Chronic ecotoxicity of copper and cadmium to the zebra mussel Dreissena polymorpha

In order to evaluate ecological consequences of the long-term presence of metals in aquatic ecosystems, we investigated the filtration rate and survival of zebra mussels (Dreissena polymorpha) during chronic exposure to Cu and Cd. The filtration rate was measured once a week in laboratory experiments lasting 9–11 weeks. The lowest Cu concentration tested (13 g/L) did not affect the filtration rate and survival of D. polymorpha, but the lowest Cd concentration (9 g/L) did affect the filtration rate, but had no effect on survival. The EC₅₀ for Cd decreased markedly from 388 g/L to 27 g/L when the exposure time was lengthened from 48 hours to 10 weeks. The largest decrease in EC₅₀ for Cd was observed during the first week of exposure. In contrast, the EC₅₀ for Cu did not decrease with increasing exposure time (chronic EC₅₀: 43 g/L). Since the chronic LC₅₀ for Cd was 130 g/L, the filtration rate appeared to be a far more sensitive endpoint for ecotoxicological laboratory experiments than mortality. D. polymorpha was capable of regulating the body concentration of the essential metal Cu at low concentrations in the water (13 g/L). Cd was accumulated at every Cd concentration in the water, suggesting that Cd could not be regulated by D. polymorpha. It is concluded that the relation between short-term and long-term ecotoxicity was different for each metal and could not be predicted from the results of the short-term experiments.