Analysis of Occurrence Mechanism of Hypoxic Water in the Western Interior Parts of the Ariake Sea Using a Two-layer Box Model

Abstract

In this study, the physical and biochemical parameters concerned with the occurrence of hypoxic water in the western interior parts of the Ariake Sea are analyzed by a two-layer box model using the Saga Prefecture research data in 1972-2004, and the occurrence mechanism of hypoxic water is investi- gated based on the above analytical results. As a result, the monthly variations of an average year value over 1972-2004 for advection velocity, vertical diffusion coefficient ( Kz) and biochemical oxygen con- sumption rate (R ) are clarified, and Kz and R contribute greatly to the temporal variation of DO concen- tration in lower layer box in summer. Next, the monthly variations of an average value for density stratifi- cation degree (P ), Kz and R in the 1970s (1972-1980), the 1980s (1981-1990) and the 1990s-early 2000s (1991-2004) are analyzed. As a result, in summer, P and Kz do not have precise change among the calcu- lated periods, but R between the 1970s and from the 1980s on is clearly different. It is considered that one factor which changes R in summer since the 1980s is an increase of organic matter amount and the change of decomposition process of organic matter in the study area. Summary