revealed significant effects of carbon enrichment on bioaccumulation of both Cd and Hg. Significant effects of oxygen level and oxygen-organic interactions were found for the bioaccumulation of Cd only.