VERTICAL DISTRIBUTIONS OF NUTRIENT AND PHYTOPLANKTERS ON LAKE WAKU -IKE IN SUMMER

Vertical distributions of nutrient and phytoplankters were investigated on as malllake Waku-ike (area :2， 338ha，d epth :8. 5m) on 31 August 1992. Thermocline layer was developed from 2t o 4m eters depth. Its water temperature difference was 11.1\u27C in this layer. Dissolved oxygen was disappeared at 3 meters depth. Nitrate nitrogen and reactive silicate were gradually increased toward the bottom. Reactive phosphorus and total phosphorus were rapidly increased below the thermocline. Standing crop of 95.6 % of phytoplankters in cell volume concentrated in surface layer from 0 to 2 meters. But the maximum peak of chlorophyll ac oncentration (207.0mg/m3)w as observed at 4m eters depth，w here the water color was milky green. The estimated cause of the green water was due to dense populations of photosynthetic sulfur bacteria，Chlorobium sp.. Phytoplankters observed were in 5 classes and 33 species. Dominant species were Ceratium hirundinella and Peridinium elpatiewsk Composition of phytoplankton species was similar to the record of 10 years ag