[A-OS15] Ocean circulation and material cycle in coastal seas

[AOS15-10] Interannual variations of nutrient concentrations and the mechanisms of nutrient supply in the western Seto Inland Sea

*Sohei Ogawa¹, Naoki Yoshie¹, Hidezirou Ohnishi¹ (1.Center for Marine Environmental Studies, Ehime University)

Keywords:the inflow of river water, the bottom intrusion, Precipitation

The mechanism of nutrient supply in the Bungo Channel part of the Western Seto inland Sea is not only from terrestrial regions by the inflow of river water, but also from the offshore regions caused by entering the bottom intrusion of the Kuroshio subsurface water into the Bungo Channel (Takeoka 2020). In the Hoyo Strait which is a narrow strait sandwiched between the Sada-misaki Peninsula and Saganoseki Peninsula and located in the northernmost in the Bungo Channel, sea water is strongly mixed horizontal and vertical with the strong tidal current. For these reasons, it can be said that the nutrient concentration around Sada-misaki is representative of the nutrient concentration in most parts of the western Seto Inland Sea, including the Bungo Channel and Iyo-nada. In this study, we investigated the relationship between seasonal and interannual variations of the nutrient concentration around Sada-misaki and variations of chlorophyll concentration, and the relationship between variations of the nutrient concentration and nutrient supplies from terrestrial and offshore regions. We have used a lot of data such as Dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved inorganic silicon (DISi) and chlorophyll concentration, which has been monitored daily at the cape of Sada-misaki from 2004 by joint research of Ehime University CMES and Misaki Fisheries Cooperative, Meteorological data from AMEDAS, first-class river flow and nutrient concentration data from the Ministry of Land, Infrastructure, Transport and Tourism and multi-layered water temperature data from the Uwa Seawater Temperature System, and analyze comprehensively. As a result, the seasonal variations of nutrient concentrations at the cape Sada-Misaki had the minimum values in the spring, which were quite different form typical variations in the mid-latitude offshore regions with the minimum values in the summer. This is because the nutrient supplies more than necessary from the both terrestrial and offshore regions from late May to September associated with the rainy and typhoon seasons in Japan. The nutrient supply from the terrestrial region through the river discharge was controlled by the amount of precipitation around Kyusyu Island. On the other hand, the nutrient supply from the offshore region was controlled by the intensity of the bottom intrusion of the Kuroshio subsurface water which was enhanced by flowing due to density difference which caused by the fresh water discharge from the inside of the Seto Inland Sea. In addition, biological production in Western Seto Inland Sea is controlled by DIN. In such area, phytoplankton breeding was further promoted for the next three months in the year when a large amount of DIN flowed due to rainfall in August. In recent years, decreasing trend of nutrient concentrations observed in the Eastern Seto Inland Sea, but it didn’t occur around Sada-misaki including the Bungo Channel and Iyo-nada.