THE INTERACTION OF PH, DISSOLVED ORGANIC-CARBON, AND TOTAL COPPER IN THE DETERMINATION OF IONIC COPPER AND TOXICITY

The bioavailability of copper to Daphnia magna was studied in a complete three factor experiment with variable total copper (TCU), pH, and naturally-derived dissolved organic carbon (DOC). Both pH and DOC were found to be important in controlling the amount of ionic copper (for a given total copper concentration) and a model was developed which explained 94% of the variability in the data. Mortality of Daphnia magna was highly correlated with ionic copper under pH 7 at 24 and 48 hours (r2 = 0.90 and 0.96) and less so at higher pH suggesting that copper complexes may be contributing to the response. When toxicity was expressed in terms of ionic copper, it was concluded that a given amount of ionic copper produced more toxicity as pH increased. The 48-h LC50 for D. magna exposed to ionic copper was in the order of 0.5 nM (0.03 ng.ml-1).