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, u sing mortality as an endpoint. Specifically, concentration-time-response su rfaces 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. Mo rtality data were log, transformed into probits and then regressed against the reciprocal of exposure time and concentration. The results obtained cle arly showed that mortality was markedly affected at exposure periods longer than 48 h. Thus, an optimal exposure period of 72 h was selected to compar e predicted lethality effects (LC) of different intensities (i.e., 10-50%) derived from concentration-time-response surfaces with measured chronic let hal levels obtained from the literature. Only the results of Cd and Cu show good agreement between predicted and measured chronic lethal concentration levers. The apparent disagreement observed for U could be attributed to di fferences in water quality, and bend, U bioavailability. The high levels of Zn toxicity observed in relation to the predicted values could indicate th at chronic mortality of Zn is mediated by toxic anorexia. Overall, the resu lts obtained show that concentration-time-response surfaces offered the pot ential to assess the effect of time on toxicity, which is desirable to cred ibly extrapolate from acute to chronic scenarios. Furthermore, by determini ng lower mortality thresholds (i.e., LC10) at different times, concentratio n-time-response surfaces were able to emulate the mode of action of the sel ected metals on chronic exposures. Therefore, the use of concentration-time -response surfaces has potential application in environmental risk assessme nt.