Impact of 17alpha-ethinylestradiol on the plankton in freshwater microcosms--I: response of zooplankton and abiotic variables.

We investigated effects of 17alpha-ethinylestradiol (EE) in vertebrate free 230 L still water microcosms. Zooplankton composition and physico-chemical variables were observed during 4 weeks of pre-application, 6 weeks of dosing via controlled release, and a 12 weeks post-treatment period. In the treated microcosms, time-weighted averages of EE concentration ranged between 7 and 220 ng/L during the dosing period, with concentration maxima up to 724 ng/L. EE exposure resulted in a decrease of species numbers and diversity (Shannon-Wiener, Simpson). Abundances of cladocerans, copepods, and, less unambiguously, rotifers declined. Strongest affected groups were the offspring of cladocerans and copepods and, on species level, the cladoceran species Daphnia longispina and Chydorus sphaericus as well as the rotifer species Keratella quadrata and Polyarthra sp. EE apparently affected the phosphate cycle as indicated by increased phosphate concentrations in the water. During post-treatment period, the treated microcosms recovered, but especially the highest treated microcosms did not fully re-approximate to the controls. Whereas EE affected cladocerans and copepods directly, shifts of rotifers may (partly) be caused indirectly, e.g. by competition with crustaceae. Although not providing an absolute proof, the traits of direct and indirect effects on different taxonomic groups and larval stages as well as the time course of the effects indicate that effects primarily resulted from endocrine activity of EE.