Population structure and dynamics of Gammarus fossarum (Amphipoda) upstream and downstream from effluents of sewage treatment plants

Two streams in Germany (Korsch and Lockwitzbach), each with two sampling sites above and below a sewage treatment plant (STP) discharging effluent, were investigated. Sampling sites were characterized, and exposure monitoring for chemicals with known or assumed endocrine disrupting potential was carried out. Both the population structure and the population dynamics of Gammarus fossarum were examined. The physicochemical parameters measured at the sampling sites of the Lockwitzbach and Korsch streams were found not to reach levels having an acute toxic impact on the development of gammarids. The calculated estrogenic potential in the stream water was 22- to 35-fold higher at the downstream site of the Korsch compared with the other sampling sites, mainly because of the concentrations of 17 alpha-ethinylestradiol on two sampling dates. At both streams, an influence of the respective STP effluent on the sex ratio of G. fossarum was not observed. Moreover, intersexuality was not induced by these effluents. Differences in the structure and dynamics of G. fossarum populations were more pronounced at the Korsch than at the Lockwitzbach. At the downstream sampling site at the Korsch, gammarids reached their highest abundances. Particularly at the downstream sampling site of the Korsch, the proportion of breeding female gammarids and the proportion of juvenile gammarids in the smallest body length class were decreased compared with upstream. Adult gammarids were larger from the Lockwitzbach downstream site, but they were smaller from the Korsch downstream site compared with the respective upstream site. At the Korsch, the earlier onset of the autumnal reproductive resting period could be caused by the influence of the STP effluent.