Abstract
Lake Koyama is a low salinity lake that is connected to the Japan Sea via Koyama River. The seawater inflow into the lake is controlled by a sluice gate set up in the river. In summer there is no seawater inflow because the lake water is used for irrigation. On the other hand, in winter, to promote the use of the lake as a fishery, there is an artificial seawater inflow around November. This paper details observations performed to see the change in water quality when seawater flowed into the lake in the winter of 2001. The following has been understood as a result.
The seawater quality was found to have already deteriorated around the mouth of Koyama River. When this water flowed into the river, it became worse, because nutrients emerged from the bottom mud and sewage was carried into the river. Moreover, when the seawater entered the lake it concentrated in the vicinity of the deepest part. As a result of this, the dissolved oxygen of the lower layer disappeared for a few days after the inflow and this made the water quality worse. This situation continued for about 20 days until destratification by a strong wind occurred.
