Change in the qualities of water and bottom sediment with the development of anoxic layer in a stratified lake.

The change in the qualities of water and bottom sediment accompanied with the development of anoxic layer was studied. A dredged water region in the South part of Lake Biwa was being used as a model of the Northern Lake with a fair water depth. An anoxic layer was observed after the development of the water stratification due to vertical difference in temperature in June. The accumulation of ammonia, phosphates, iron, manganese, and sulfides was observed in the hypolimnion. As a result, considerable amount of these chemical components was found to liberate from bottom sediment into the bottom water. Active liberation of sulfides began not after the development of an anoxic layer but at almost the same time as the stratification. Sulfide production in the bottom sediment with a large amount of organic matter accumulation was accelerated by the presence of organic acids such as formic, butyric, and lactic acids. The production of these acids in the bottom sediment was closely related with the supply of dissolved oxygen to the bottom surface, which might be governed by the vertical diffusion of lake water.