INSECTICIDE (CARBARYL, 1-NAPTHYL-N-METHYL CARBAMATE) EFFECTS ON A FRESH-WATER PLANKTON COMMUNITY - ZOOPLANKTON SIZE, BIOMASS, AND ALGAL ABUNDANCE

Natural plankton communities were enclosed in mesocosms and exposed to nine dose levels (0 to 100 mu g L(-1)) of the insecticide Carbaryl. Plankton responses were determined after 4 day in situ incubations. Total zooplankton biomass and mean individual biomass declined significantly across the range of treatments. Daphnia were not found at concentrations above 20 mu g L and all cladocerans were absent at > 50 mu g L(-1) Carbaryl. In the higher dose treatment, copepods (Skistodiaptomus and Mesocyclops) became extreme dominants. The decline in zooplankton biomass, and the selective elimination of Daphnia led to increases in algal biomass, especially among small flagellates which are unavailable to the raptorial copepods. Across the range of insecticide dose levels, there was a repartitioning of biomass from zooplankton to phytoplankton, while total plankton biomass was unchanged.