Response of lower trophic ecosystem in the Japan Sea to horizontal nutrient flux change through the Tsushima Strait

Abstract

The Japan Sea is a semi-closed sea with straits such as the Tsushima Strait (Korea Strait). The nutrient transport through the Tsushima Strait impacts on the ecosystem around the coast of Japan. We analyzed hydrographic data from the Tsushima Straits to reveal interannual variations in the volume transport, nutrient concentration, and nutrient transport. The nutrient transport in the strait has large interannual variations of ±50% of average transport. We developed a coupled ocean circulation and ecosystem model, in which the lateral boundary condition was applied based on our observation data, and investigated the biogeochemical responses in the Japan Sea to the variations in horizontal nutrient flux through the Tsushima Strait.We reproduced the lower trophic ecosystem in the Japan Sea and evaluated the contribution ratio of dissolved inorganic nitrogen supplied from the Tsushima Strait to the biogeochemical cycles in the Japan Sea. A primary production of more than 70% in the region of 300–400 km from the coast of Japan is determined by the nutrient inflow through the Tsushima Straits. We investigated the response of the lower trophic ecosystem in the Japan Sea associated with nutrient transport change (±50%) through the Tsushima Strait. We found large impacts on primary production related to horizontal nutrient flux change in the central regions in the Japan Sea and near the East Korea Warm Current region. The increase of chlorophyll a in the central regions is caused by the transportation of nutrients through the sub-surface layer of the Tsushima Straits from 50 m to 100 m. Horizontally transported nutrients in the sub-surface layer are not utilized by phytoplankton during the summer because of stratification, but they are supplied in the surface layer in autumn and winter by vertical mixing. These processes also increase primary production in broader regions (132° E, 37–41° N). The case study on the increased time for nutrient transport showed that the nutrient flux change in summer had the highest impact on the lower ecosystem changes. The present study suggests that the interannual variations in the nutrient transport through the Tsushima Straits impact the productivity of autumn blooms in the broader regions of the Japan Sea.