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 (log1096-h Cu LC50 = −0.308 + 0.192 pH + 0.136 (pH∙log10DOC)) 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.
SUMMARYZebrafish of eleven age groups were exposed to four concentrations of zinc sulphate (5, 10, 20 and 40 p.p.m. Zn) in soft water (CaCO3 10 p.p.m.) at 25d̀ C. In all four concentrations, newly laid eggs generally survived the longest time. Survival time decreased with age until hatching on the fourth day. Newly hatched fish (4–13 days old) survived the shortest time. Forty‐day‐old fish and adults (100 days old) survived slightly longer in the two highest concentrations. In the two lowest concentrations, 40‐day‐old fish survived as long as newly laid eggs, and many adults survived indefinitely. The threshold concentration of zinc was approximately 10 p.p.m. for adults and 1.3 p.p.m. for 40‐day‐old fish.
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