1Graduate School of Fisheries Science and Environmental Studies, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki-city, Nagasaki, 852-8521, Japan
2Hydrospheric Atmospheric Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8601, Japan
3Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8 Taira-machi, Nagasaki 851-2213, Japan
4Center for Ecological Research, Kyoto University, Japan, Hirano 2-509-3, Otsu city, Shiga, 520-2113, Japan
5Faculty of Fisheries, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki-city, Nagasaki, 852-8521, Japan
Abstract. The northern East China Sea (ECS) serves as a spawning and nursery ground for many species of fish and squid. To clarify the basis of the food web in the northern ECS, we examined the nitrate (NO3) dynamics along four latitudinal transects based on stable nitrogen and oxygen isotopes of NO3 (δ15NNO3 and δ18ONO3) and temperature-salinity dynamics in both winter (February 2009) and summer (July 2009 and July 2011). The δ15NNO3 and δ18ONO3, which were distinctly different among the potential NO3 sources, were useful for clarifying NO3 sources and its actual usage by phytoplankton. In winter, Kuroshio Subsurface Water (KSSW) and the Yellow Sea Mixed Water (YSMW) predominantly contributed to NO3 distributed in the shelf water. In the surface water of the Okinawa Trough, NO3 from the KSSW, along with a temperature increase caused by an intrusion of Kuroshio Surface Water (KSW), seemed to stimulate phytoplankton growth. In summer, Changjiang Diluted Water (CDW), Yellow Sea Cold Water Mass (YSCWM), and KSSW affected the distribution and abundance of NO3 in the northern ECS, depending on precipitation in the Changjiang drainage basin and the development of the YSCWM in the shelf bottom water. Isotopic fractionation during NO3 uptake by phytoplankton seemed to drastically change δ15NNO3 and δ18ONO3, which may indirectly indicate the amount of primary production. And δ15NNO3-ln([NO3]) dynamics and relatively lighter δ15NNO3 suggested that atmospheric nitrogen and nitrification may have contributed to NO3 dynamics, too, in surface and subsurface layers, respectively, during summer, suggesting a tightly coupled nitrogen cycle in the shelf water of the northern ECS.