[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-P03] Origin of dissolved inorganic nitrogen in the Tsushima Strait and its seasonal and interannual variation

*Ryota Shibano¹, Akihiko Morimoto², Katsumi Takayama³, Tetsutaro Takikawa⁴, Joji Ishizaka⁵, Xinyu Guo² (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Center for Marine Environmental Studies, Ehime University, 3.Research Institute for Applied Mechanics, Kyushu University, 4.Graduate School of Fisheries and Environmental Sciences, Nagasaki University, 5.Institute for Space-Earth Environmental Research, Nagoya University)

Keywords:East China Sea, nutrient cycle, river discharge

Tsushima Strait is shallow and narrow straits connecting the East China Sea and the Japan Sea. Along the coast of Japan in the Japan Sea, a warm and saline water mass is supplied from the East China Sea (ECS) and the Pacific Ocean by the Tsushima Warm Current (TWC). This current transports water, heat, and nutrients from the southern regions to the Japan Sea. TWC is originated the transports from the six sources, the Pacific through the Kuroshio, the Taiwan Strait, the Changjiang and Yellow Rivers, the dust from the atmosphere, and the bottom layer in the East China and Yellow Seas. The transports by TWC have the large variations in the seasons and/or years according to the observations in the Tsushima Strait. To investigate the dynamics in the Tsushima Strait, the estimations on the nutrient cycles in ECS and the Yellow Sea.
We developed a physical and ecosystem coupled model for the East China, Yellow, and Japan Seas to investigate the origin of dissolved inorganic nitrogen (DIN) in the Tsushima Strait. Assuming the potential six sources of DIN in the Tsushima Strait, we calculated the contribution ratio of each source of DIN in the Tsushima Strait using the numerical model. The most dominant source of DIN in the strait was DIN from the Pacific Ocean, with a contribution ratio of 40–50%. The second most dominant source was that from the Taiwan Strait, with 10%. The contribution from the rivers was less than 5%. Although more than 70% of the freshwater discharged from the Changjiang River enters the Japan Sea through the Tsushima Strait, most of the DIN input from the river sinks to the bottom of the ECS. A small portion that is decomposed at the bottom of the sea is transported in subsurface and lower layers to the Tsushima Strait. Interannual variation in DIN concentration was caused by variation in the DIN flux across the continental shelf in the ECS, and the influence of DIN originating from the Changjiang River was small. However, a drastic increase in DIN concentration could possibly occur if the N/P ratio in the Changjiang River were to become much higher than the current ratio.