Drawback of Water Quality and Lake Current in Lake Chūzenji

新井, 正, 岡田, 浩美, 植田, 芳明, ARAI, Tadashi, OKADA, Hiromi, UEDA, Yoshiaki

Lake Chuzenji in Nikko National Park has been considered as a clear oligotrophic lake. In recent years, some drawbacks of water quality have been reported even in this lake. The drawback was brought by the bloom of Uroglena americana, a kind of plankton which brought freshwater red-tide in several lakes in Japan. In the case of this district, bloom of Uroglena is first observed in Lake Yunoko in May. Lake Yunoko is a small and shallow polluted lake in the upper reaches. Then the plankton flows into Lake Chuzenji through River Yukawa and brings the bloom in June and July in this lake. In early summer, the lake is thermally stratified and the inflowing water mass submerges below the surface as density current. As the vertical diffusion of the lake water in this season is limited by thermal stratification, and thermal and optical conditions favorable for the growth of Uroglena are also limited, the bloom is observed in a narrow water layer between 5 and 15 m below the surface. Observation of lake current was made in June and July, 1983, to search the diffusion of inflowing materials from the river into the whole lake areas. The result is shown in Fig. 4, where evident anti-clockwise curent is obtained. The observation was made by use of current cross (drag) and tracing the movent of buoy connected to the cross at each half day interval. The result shows that the water mass from the river may be distributed to the whole lake area in several days. The local waterworks intake the water from 10m depth in the lake which is regarded as an unfavorable site for water quality. Our committee recommended to change the depth more deeper layer.

Drawback of Water Quality and Lake Current in Lake Chūzenji

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