Characteristics of water conditions in the inner part of Ariake Bay with hypoxic water and red tide and causes of decline of the ecosystem on the tidal flats faced to the bay.

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The water conditions were monitored monthly or biweekly at 9 to 12 sampling stations in the inner part of Ariake Bay from April 2005 to December 2007. At four of these sampling stations, which are located in the innermost areas of the bay, the benthic communities were assessed quantitatively at the same sampling occasions. In this bay, hypoxic water occurred between the end of July after the rainy season and August in the innermost areas of less than 20 m in depth every year. On August 5, 2006, the most severe decrease of DO occurred in these areas. In the middle layer of the water, anoxic water was formed, and DO of the bottom water also fall to the levels of less than 1 mg/L. In this time, the surface water stood over 30℃, and the phytoplankton, mainly diatoms that bloomed during the rainy season, died due to the extremely high water temperature. The dead body of the phytoplankton became large amounts of dead organic particles suspended in the water, and was being decomposed, consumi … More ng DO in the water. This phenomenon indicate that the hypoxic water has occurred by the increase of DO consumption from the sea floor with organic enrichment of he sediment, but also the increase of water temperature due to the recent global warming is becoming another factor that increase oxygen consumption in the water and results in the progress of hypoxia or anoxic in the bay. The benthic communities in the innermost areas of the bay were declining markedly year to year due to the occurrence of the severer hypoxic or anoxic water. They repeated a cyclic fluctuation in density and biomass, that they decreased rapidly in summer, and recovered from autumn to winter. However, the benthic communities did not recover fully in the cold seasons. The benthic communities consisted of restricted members including small spionid polychaetes and small bivalves that had high adaptability against the environmental fluctuations. Therefore, the total biomass of the benthic communities were very poor (less than 10g /m^2) in spring, when they tended to reach a peak. If the present benthic conditions with increasing organic discharge from the occurrence of large scales of red tide and developing hypoxic water in levels and areas continued in summer, the benthic ecosystem in the inner parts of the bay would be seriously destroyed in near future. Less