Statistical models to predict the toxicity of major ions to Ceriodaphnia dubia, Daphnia magna and Pimephales promelas (fathead minnows)

Toxicity of fresh waters with high total dissolved solids has been shown to be dependent on the specific ionic composition of the water. To provide a predictive tool to assess toxicity attributable to major ions, we tested the toxicity of over 2,900 ion solutions using the daphnids, Ceriodaphnia dubia and Daphnia magna, and fathead minnows (Pimephales promelas). Multiple logistic regression was used to relate ion composition to survival for each of the three test species. In general, relative ion toxicity was K⁺ > HCO₃/^- ≃ Mg²⁺ > Cl^- > SO₄/^2-; Na⁺ and Ca²⁺ were not significant variables in the regressions, suggesting that the toxicity of Na⁺ and Ca²⁺ salts was primarily attributable to the corresponding anion. For C. dubia and D. magna, toxicity of Cl^-, SO₄/^2-, and K⁺ was reduced in solutions enriched with more than one cation. Final regression models showed a good quality of fit to the data (R² = 0.767-0.861). Preliminary applications of these models to field-collected samples indicated a high degree of accuracy for the C. dubia model, while the D. magna and fathead minnow models tended to overpredict ion toxicity.