Comparing metal toxicity among Daphnia magna clones: an approach using concentration-time-response surfaces

This study investigates the use of concentration-time-response surfaces as a tool to predict potential long-term effects of metals to Daphnia magna, using mortality as an endpoint. Specifically, concentration-time-response surfaces were determined for four D. magna clones exposed to four metals (Cd, Zn, Cu or U) in moderate-hard or hard synthetic freshwater for 24-96 h. Mortality data were log(e) transformed into probits and then regressed against the reciprocal of exposure time and concentration. The results obtained clearly showed that mortality was markedly affected at exposure periods longer than 48 h. Thus, an optimal exposure period of 72 h was selected to compare predicted lethality effects (LC) of different intensities (i.e., 10-50%) derived from concentration-time-response surfaces with measured chronic lethal levels obtained from the literature. Only the results of Cd and Cu show good agreement between predicted and measured chronic lethal concentration levels. The apparent disagreement observed for U could be attributed to differences in water quality, and hence, U bioavailability. The high levels of Zn toxicity observed in relation to the predicted values could indicate that chronic mortality of Zn is mediated by toxic anorexia. Overall, the results obtained show that concentration-time-response surfaces offered the potential to assess the effect of time on toxicity, which is desirable to credibly extrapolate from acute to chronic scenarios. Furthermore, by determining lower mortality thresholds (i.e., LC(10)) at different times, concentration-time-response surfaces were able to emulate the mode of action of the selected metals on chronic exposures. Therefore, the use of concentration-time-response surfaces has potential application in environmental risk assessment.