| Major anthropogenic sources of metals into natural water systems include mining, industry, municipalities, and agriculture. Although potentially toxic metals usually occur in mixtures instead of alone in water, the toxicity of metal mixtures currently is difficult to predict accurately. As part of a project to provide mixture-toxicity data for development of multi-metal toxicity models, the toxicity of binary Cu-Ni, Cd-Ni, and Ni-Zn mixtures was tested. To analyze the interactions of these metals, Daphnia magna neonates were exposed to the metals individually and in binary combinations in standard 48-h toxicity tests conducted in USEPA moderately hard reconstituted water to which 6 mg/L of Suwannee River fulvic acid was added (resulting in dissolved organic carbon concentrations of approximately 3 mg/L). For each combination of metals in the binary mixtures, one metal was held constant at a specified concentration while the second metal was varied through a series that ranged from nonlethal to lethal concentrations; then the roles of the two metals were reversed in a separate series of tests. The Ni-Zn binary combinations tested provided evidence of response-additive toxicity that is easily predicted from known single-metal toxicities. On the other hand, sub-lethal concentrations of Ni protected against the toxicity of Cd, with mortality only occurring at considerably higher Cd concentrations than in paired Cd-only tests (i.e., less-than-additive toxicity). In contrast, based on dissolved-metal concentrations, a synergistic (i.e., greater-than-additive) toxicity occurred in binary Cu-Ni mixtures; with mortality occurring at concentrations of each of the two metals that were lower than in the paired single-metal tests. These findings provide evidence for the dominance of competition between metals for complexation to biological ligands or to dissolved organic matter, depending on the metal combination in question. |
