The changes of chemical and biological water environment of Lake Sapporo were investigated for over eight years from the beginning of water storage, and this is the first detailed report concerning the reservoirs situated in sub-frigid zones in Japan. The pollution load flowing into this reservoir is only nature-derived. At Lake Sapporo, the large inflow of water from the melting snow in spring supplies the middle and bottom layers with oxygen, and replaces water in the reservoir with new water. Under these circumstances, after effects of the initial deterioration were detected during two years in water quality, three years in phytoplankton composition and four years in zooplankton composition.

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