The development of oxygen-deficient water mass in Dokai Bay was investigated temporally and spatially during the summer of 1994. It was found that oxygen-deficient water mass(<2mg O2/ι) was generated from the bottom layer of the inner part of the bay, and then spread not only vertically toward the surface layer, but also laterally to the middle part of the bay. The oxygen-deficient water mass persisted for about two months and its maximum volume was estimated to be 20% of the whole volume of the bay.
A pycnocline, due to the inflow of river water, was found at the inner part of the bay where the recurrent formation of a red tide resulted in a thin surficial euphotic layer. Such conditions enable the formation of oxygen-deficient water mass expanding from the bottom up to 1.5m below the surface, where wind over 3m/sec prevents this water mass from expanding further vertically. These phenomena of oxygen-deficient water mass in Dokai Bay are different conpared to other eutrophic sea areas in Japan.
