Acute toxicity of waterborne Cd, Cu, Pb, Ni, and Zn to first-instar Chironomus riparius larvae.

The acute toxicities of waterborne Cd, Cu, Pb, Ni, and Zn were determined in the first-instar larvae (generally considered to be the most sensitive) of Chironomus riparius, under standardized conditions. Toxicity tests were conducted in soft water (hardness, 8 mg/L as CaCO(3) equivalents) in the absence of food and were limited to 24 h to avoid control mortality associated with food deprivation. For each metal, a logarithmic range of concentrations was tested between 0 and 25 mg/L. First-instar C. riparius are most sensitive to Pb, with a 24-h LC50 of 0.61 mg/L (CI of 0.26-1.15 mg/L), and to Cu, with a 24-h LC50 of 2.09 mg/L (CI of 1.57-2.96 mg/L). The LC50 for Cd was 9.38 mg/L, while the LC50s for Zn and Ni were both higher than the highest tested concentration of 25 mg metal/L. Our results found that even first-instar chironomid larvae are well protected by both the current CCME Canadian water quality guidelines for the protection of aquatic life and the USEPA Water Quality Criteria, as LC50s were at least 25 times higher than the guideline concentrations.