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 sho wn to be dependent on the specific ionic composition of the water. To provide a predictive tool to assess toxicity attributable to major ion s, we tested the toxicity of over 2,900 ion solutions using the daphni ds, Ceriodaphnia dubia and Daphnia magna, and fathead minnows (Pimepha les promelas). Multiple logistic regression was used to relate ion com position to survival for each of the three test species. In general, r elative ion toxicity was K+ > HCO3- approximate to Mg2+ > Cl- > SO42-; Na+ and Ca2+ were not significant variables in the regressions, sugge sting that the toxicity of Na+ and Ca2+ salts was primarily attributab le to the corresponding anion. For C. dubia and D. magna, toxicity of Cl-, SO42-, and K+ was reduced in solutions enriched with more than on e cation. Final regression models showed a good quality of fit to the data (R-2 = 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 o verpredict ion toxicity.