At the head of Tokyo bay, blue tides often occur from summer to autumn by the bottom layer upwelling. It is a factorof blue tides that the anoxic water in the bottom of dredged trench produces sulfide in high concentration. The purposeof this paper is to investigate behavior of the bottom water in Tokyo bay and its influence on the bottom water of thedredged trench. We surveyed water qualities from Aug.(2007) to Dec. across Tokyo bay. As a result, intrusion of oceanicwater to the head of the bay develops density stratification, and produces the situation in which a blue tide occurs easily. The bottom water of dredged trench stays when density of the water is higher than the intrusive water. Additionally, thelow salinity layer is newly observed near a density interface in the dredged trench.
