Influence of Fenitrothion-Exposure on the Metabolic Activity of a Rotifer (Brachionus plicatilis) to Organophosphorus Insecticides

「INTRODUCTION」 Zooplankton are vital secondary producers in aquatic ecosystems and are recognized as important bioindicators of water quality. 1) The use of insecticides is affecting such aquatic biota. The nature of the impact of short- and long-term exposures to chemicals, even in sublethal concentrations, needs to be ascertained clearly. According to OECD Test Guidelines 202 for chemicals, Daphnia magna are to be exposed for a minimum of 14 days to determine the effects of chemicals on reproduction. However, this period is not always sufficient to estimate the impact of pollution on multiple generations of zooplankton. The influence of some insecticides on zooplankton and phytoplankton communities has previously been investigated in enclosed ponds. Treatments of carbaryl and fenthion, carbamate and organophosphorus insecticides, respectively, induced dominancy by rotifers in a pond through reduction in density of cladocerans, which prey upon rotifers;2-5) generally rotifers have higher tolerance than cladocerans to chemicals. 6) Normally, the dominant zooplanktons in ponds are cladocerans. After the cladoceran population peaks, cladoceran then decreases in density while some inedible algal species increase. Thereafter, rotifers become dominant in the zooplankton community. However, cladocerans then increase again and regain dominance, suppressing the rotifers by predation.