Modulation of sex ratios in Daphnia magna following multigenerational exposure to sewage treatment plant effluents.

The influence of sewage treatment plant effluents on sex ratios in Daphnia magna was investigated. Female daphnids were acclimated for several generations to effluents from a municipal sewage treatment plant and a residential oxidation lagoon and then placed under conditions to maximize male offspring production. Both effluents resulted in a statistically significant decrease in male production and a shift in male broods from earlier broods to later broods near the end of the adult life cycle. For example, sex ratios in control daphnids ranged from 0.43 to 0.67 in broods 3-4 compared with 0.0-0.13 in daphnids exposed to the residential oxidation lagoon. Secondary sexual characteristics of both sexes were statistically significantly increased by the sewage lagoon effluent but not the municipal effluent. These preliminary results suggest that alteration in timing of sexual determination due to exposure to sewage treatment plant effluents could severely impact the survival of daphnid populations.