Dissolved Oxygen, Bloom of Microcystis and Susu-mizu of Summer-autumn Season in Lake Suwa

Vertical distribution of dissolved oxygen and chlorophyll in summer-autumn season were determined in the offshore of Osaka in Lake Suwa. In mid-summer, when whole the surface of the lake was covered with blooming Microcystis, dissolved oxygen in the surface water layer could attain over-saturation to the utmost extent (160%). Late in August the amount of dissolved oxygen markedly decreased less than 90% saturation, in spite of a large amount of Microcystis existed. It might be related to the declining of the photosynthetic activity of the alga. Moreover, if drifting of the upper water containing a great part of Microcystis by wind (Susu-mizu) occurred, serious decrease of dissolved oxygen (less than 50% saturation) was observed. Wind and waves usually were not favorable to increase oxygen into the surface water. In such a situation carps cultured in the floating nets may be exposed to clanger of asphyxia unless a large amount of dissolved oxygen is supplied from surrounding water outside of the nets during the night. There was a relationship between the pH value and the saturation percentage of dissolved oxygen in the water of 0 to 2 m depth.Article信州大学理学部紀要 2(2): 139-148(1968)departmental bulletin pape