The influence of salinity and dissolved organic carbon on the toxicity of copper to the estuarine copepod, Eurytemora affinis

The objectives of this study were to determine the influence of salinity (2.5, 5, 15, and 25 ppt) at dissolved organic carbon (DOC) concentrations of 1.3-3.3 mg/L and DOC concentrations of 2, 4, 6, and 8 mg/L at a fixed salinity of 10 ppt on the acute toxicity (96-h LC50s) of copper to the sensitive estuarine copepod, Eurytemora affinis. For both salinity and DOC experiments, various other chemical constituents such as DOC, Ca2+, Cl-, Mg2+, Na+, K+,SO42-, hardness, alkalinity, salinity, pH, temperature, and dissolved oxygen were measured at selected copper concentrations at test initiation and test termination. Dissolved copper, copper speciation, and organic copper complexation were measured at various test conditions during the salinity and DOC experiments. Ninety-six-hour dissolved copper LC50 values for the four salinities ranged from 58 mu g/L (25 ppt) to 104 mu g/L (5 ppt) with intermediate values of 71 mu g/L (2.5 ppt) and 68 mu g/L (15 ppt). The 58, 68, and 71 mu g/L LC50 values were not significantly different. Copper LC50 values at 5 ppt were higher than at both 15 and 25 ppt. The isosmotic salinity of E. affinis is approximately 5-10 ppt, which was a likely factor for why the LC50 value increased for copper at 5 ppt. The dissolved copper 96-h LC50s for E. affinis increased from 76 to 166 mu g/L as DOC increased from 2 to 8 mg/L. This result is not surprising and is consistent with reported values for other saltwater species.