Transtation of Water Quality Class in Ogochi Dam Reservoir in Terms of Status Changes of Phytoplankton in Production Layer

Abstract

Lake Okutama is an artificial lake formed by the Ogochi Dam that blocks the upstream of the Tama River. Analyzing the plankton data of 30 years (1974-2003) at the Ogochi Dam point, this paper characterized the water quality transition that can be divided into three patterns and three periods in terms of the increase of dominant plankton species. This paper revealed that the recent water pollution phenomenon is caused by the third pattern, which indicates an increase of cyanbacteria. After the enforcement of "selective withdrawal" in 1992, Anabana and the Microcystis became the dominant species in 1999. The selective withdrawal has changed the flow mechanism of the reservoir. It was considered that management of dam reservoir operation was related to the appearance of Anabaena and Microcystis, which cause the water pollution.