OCCURRENCE OF HYPOXIC WATER MASSES CAUSED BY THE LARGE-SCALE FRESH WATER OUTFLOWS IN THE ARIAKE BAY IN 2020 AND 2021

Abstract

 In both 2020 and 2021, hypoxic water masses were formed in the Ariake Bay from the inner part of the bay to the Isahaya Bay. In 2021, the distribution of hypoxic water masses were observed before and after the freshet of the Chikugo River, indicating that the area of hypoxic water masses have approximately doubled due to large-scale outflow of fresh water. Furthermore, the duration of density stratification due to the freshet was approximately one month, which is longer than the time scale of hypoxia. It is considered that the hypoxic water mass formed mainly in the tidal flat marginal area in the western part of the inner part of the bay expanded further to the southern part of the inner part of the bay due to the large scale fresh water discharge, resulting in the largest hypoxic water mass that was combined with the hypoxic water mass formed in the Isahaya Bay.