A number of risk assessments of the adverse effects of Cu and its compounds have been conducted since it is one of the leading substances responsible for water contamination. However, with the exception of standard organisms, ecotoxicological knowledge is still scarce. Here, we examined the influence of Ca2+ and Mg2+ concentrations on acute toxicity of Cu toward a cladoceran, Daphnia galeata Sars. We found that the protective effects of Ca2+ against copper toxicity were larger than those of Mg2+. BLM parameters (log Ks) for Ca and Mg estimated based on our 48-h LC50 (50 % lethal concentration), respectively, were 3.14 and 2.29. The log K of Ca was similar to that reported in previous studies using Daphnia magna; however, the log K of Mg was lower by one order of magnitude. Our results suggest that there is some mechanistic difference related to the Mg uptake between D. magna and D. galeata. The results obtained from the present study will contribute to the water quality criteria of copper in soft water because D. galeata is a widespread (distributed in Eurasia and North America) and common species, even in soft water lakes.
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