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 larval fathead minnow (Pimephales promelas) from surface water pH and dissolved organic carbon (DOC) concentrations with an independent data set of 18 softwater lakes. These lakes ranged widely in concentrations of 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 lethal 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 Cu toxicity; however, this could be corrected by including Ca in a modified model: log 96-h Cu LC(50) = 0.981 + 0.192 pH + 0.136 (pH x log DOC) + 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(pH x log colour) + 0.078 hardness (r(2) = 0.83) because colour and hardness are easily measured surrogates for DOC and Ca concentrations. This research demonstrates that acute lethality of Cu to larval fathead minnows varies in a predictable way in soft water, on the basis of DOG, pH, and Ca concentrations.