Exposure-pathway models explain causality in whole sediment toxicity tests.

Measurements of lethal effect concentrations (LC50) and bioaccumulation following water-only and whole-sediment exposures of the amphipod, Melita plumulosa, and the bivalve, Tellina deltoidalis, to copper, were combined with bioenergetic-based kinetic models of exposure pathways to explain causality in whole-sediment toxicity tests. For both organisms, lethal body concentrations (LBCs) were greater for water-only exposures than for sediment exposures and indicated that the rate of copper accumulation and/or the mode of toxicity of copper assimilated were different for dissolved and particulate phases. The net assimilation of copper, expressed as a lethal exposure concentration (LEC) that was independent of the postexposure copper efflux, was shown to better explain the observed toxicity. The LEC of copper was the same for both water-only and whole-sediment toxicity tests. It is predicted that, for each species, a large range of effect concentrations may be measured for sediments having the same total copper concentration. These are conditional effect concentrations, as their value will be determined by total copper concentrations, partitioning (Kd) relationships (sediment properties), organism physiology (uptake rates from waters, assimilation efficiencies from solids), and organism feeding behavior (feeding selectivity). The importance of these factors to the development of sediment quality guidelines for metals based on species sensitivity distributions is discussed.