Seasonal shifts in the contributions of the Changjiang River and the Kuroshio Current to nitrate dynamics in the continental shelf of the northern East China Sea based on a nitrate dual isotopic composition approach

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 (δ¹⁵N_NO3 and δ¹⁸O_NO3) and temperature–salinity dynamics in both winter (February 2009) and summer (July 2009 and July 2011). The δ¹⁵N_NO3 and δ¹⁸O_NO3, 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. Although isotopic fractionation during NO₃ uptake by phytoplankton seemed to drastically increase δ¹⁵N_NO3 and δ¹⁸O_NO3 in summer, relatively light nitrate with δ¹⁵N_NO3 lower than expected from this fractionation effect might be explained by contribution of atmospheric nitrogen and/or nitrification to NO₃ dynamics in the surface and subsurface layers. If the latter were a dominant process, this would imply a tightly coupled nitrogen cycle in the shelf water of the northern ECS.