Multigeneration zinc acclimation and tolerance in Daphnia magna:: Implications for water-quality guidelines and ecological risk assessment

Development of zinc tolerance is described for the cladoceran Daphnia magna Straus. Zinc tolerance (i.e., toxicity and deficiency) was monitored during successive generations of D. magna acclimated to different zinc concentrations. Survival, reproduction, carapax length measurements, and cellular energy allocation assessments were used as test endpoints. Special attention was paid to the consequences of zinc deficiency. The, zinc acclimation concentration clearly influenced the overall fitness of the organism. After several generations of acclimation, an optimal concentration curve was observed, with an optimum zinc concentration between 300 and 450 mug/L. Zinc deficiency resulted in a lower zinc tolerance, a higher coefficient of variation for brood size, and an increased pH sensitivity. These results clearly indicate that (background) zinc concentrations present in test and culture media have to be considered in the evaluation of toxicity test results, especially when the toxicity data are used for water-quality guideline derivation and/or ecological risk assessment. Culture and test media containing very little or no zinc do not provide a basis for useful ecotoxicological data.