Mechanisms of occurrence of hypoxic water in the inner parts of Ariake Bay

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Recently, the contents of mud and organic matter produced by the red tide plankton have recently increased in the surface layer of the sediment in the central and eastern areas of the inner part of Ariake Bay. These facts indicate the decrease of tidal current and vertical mixing potentials of the water in these areas, and suggest that the stratification of the water tends to be enforced following the increase of river water input in the long rainy season in mid June to mid July and the short one in autumn. The results of this study shows the mechanisms of frequent occurrence of red tide, progress of organic enrichment of the sediment, and the occurrence of a large scale of hypoxic water in the inner part of Ariake Bay without increase of nutrient loading.