The coastal brackish lakes, Lake Namako-ike, Lake Kai-ike and Lake Kazaki-ike lie to the north-east shore of Kamikoshiki Island in Kagoshima Prefecture of Japan. They are separated from the sea by along gravel bar called as Nagameno-hama Quality of water(salinity, electric conductivity, pH, Cl^-, dissolved oxygen, H_2S), change of water levels and distribution of diatom assemblage and analysis of total organic carbon(TOC), total nitrogen(TN) and total sulfur(TS) contents in the surface sediments were carried out, for clearing water environment of the three lakes.
The three lakes have different salinity. At Lake Namako-ike, the salinity of surface water was 30～34%o. At Lake Kai-ike, 20～29%o. At Lake Kazaki-ike, 5～9%o. Lake Kai-ike is stratified, the layer of high salinity is covered by the layer of low salinity, and the deeper layer contained a significant amount of H_2S. It has a bacterial plate in the upper H_2S layer. Analysis of TOC, TN and TS content in the surface sediments and distribution of diatom assemblage are corresponded to the difference of environment in the three lakes.
Quality of water has been kept up at least 13 years in comparison with Matuyama.(1985) and Yamamuro.(1978) Inflow and outflow of seawater has been kept up at least 50 years in comparison with Aramaki et al.(1976) and Yasui and Shinoda.(1938).