Age and exposure duration as a factor influencing Cu and Zn toxicity toward Daphnia magna

Standardized toxicity tests are generally performed with juvenile test organisms, e.g., in Daphnia magna assays neonates<24 h old are used. The purpose of this research was to investigate the influence of a delayed exposure to Cu and Zn on population parameters and toxicity values derived from these endpoints. Juveniles (<24 h; T0) and 7-d old daphnids (T7) were exposed for 21 and 14 d, respectively. For Cu, juveniles were significantly more sensitive than 7d old organisms following acute (48 h) as well as chronic (14 d) exposure. After 14 d of exposure to 130 microg/L Cu, mortality was 80% and 10% in T0 and T7, respectively. Juveniles per surviving female at this concentration decreased by 78% and 14% compared to the control. 14 d-NOEC and LOEC values (based on juveniles per surviving female) were 75 and 90 microg/L Cu for T0 and both>130 microg/L for T7. For Zn, survival in T0 and T7 was similar. Although T7 organisms produced significantly more offspring, 14 d-NOEC and LOEC values were equal to those of T0, i.e., 80 and 115 microg/L, respectively. For Cu as well as for Zn effect concentrations based on 14 and 21 d exposure were similar (results from T0). It can be concluded that acute and chronic toxicity data obtained from juvenile D. magna are more sensitive or equally sensitive than obtained from 7d old organisms. As differences are observed between the two metals extrapolations of these conclusions to other toxicants and other aquatic species cannot be made without further investigation.