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 toxi city of Cu to larval fathead minnow (Pimephales promelas) were determi ned using natural soft water from two Precambrian Shield lakes in sout h-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 mug.L-1 (pH 5.6, DOC 0.2 mg.L-1) to a high of 182 mug.L-1 (pH 6.9, DOC 15.6 mg.L-1). A mult iple regression model (log(10)96-h Cu LC50 = -0.308 + 0.192 pH + 0.136 (pH.log10DOC)) was used to describe the relationship between Cu toxic ity, pH, and DOC. The model was significant (p < 0.00001) and explaine d 93% of the variability in the toxicity data. These results suggest t hat current water quality objectives for Cu, and possibly for other me tals, may not be sufficiently protective of aquatic life in soft, mode rately acidic water containing low levels of DOC.