No abstract

To clarify the mechanism of hypoxia in the western interior parts of the Ariake Sea (WIAS), field observation data collected in the period of 1972-2004 were analyzed using a two-layer box model. Monthly averages of advection velocity, vertical diffusion coefficient (K(z)), and biochemical oxygen consumption rate (R) in WIAS were evaluated quantitatively during the above period. The estimated advection velocity comparatively corresponded to the observed residual flow pattern of bay head in summer and winter. The estimated K(z) was relatively high (0.6-5.3 cm(2) s( -1)) from September to March but lower (0.2-0.4 cm(2) s( -1)) from April to August. The estimated R ranged from 0.30 to 0.46 mg L( -1) day( -1) during May to August. In summer, the temporal variation of dissolved oxygen (DO) concentration in the lower layer was controlled largely by K ( z ) and R. Monthly variations of K(z), R, and degree of density stratification (P) in the 1970s, the 1980s, and the 1990s-early 2000s were analyzed. P, K ( z ), and R were not significantly different among the calculated periods (p = 0.93, 0.23, and 0.49). However, the variations of R in summer between the 1970s and the other calculated periods changed. DO consumption period was longer in the 1980s and the 1990s-early 2000s than in the 1970s. R in the 1980s was highest among the calculated periods. The increase in R in the 1980s was caused by the increase in organic matter load originating from red tide phytoplankton due to a decrease in the suspension feeders.

From the data measured in the interior western parts of the Ariake Sea in summer of 2006, vertical diffusion coefficient profiles in the water column under the formation of density stratification were clarified. Oxygen consumption was mainly caused by high consumption rate in the bottom water below the pycnocline, while the contribution of the bottom sediment to the oxygen consumption was small. The seasonal variations of vertical diffusion coefficient and oxygen consumption rate in the study area were analyzed by the two-layer box model. As a result, vertical diffusion coefficient tended to increase in summer-autumn and to decrease in winter-spring, and its seasonal variations were closely related to that of density stratification parameter. On the other hand, oxygen consumption rate tended to be positive (O 2 consumption) in spring-summer and to be negative (O 2 production) in autumn-winter.