What is the future of Lake Kussharo, which has been naturally recovering from acid lake of pH 4, eutrophication or acidification once again?

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Water sampling and observation with CTD were repeated in summer and winter ice-covered seasons. Warm water was supplied from the lake bottom and it was mixed quickly, as was observed last year. Bottom water was cooled to 2℃ in winter, whose density was lighter than the maximum density of water. It was recognized that convection continues even in winter and anormal temperature profile was formed. Flow rate measurement and sampling from more than 10 inflow rivers and one outflow river (Kushiro River) were repeated. With a help of volunteer working around the lake, monthly sampling and observation of Kushiro River started.Loggers for water temperature, conductivity, chlorophyll and turbidity moored at the deepest point of Lake Kussharo from September 2003 were recovered in August 2004. It was shown that lake water mixed throughout the water column in autumn and spring mixing periods, and then the water temperature changed continuously without stabilization at the temperature of maximum water density. Chlorophyll concentration increased just after the ice melting. The logger for water temperature and conductivity was moored at the river mouth of Yukawa, which is acidic and rich in ionic components and the most important site to calculate the acid budget.Alkalinity and ionic components were determined. As a result, slight decreases in the concentration for ionic components and increase in that of alkalinity were detected through precise measurements. For the alkalinity does not decrease at present, re-acidification of Lake Kussharo is not expected. On the other hand, chlorophyll bloom in the spring might be a trigger or symptom for the eutrophication. Continuous observation as well as utilization of public monitoring data for nutrients is required to detect the change in the trophic stage of Lake Kussharo.