Studies on the Occurrence of Hypoxic Water in the Interior Parts of the Ariake Sea and its Preventive Measures

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The actual conditions of hypoxic water which occurred in the western interior parts of the Ariake Sea were clarified based on the data of dissolved oxygen concentration (DO), velocity (V), salinity (S) and water temperature (T) at the 30 cm height from the sea-bottom measured hourly in summer of 2005-2007 using two multi-items measures with DoPa communication system. The relationships between the occurrence of hypoxic water and vertical structure of density calculated from Sand Talong the measurement lines in longitudinal and lateral directions set in this sea area were also grasped, and it was shown that the density pycnocline were always formed at the time of the outbreaks of hypoxic water and the vertical diffusion coefficient decreased rapidly near the density pycnocline. Moreover the consumption rates of DO in the bottom layer depended largely on the amount of suspended organic mutter there. On the other hand, the analysis results of DO data during 34 years by a two-layer box model indicated that the vertical diffusion coefficients decreased rapidly, and on the contrary, the consumption rate increased largely as the density stratification parameter rose greatly in summer. The occurrence of hypoxic water in summer was due to the fact that with the formation of the density pycnocline, the vertical diffusion flux of DO from the surface layer to the bottom one in the sea-water column became more than the consumption of DO in the bottom layer. It was also presented experimentally that the installation of a triangle block with special shape on the sea-bottom was effective for prevention of the occurrence of hypoxic water in the study sea area