A microplate freshwater copepod bioassay for evaluating acute and chronic effects of chemicals

Toxicity test protocols for establishing the effect of zinc on the freshwater copepod Bryocamptus zschokkei are reported. In the absence of food, larval life-history stages were more sensitive than adult females to acute zinc exposure with a 96-h median lethal concentration of 0.62 mg Zn/L (0.52-0.73 mg Zn/L, 95% confidence intervals) for copepodids. The acute toxicity of zinc to adult females was also significantly reduced in the presence of food (a leaf disc). The main toxic effect of zinc in a life-cycle test was a reduction in the number of offspring per brood at 0.48 mg Zn/L. As this endpoint corresponded with zinc concentrations causing reduced juvenile survival, prolonged embryonic development times, and mortality during hatching, it appears that zinc had a direct toxic effect on the current brood rather than an indirect effect on egg production via maternal reallocation of resources. The lowest-observed-effect concentration for reproduction of measured zinc concentrations of 0.48 mg Zn/L corresponded with environmental concentrations of zinc causing reduced population densities of this copepod in the field. It is proposed that B. zschokkei is an ecologically relevant test species for evaluating the potential effects of contaminants on freshwater invertebrates and for giving insight into the mode of action of a test chemical.