In recent years, marine products industry has been damaged seriously by red tide in the Yatsushiro Sea.Furthermore, the coastline of the Yatsushiro Sea has been changed by land reclamation is the case in the Ariake Sea. In this research, we tried to reveal the behavior of low-salinity water masses in summer season and the shift of physical fields triggered by the change of coastline. As a result, the followings were clarified: 1) In case of large discharge such as over 1,000m 3 /s from Kuma River is happened, surface layer's salinity across the Yatsushiro Sea falls to a low level; 2) The change of coastline caused by land reclamation from1900s to 2000s, so that 30% decrease of tidal current in closed-off section and 10% decrease of tidal current in the southern part of the Yatsushiro Sea are caused.
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.
We conducted highly-frequent water sampling for mercury speciation in sea water and suspended solid (SS) in Minamata Bay with continuous current and SS measurements. As a result, the followings were clarified: i) Particulate total mercury (T-Hg) in bottom layer was higher than that in surface and middle layer; ii) Dissolved T-Hg and methylmerculy (MeHg) were almost uniform vertically. Negative correlation between T-Hg and MeHg was shown; iii) Annual particulate T-Hg transport from Minamata Bay to the Yatsushiro Sea was estimated as 5-13kg. These results suggest that highly-frequent mercury measurement plays an important role in an accurate estimation of mercury mass balance.
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