Effect of pH and Dissolved Organic Carbon on the Toxicity of Copper to Larval Fathead Minnow (Pimephales promelas) in Natural Lake Waters of Low Alkalinity

The impacts of pH and dissolved organic carbon (DOC) on the acute toxicity of Cu to larval fathead minnow (Pimephales promelas) were determined using natural soft water from two Precambrian Shield lakes in south-central Ontario. By artificially manipulating the pH and DOC levels of the water, we demonstrated that both acidification and the removal of DOC increased the toxicity of Cu. The 96-h Cu LC50s were determined over a pH range from 5.4 to 7.3 and a DOC concentration range from 0.2 to 16 mg∙L −1 . The LC50s ranged from a low of 2 μg∙L −1 (pH 5.6, DOC 0.2 mg∙L −1 ) to a high of 182 μg∙L −1 (pH 6.9, DOC 15.6 mg∙L −1 ). A multiple regression model (log 10 96-h Cu LC50 = −0.308 + 0.192 pH + 0.136 (pH∙log 10 DOC)) was used to describe the relationship between Cu toxicity, pH, and DOC. The model was significant (p < 0.00001) and explained 93% of the variability in the toxicity data. These results suggest that current water quality objectives for Cu, and possibly for other metals, may not be sufficiently protective of aquatic life in soft, moderately acidic water containing low levels of DOC.