Abstract
In Isahaya Bay, the secular variation of hypoxic water mass and the correlation of concentration of dissolved oxygen (DO) near the seabed were investigated using the data obtained by the Kyushu Regional Agricultural Administration Office. The factors controlling the DO concentration and the characteristics of the spatial variation were also investigated by means of multiple regression analysis and principal component analysis, respectively. The rate at which Isahaya Bay was covered with the hypoxic water mass (DO . 4 mg/l) was largest in 2006. The hypoxic water mass occurred most frequently near the center of the bay, and the probability decreases toward the inner area and the Shimabara Peninsula. The multiple regression analysis indicated that the density stratification decreases the DO concentration near the seabed, and the Chl.a concentration increases the DO concentration. The three main modes in the spatial variation of DO in Isahaya Bay are shown using the principal component analysis.
