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A Test of the Additivity of Acute Toxicity of Binary‐Metal Mixtures of Ni with Cd, Cu, and Zn to Daphnia Magna, Using the Inflection Point of the Concentration‐Response Curves

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Mixtures of metals are often present in surface waters, leading to toxicity that is difficult to predict. To provide data for development of multi‐metal toxicity models, Daphnia magna neonates were exposed to individual metals (Cd, Cu, Ni, Zn) and to binary combinations of those metals in standard 48‐h lethality tests conducted in USEPA moderately hard reconstituted water with 3 mg DOC/L added as Suwannee River fulvic acid. Toxicity tests were performed with 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. For each combination of metals in the binary mixtures, the concentration of one metal was held constant while the second metal was varied through a series that ranged from nonlethal to lethal concentrations; then the roles of the metals were reversed. Inflection points of the concentration‐response curves were compared, to test for additivity of toxicity. Sub‐lethal concentrations of Ni caused less‐than‐additive toxicity with Cd, slightly less‐than‐additive toxicity with Zn, and greater‐than‐additive toxicity with Cu. One explanation of these results might be competition among the metals for binding to biological ligands and/or dissolved organic matter. Therefore, models might have to incorporate sometimes competing chemical interactions in order to accurately predict metal‐mixture toxicity. This article is protected by copyright. All rights reserved

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