Toxicities of total and chelex-labile cadmium to salmon in solutions of natural water and diluted sewage with potentially different cadmium complexing capacities

The LC₅₀ for total Cd averaged 4.8 and 8.0 μg1⁻¹ in river water and 33% sewage-treatment-plant effluent (STPE), respectively, and for Chelex-labile Cd, 3.9 and 5.6 μg1⁻¹, respectively. The LC₅₀ values for total Cd were significantly (P < 0.05) different, indicating a reduction in toxicity of Cd in the presence of 33% STPE, presumably due to complexation of Cd²⁺. The similarity of LC₅₀ values for Chelex-labile Cd indicates that that fraction contained toxic species of Cd at approximately the same concentration(s) in both river water and 33% STPE; it is therefore considered a better measure of Cd toxicity than total Cd. Furthermore, mortality was correlated with the concentration of Chelex-labile Cd but not with that of Chelex-nonlabile Cd. Measurements of Cd²⁺-complexing capacity by the Chelex method indicated that toxicity was due, at least in part, to Cd²⁺. Values were less than those obtained by the ion-selective-electrode method; these indicated that toxicity was due only to complexed Cd. Values from both methods were uncorrelated with LC₅₀ values.