Influence of Chaoborus density on the effects of an insecticide on zooplankton communities in ponds

Zooplankton communities with a high or low density of Chaoborus larvae were established in outdoor concrete ponds, to which a carbamate insecticide, carbaryl, was applied at 0.1 or 0.5 mg l⁻¹. The lower concentration of the chemical was harmful only to Cladocera. The higher concentration damaged Chaoborus, Copepoda, and some rotifer species, as well as Cladocera. In the ponds with a low density of Chaoborus, chemical application altered the cladoceran community from dominance by Daphnia to that by Bosmina and Moina. In the ponds with a high density of Chaoborus, Chaoborus excluded cladocerans from the zooplankton community presumably by predation, and supported the dominance of rotifers. Cladocera did not recover after application of the chemical, even when Chaoborus was eliminated by the higher concentration of chemical. The relatively rapid recovery of Chaoborus seemed to interrupt the recovery of Cladocera.