RELATIONSHIP OF INTERNAL WAVES WITH WATER MOVEMENT AT THE HOLLOW IN YONAGO BAY OF LAKE NAKAUMI

Average depth of Lake Nakaumi is 5.4m in depth, and a lot of hollows of 10m or more in depth exist in the Yonago bay. The deepest place is 16m in depth. In Lake Nakaumi, a stable stratification with a halocline is formed by the density difference between seawater flowing in through the Sakai Channel from the Sea of Japan and freshwater from the many rivers which flow into the lake. This stable two layer structure produces a peculiar aquatic environment. The highly saline water mass below this halocline can easily become anoxic due to both weak vertical mixing between the upper and lower layer, and generate the nutrient release in the lower layer. It is well known that internal waves with a large amplitude of 2-3m and period of 24 hours are often driven by tidal and wind drift effects around Yonago Bay of Lake Nakaumi (Fujii, 1997).Then, we carry out the observation to detect the internal oscillation and flow and its effect on the vertical distribution of water properties, and found a dynamical relation between the internal oscillation and the anoxic water movement in the hollows. As a result, internal oscillation with a large amplitude of 2-3m and period of 20 hours are observed in the hollow and changed periodically. Therefore, it is though that the water of the hollow periodically creeps up towards the neighboring water regions by internal oscillation, greatly affecting the general water quality and biological environment.