| Summary: | In this study, an artificial stream mesocosm consisting of a head tank, faster-flowing riffle section, gravel section, pool section, lower-run section, and tail tank was installed to simulate a chemical spill in a river. The responses of freshwater periphyton algae, crustacea (<i>Moina macrocopa</i>), freshwater worm (<i>Limnodrilus hoffmeisteri</i>), benthic midge (<i>Glyptotendipes tokunagai</i>), and fish (<i>Zacco platypus</i> and <i>Aphyocypris chinensis</i>) were observed after exposure to benzyl chloride (classified as an accident preparedness substance, APS) at concentrations of 1, 2, and 4 µL/L for 22.5 h. Higher concentrations increased the inhibition (photosynthetic efficiency decrease) of periphyton algae and the mortality of <i>M. macrocopa</i>, whereas the reproduction of the female cladoceran decreased in the 4 µL/L treatment. Mortality of fish did not occur or was lower (≤20%) at all concentrations; however, toxic symptoms were observed for some time after chemical exposure termination and later, symptoms receded. <i>G. tokunagai</i> mortality increased at all concentrations except the control after seven days, and no significant toxic effects were observed in <i>L. hoffmeisteri</i>. The hazardous concentration of benzyl chloride was calculated as 94 µg/L. This study showed the different sensitivities of each species to benzyl chloride. The findings can assist in environmental risk assessment of APSs after chemical spills to protect Korean aquatic species.
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