Distribution of Some Physicochemical Parameters and Trophic Status of Lake Onogawa

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Abstract : The first data set of depth time profiles on some physicochemical parameters of Lake Onogawa over a two-year period is presented. Lake Onogawa is a dimictic lake. Hypolimnetic dissolved oxygen was not detected from August to October. This is different from Lake Hibara, a sister lake, where hypolimnetic oxygen was not exhausted in summer. Nitrate was depleted in the anoxic zone of Lake Onogawa, suggesting the operation of denitrification. Hypolimnetic ammonia consistently increased from spring to summer, while nitrate decreased during that same period. Based on the mass balance between ammonia and nitrate, apparent nitrification was not evident during this period. Chl. a concentration ranged from less than 3 μg・1~-1 to 48 μg・1~-1. It reached an annual maximum in early autumn. The Chl. a maximum was about 3 times greater than that of Lake Hibara. Mean transparency was about 3 m during the vernal and autumnal circulation periods and 5 m in the summer stratification periods. Mean concentrations of total phosphorus and Chl. a, and mean transparency in the circulation period indicate that the lake is oligotrophic, eutrophic and mesotrophic, respectively. The anoxic hypolimnion also indicates that the lake is eutrophic. Based on these facts, the lake is classified as on the boundary between meso- and eutrophic.　Key words : depth-time profiles, dammed lake, nutrients, nitrogen metabolism