Enclosures (1m diameter and 1.5m deep) established five at dam site during bloom of Peridinium bips in Lake Soyang. The changes of phytoplankton community were surveyed that the each enclosures controled by acid (pH 4.0 by HC1), CO_2(6.0∼7.01/min), aeration (21/min) and phosphate (0.1mg/1 by KH_2PO_4). The results indicated that Peridinium bipes appeared in the control and added phosphate bag all the time during the study periods. In addition, in the bag of added CO_2, Peridinium bipes disappeared during the adding CO_2, but it appeared again after stopped the adding CO_2. In added aeration and acid bag, Peridinium bipes was replaced with the Asterionella after a few days. And, although under the condition of low pH, acid bag after added the phosphate dominanted by Chlorophyte, Crytomonas of small cell size.