Modelling cadmium accumulation by benthic invertebrates in situ: The relative contributions of sediment and overlying water reservoirs to organism cadmium concentrations

An in situ experiment was designed (1) to determine the relative importance of Cd in the sediment versus Cd in the overlying water for its accumulatio n in benthic animals and (2) to test in situ the acid volatile sulfide (AVS ) model. Sediments of a low-Cd shield lake were artificially contaminated w ith various amounts of Cd and placed at a littoral site in open plastic con tainers in the lake bottom for 11 months to allow colonization by indigenou s lake benthos. Gradients in sedimentary and interstitial water Cd concentr ations were, thus, created in situ, although Cd concentrations in the overl ying water remained low. Accumulation of Cd by benthic invertebrates was si gnificantly and linearly correlated with sediment Cd concentrations across treatment levels for most taxa. The magnitude of the response to the sedime nt Cd gradient was taxon specific and consistent with differences in animal ecology. Despite the high Cd contamination levels attained in the experime nt, the population densities of most taxa appear to have been unaffected by the presence of Cd. We used the results from the experimental containers t o predict the sources of Cd for benthic animals in the environs. For almost all of the taxa studied, individuals living in the low-Cd sediments outsid e the experimental containers accumulated their Cd almost exclusively from the overlying water; only the burrowing, sediment-feeding taxa Chironomus s taegeri and members of the Tubificidae obtained substantial Cd from the sed iment. Cadmium activities in the pore waters of the containers were in agre ement with the AVS model, but the Cd concentrations found in the benthic or ganisms did not. Our results from this shield lake suggest that predictions of Cd concentrations in most benthic animals would be mote accurate if the y were based on water column rather than on sedimentary Cd concentrations.