AN OUTDOOR MESOCOSM STUDY TO ASSESS ECOTOXICOLOGICAL EFFECTS OF ATRAZINE ON A NATURAL PLANKTON COMMUNITY

Pond mesocosms were used to assess the effects of a widely used pestic ide on a natural plankton community. Atrazine, whose residuals in surf ace waters can occur at levels potentially hazardous to aquatic organi sms, was applied at six test concentrations during a 6-week study. Str uctural and functional parameters were measured twice a week. Oxygen c oncentration was the most sensitive parameter, with marked reductions indicating adverse affects by atrazine on photosynthesis. Taxa in the phytoplankton community showed varying sensitivity to the pollutant, a lthough threshold concentrations for effects on most taxa were exceede d at 182 mu g/L. Indirect effects due to food limitation were the most likely cause of changes in the abundance of several zooplankton speci es. It is concluded that mesocosms provide realistic effects of atrazi ne on aquatic systems, but at the same time some difficulties with thi s approach are apparent when applied to seasonally dynamic planktonic systems.