Identifying cause in sediment assessments: bioavailability and the Sediment Quality Triad

Application of the Sediment Quality Triad approach to non-pH-stressed Sudbury area lakes clearly demonstrated increased metal (Cd, Co, Cu, and Ni) contamination, decreased abundances of some benthic invertebrates (amphipods, pisidiid clams, and tanytarsiid midges), and severe sediment toxicity to amphipods and mayflies. However, the bioavailability of metals and the cause of biological effects cannot be identified from these data. Bioaccumulation measurements, however, clearly demonstrated increased Cd, Co, and Ni bioavailability to Hyalella. Copper concentrations in Hyalella were not elevated, in spite of much higher sediment Cu concentrations. Nickel was the only metal accumulated in sufficient amounts to cause toxicity. Toxicity in Hyalella caged above the sediments was equivalent to that of animals exposed directly in sediment, demonstrating that effects were due to dissolved metal and not metals in the solid phase. Copper concentrations in overlying water were much lower than Ni concentrations and insufficient to account for toxicity. Including bioaccumulation data with traditional measurements of sediment chemistry, benthic community composition, and sediment toxicity allows quantification of metal bioavailability and identification of the cause of toxicity. This approach results in a much more complete assessment of the biological impacts of metals in sediments.