Effects of tetradifon on Daphnia magna during chronic exposure and alterations in the toxicity to generations pre-exposed to the pesticide

Daphnia magna was exposed to some nominal sublethal tetradifon concentratio ns (0.10, 0.18, 0.22 and 0.44 mg/l) during 21 days. Chronic toxicity tests were carried out using neonates of F-1 first brood (F-1-1st) and F-1 third brood (F-1-3rd) offspring generations from parentals (F-0) pre-exposed to t he pesticide. The effect of tetradifon on survival, reproduction and growth was monitored for the selected daphnid generations. The parameters used to evaluate pesticide effect on reproduction were: mean total young per femal e, mean brood size, time to first reproduction, mean number broods per fema le and intrinsic rate of natural increase (r). Survival and growth (body le ngth) were also determined after 21 days of exposure to the pesticide. Repr oduction was significantly reduced when tetradifon concentration increased in the medium. This effect was higher in F-1-1st and F-1-3rd offsprings com pared to the parental generation (F-0) daphnids. The intrinsic rate of natu ral increase (r) decreased with increasing concentrations of tetradifon esp ecially in those animals from F-1-3rd. However, the growth of the exposed o rganisms decreased in the same order of magnitude in all the generations te sted. Survival was not affected after exposure to the selected tetradifon c oncentrations. The maximum acceptable toxicant concentration (MATC) was cal culated for F-0, F-1-1st and F-1-3rd generations exposed to the pesticide u sing as parameter of evaluation the intrinsic rate of natural increase (r). The interpolation of these results gave MATC values of 0.13 mg/l pesticide for F-0 and F-1. The EC50 values have been derived for some selected param eters on D. magna exposed to tetradifon. EC50 values decreased in F-1-1st a nd F-1-3rd generations compared to the parental generation F-0, these resul ts mean that less amount of toxicant would be necessary to reduce the selec ted parameters to 50% in the first and third broods of D. magna. Expanding the reproduction tests to several broods of a parental generation revealed important information on chronic toxicity that could be used in the protect ion of aquatic environment. (C) 2000 Elsevier Science B.V. All rights reser ved.