Surface water is the main drinking water source in Korea. Algal bloom caused byphytoplankton in reservoir is common event in every summer season. To prevent or control thealgal blooms, artificial circulation system has been adopted in many reservoirs, includingHoengseung reservoir. Total 7 hydraulic-gun-aerators were installed around the intake tower inHoengseung reservoir since 2000. This study is to elucidate the effects of hydraulic-gun-aeratorson phytoplankton bloom, pH, DO, temperature and evaluate the selective withdrawal andvertical distribution of phytoplankton by means of submersible fluorescence probe, whichfeatures high correlation with a standard ISO method (r=0.90, P<0.0001) for chlorophyll-aquantification.