DEVELOPMENT AND CHARACTERISTICS OF OXYGEN -DEFICIENT WATER MASS IN HYPERTROPHIC DOKAI BAY, JAPAN

The development of oxygen-deficient water mass in Dokai Bay was invest igated temporally and spatially during the summer of 1994. It was foun d that oxygen-deficient water mass (<2 mg O-2/l) was generated from th e 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 t wo 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 th e bottom up to 1.5 m below the surface, where wind over 3 m/sec preven ts 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.