Lake Fukami-ike is a small monomictic and eutrophic lake, located in southern NaganoPrefecture in central Japan. Water quality improvement was expected from maintenance of farm villagedrainage and waterfront function was carried out for town activation in 1992. However, no blue-green algalbloom outbreak had occurred before. We studied the water quality and compositions of the zooplankton andphytoplankton before and after building a bulkhead maintenance construction. Large decreased values forNO3-N, NH4-N and NO2-N were 1.87→0.50 gNm-2(26.7%), 1.49→0.78 gNm-2(52.3%) and 0.085→0.030gNm-2(35.3%), respectively. Aphanizomenon flos-aquae (Cyanophyceae) were dominant in the summer;some species of the genus Fragilaria were dominant among phytoplankton during study periods. Thedominant species of zooplankton during study periods were Keratella cochlearis (Rotatoria); smaller thanCyclops vicinus (Crustaceae) were dominant species of zooplankton before the construction.