Low exposure concentrations of atrazine increase male production in Daphnia pulicaria

Results of 24 bioassays with the water flea (Daphnia pulicaria) show an exp osure-response relationship between Daphnia sex ratio and the herbicide atr azine. Exposure of Daphnia to atrazine during embryogenesis resulted in a s hift in sex determination reward males. The shift was detectable at 0.5 ppb (nominal concentration) and pronounced at 10 ppb or more. The shift occurr ed in the upper part of the range of atrazine concentrations commonly found in aquatic environments (such as lakes, streams, wells, and rainwater). Ou r results suggest that Daphnia sex ratio is one to two orders of magnitude more sensitive to atrazine than are survivorship or fecundity.