Study on the Occurrence Mechanism of Hypoxic Water in the Interior Western Parts of the Ariake Sea Using the Box Model

Abstract

In the present study, the occurrence mechanism of hypoxic water that occurs frequently in the interior western parts of the Ariake Sea was investigated by the two-layer box model on the base of field observation data and Saga Prefecture research data in 1972-2000.
As a result, seasonal variations of advection velocity, vertical diffusion coefficient and biochemical oxygen-consumption rate in the study area were clarified. The physical process (i. e. advection and vertical diffusion) and biochemical oxygen-consumption process, which related to the temporal variation of dissolved oxygen (DO) concentration in lower layer box, were evaluated. The analytical results showed that vertical diffusion and biochemical oxygen-consumption contribute greatly to the temporal variation of DO concentration in lower layer box during summer season, and 83% of DO was supplied to lower layer box by vertical diffusion. In addition, it was considered that the occurrence of hypoxic water in the study area during summer season was caused by the fact that the biochemical oxygen consumption in the bottom layer was more than DO supply from the surface layer to the bottom one due to vertical diffusion.