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 7 d old organisms following acute (48 h) as well as chronic (14 d) exposure. After 14 d of exposure to 130 μg/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 μg/L Cu for T0 and both>130 μg/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 μg/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 7 d 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.