Mixtures of potentially toxic metals including nickel, zinc and copper are often present in natural water systems due to a variety of point and non-point sources, leading to toxicity that is difficult to predict. As part of a project to provide mixture-toxicity data for development of multi-metal toxicity models, we tested the toxicity of some binary and ternary metal mixtures. To analyze the interactions, we exposed <i>Daphnia magna</i> neonates to individual metals (Cd, Cu, Ni, Zn), and to those metals in binary and ternary combinations in standard 48-h toxicity tests conducted in USEPA moderately hard reconstituted water with 3 mg DOC/L added as Suwannee River fulvic acid. Toxicity tests were performed with binary metal mixtures of Ni and (1) Cd, which is considerably more toxic than Ni, (2) Cu, which is less toxic than Cd but more toxic than Ni, and (3) Zn, which has a toxicity threshold similar to Ni. Although sub-lethal concentrations of Ni protected considerably against the toxicity of Cd (i.e., a large less-than-additive toxicity), only slightly less-than-additive toxicity occurred in mixtures of Ni and Zn, and greater-than-additive toxicity occurred in binary mixtures of Cu and Ni. We hypothesize that these results provide evidence of competition among the metals for binding to biological ligands and/or dissolved organic matter; however, the relative importance of these two geochemical mechanisms is unknown when a mixture contains more than two metals. To address this question, we conducted toxicity tests with ternary mixtures in which two metals were held constant at specified concentrations while the third metal was varied through a series that ranged from nonlethal to lethal concentrations. In ternary mixtures with Ni, Cd and Cu, greater-than-additive effects ‘overshadowed’ the protective effect that had previously been observed between Ni and Cd.  In Cd-Ni-Zn mixtures, an only slightly less-than-additive toxicity of Ni-Zn overshadowed the much greater protection that Zn or Ni exhibited against Cd toxicity in binary mixtures. These results demonstrate that models will have to be able to quantitatively incorporate sometimes competing geochemical interactions among metals in order to predict metal-mixture toxicity correctly.

Toxicity of binary and ternary mixtures of nickel, copper, zinc and cadmium to <i>Daphnia magna</i>

249th National Meeting (2015)

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Toxicity of binary and ternary mixtures of nickel, copper, zinc and cadmium to Daphnia magna