Influence of duration of exposure to the pyrethroid fenvalerate on sublethal responses and recovery of Daphnia magna straus

This study compares lethal and sublethal responses of Daphnia magna Straus exposed to fenvalerate continuously (21 d) and as a pulse (24 h). Survival was reduced more severely in the continuous- than in the pulse-exposure regime. Complete mortality occurred at 1 microg/L for continuous exposure and at 3.2 microg/L for pulse exposure. Regarding reproductive endpoints, fenvalerate delayed the age at first reproduction. At the beginning of the reproductive phase (day 10), this delay resulted in a reduction of the neonates per living female at similar concentrations in both exposure regimes (0.3 and 0.1 microg/L for continuous and pulse exposure, respectively). The population growth rate was inhibited in continuous and pulse exposure at 0.3 and 0.6 microg/L, respectively. However, the effects of fenvalerate in the pulse exposure were transient. After 21 d, a recovery to values close to the controls occurred with respect to the total neonates per female and the population growth rate over a broad range of concentrations from 0.1 up to 1 microg/L. In contrast, no substantial recovery occurred in the continuous-exposure regime.