ESTIMATING ACUTE COPPER TOXICITY TO LARVAL FATHEAD MINNOW (PIMEPHALES-PROMELAS) IN SOFT-WATER FROM MEASUREMENTS OF DISSOLVED ORGANIC-CARBON, CALCIUM, AND PH

We tested an empirical model that predicts copper (Cu) toxicity to lar val fathead minnow (Pimephales promelas) from surface water pH and dis solved organic carbon (DOC) concentrations with an independent data se t of 18 softwater lakes. These lakes ranged widely in concentrations o f DOC (1.7-14.8 mg L(-1)), pH (5.5-6.9), and calcium (Ca) (1.9-4.6 mg L(-1)), all factors known to modify metal toxicity. Acute median letha l concentrations (96-h LC(50)s) for total Cu were 5.3-169.5 mu g . L(- 1) in the 18 lakes. Although measured Cu toxicity was correlated with predicted toxicity (r(2) = 0.78), the model overestimated the actual C u toxicity; however, this could be corrected by including Ca in a modi fied model: log 96-h Cu LC(50) = 0.981 + 0.192 pH + 0.136 (pH x log DO C) + 0.166 Ca (r(2) = 0.84). Similar parameters were observed when the analysis was restricted to the 18 study lakes (r(2) = 0.85). Toxicity was also expressed as log 96-h Cu LC(50) = 0.033 + 0.025 pH + 0.015(p H x log colour) + 0.078 hardness (r(2) = 0.83) because colour and hard ness are easily measured surrogates for DOC and Ca concentrations. Thi s research demonstrates that acute lethality of Cu to larval fathead m innows varies in a predictable way in soft water, on the basis of DOG, pH, and Ca concentrations.