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We have been attempting to develop the numerical model for mercury fate in Minamata Bay and Yatsushiro Sea. In this paper we compared numerical results of bottom sediment transport from the bay to the outer sea area for one year with and without sea water and freshwater stratification. As a result, it was clarified that long term bottom sediment transport was determined by both of barotropic currents and baroclinic ones. Thus, accurate numerical model for particulate mercury fate must take into account baroclinic structure in actual coastal area due to freshwater inflow.

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We have been conducting monthly water sampling to detect particulate/dissolved mercury in sea water in Minamata Bay from 2006. After 2010, LISST-100X was applied for measurement of vertical profile of suspended solid grain size distribution. In addition, core sampling for bottom sediment was conducted twice. As a result, the followings were clarified: i) There is significant difference of SS grain size distribution between upper layer and lower one in density stratification; and ii) Origin of SS in upper layer can be estimated to exist in except for bottom sediment and load from rivers.