Combined effects of pH, hardness and dissolved organic carbon on acute metal toxicity to Daphnia magna

Effects of pH, dissolved organic carbon (DOC) and hardness on toxicities of copper and chromium to Daphnia magna were investigated using a three-factor and five-level central composite design (CCD). There were no interactive effects on Cu(II) and Cr(VI) toxicities between water quality parameters. For Cu(II) toxicity, both pH and DOC showed significant effects, and DOC was the most important parameter, representing 89.3% of variation of 48-h EC₅₀ values. In contrast with Cu(II) toxicity, hardness was the most significant parameter to Cr(VI) toxicity, explaining 78.8% of variation of 48-h EC₅₀ values. In addition, pH explained 17.3% of variation of Cr(VI) toxicity. On the basis of toxicity data, empirical models were developed to predict Cu(II) or Cr(VI) toxicity under different physicochemical conditions studied in this work using a backward regression analysis.