An attempt of modification of Carlson’s trophic state index (TSI) for brackish lakes in Japan

At the beginning of this study, as the first stage of trophic state index (TSI) related studies for brackish lakes in Japan, existing studies related to Carlson’s TSI were briefly reviewed focusing on brackish lakes in Japan. Carlson’s TSI and its modified indices had been applied to some brackish lakes in Japan. However, it had been difficult to find an applicable equation for TSI for these brackish lakes in Japan. Theoretically, the water quality of surface layers should be used for TSI, however, the regression coefficient using surface layer water quality data and all layers water quality data were almost the same to the extent of the preliminary analysis based on the existing study. In this study, TSI was applied to brackish lakes in Japan by following two methods. First, some characteristics of a few Japanese brackish lakes were summarized from lake databases to establish TSI for brackish lakes in Japan. However, it was difficult to find an applicable equation for TSI for brackish lakes in Japan. Second, equations to calculate TSI for brackish lakes in Tottori and Shimane Prefectures were tentatively presented after the regression analysis of publicly monitored water quality data. Preparing database of water quality, water quantity and physiographical parameters for the brackish lakes in Japan, and further consideration on the equations of TSI would be needed for better understanding of these parameters.