Entering of the open ocean water has affected the water quality of the Seto Inland Sea. In this study, the entering path and the behavior of the open ocean water masses after coming in were analyzed using three-dimensional flow simulation and Lagrangian particle tracking. The analysis revealed that the entering paths changed significantly depending on the Kuroshio current paths. The behavior suggested an influence on the water quality and primary production of the Seto Inland Sea.
Understanding water exchange with the open ocean correctly is very crucial for managing water quality in enclosed sea areas. The Seto Inland Sea (SIS) is the largest semi-enclosed sea in Japan, with a water area of about 23,200 km2 and an average depth of 38 m, and has a complexly shaped coastline (Fig. 1). The SIS consists of 12 bays and nadas, and is connected to the Pacific Ocean by two waterways, the Bungo Channel and the Kii Channel.
Due to industrial development and population growth in the SIS, pollutant load from the land area increased, and eutrophication became a serious problem. To solve this issue, the inflow load from land areas has been reduced. As a result, the concentrations of nitrogen and phosphorus have decreased, and the frequency of red tide has also declined. The nutrient concentrations have recently decreased significantly, resulting in the problem of oligotrophication, and there is concern about aquatic organisms and fisheries resources. Therefore, measures have been taken in recent years to increase nutrient concentrations in seawater in several bays and nadas in the SIS to increase primary production and fishery productivity. To achieve this goal, understanding the origin and dynamics of nutrients in the seas is essential. Previous studies showed that the water quality of the SIS varies greatly depending on the entering water mass of the open ocean (Takashi et al., 2006; Kobayashi and Fujiwara, 2008). Leng et al. (2023) conducted a three-dimensional numerical simulation and estimated that 61% of the dissolved inorganic nitrogen existing in the SIS originated from the open ocean, thus the influence of the open ocean water is very significant. Therefore, clarifying the characteristics and behavior of the open ocean water entering the land shelf is important for assessing the potential and limitations of water quality management in the SIS.
