Acoustic Equipment and ROV Investigations of Mercury Diffusion in Minamata Bay, Yatsushiro Sea, Japan

The purposes of this study are to understand mercury accumulation and diffusion based on topographic, sedimentologic, and biologic information obtained by acoustic equipment and an ROV after dredging in Minamata Bay and Yatsushiro Sea. Differences in contrast of acoustic reflections made clear the sediment distribution and the ROV investigation provided the vertical distribution of the biota and substrate. Rocks and blocks are distributed above 5 m water depth, coarse sand with rich molluscan fragments between 5 and 12m, and mud below 12m, with these sediments controlling the biota. The video shows that many depressions by ray's predation are distributed on sandy bottoms and burrows of benthic organisms on muddy bottoms. These facts suggest the possibility that a supply of mercury-free sediment moves from the sub-surface to the surface by burrowing, that mercury is concentrated by the benthos inhabiting the sediment surface, and that diffusion occurs by the predation of nekton.