Physicochemical factors affecting the sensitivity of Ceriodaphnia dubia tocopper

The effects of physicochemical conditions, such as pH, water hardness, flow rates and natural organic substances on the sensitivity of Ceriodaphnia du bia to the toxic effects of copper were investigated using static bioassay cups and specially designed flow-through bioassay chambers. We found that C . dubia was very sensitive to pH changes and the total copper LC50 values o f C. dubia neonates increased by 15-fold as the pH increased from pH 7 to 1 0. It was also observed that the LC50 values increased sharply upon increas ing the water hardness value to 2.4 meq. In addition, increasing flow rates from zero to 50 mL hr(-1) also increased its sensitivity to copper, which was possibly due to hydrodynamic stress. The presence of natural organic su bstances (humic acid and dissolved organic matter) and suspended particles decreased the toxic effect of copper. This significant decrease in the toxi city of copper in the presence of natural organic materials can be explaine d by a reduction in the free ion concentration due to complexation. Further more, we observed that the kinetics of copper interactions with natural org anic materials are a significant factor in the toxic effect of copper and t hat the acute LC50 values increased with increasing reaction time between s olubilized copper and water-borne organics.