Influence of Summer Effluent on the Bottom Water DO Variation in the Yatsushiro Sea, Japan

Abstract

 In recent years, the water environment of the Ariake Sea and the Yatsushiro Sea in western Kyushu has been deteriorating, and the damage to fisheries has continued to expand. Dissolved Oxygen (DO) plays a vital role in maintaining the health of ecosystems and participates in the primary process of the ecological cycle. In the Yatsushiro Sea, it was first reported that the DO occurred a downward trend in the summer of 2016 and reached a level of 4 mg/L in part of the sea areas in the summer of 2019. DO reduction is more inclined to occur in the summer, as heavy rainfall causing a large amount of freshwater flowing into the bay may cut down the vertical mixing of the water column, thus further triggering the death of benthic communities. In this study, a coupled numerical model was used to simulate the different physical and biological processes in a semi-enclosed coastal area to evaluate the impact of large-scale effluent on changes in the bottom water DO in 2020. We also evaluate the effects of the extent of the effluent on the development of hypoxia. The results indicate that large-scale effluent is responsible for the occurrence of hypoxia. Notably, on July 31, 2020, hypoxia occurred in the northern part of the Yatsushiro Sea, and the state of low DO concentration (less than 4 mg/L) lasted for more than two weeks. It is an effective way to provide the basis of environmental adaptation by evaluating DO dynamics in the Yatsushiro Sea.