This study was conducted to examine the toxic effects of paraquat on Daphnia magna and its behavior in Korean agricultural conditions. Acute immobilization toxicity tests on D. magna were performed for 48 h at three different exposure systems: water-alone (System A), water-sediment system (System B), and an equilibrated water-sediment system (System C). The concentration of paraquat in the System A was very stable over 48 h and the EC50 was 6.39 ppm. In contrast, the concentration of paraquat in water phase of the System B drastically decreased. The dissipation time in water (DT50) in the System B ranged from 5.45 h to 13.05 h, while the EC50 increased 3 times to 20.8 ppm. The concentration of paraquat in the water phase of the System C decreased over 90%, and no adverse effect was observed at 80 ppm. Considering that the toxicity of paraquat greatly decreased in the System B, there was a large margin between predicted environmental concentration (PEC) and the concentration that would cause adverse effects on D. magna in environmentally relevant conditions. This study clearly indicated that paraquat equilibrated in the water sediment system did not acutely affect D. magna dwelling in the aquatic ecosystems, including rice paddies of Korea.

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