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

Y Umezawa,A Yamaguchi,H Yoshimura,N Yamawaki,T Aoshima,I Tayasu,Y Morii,C Yoshimizu,T Hasegawa,J Ishizaka

doi:10.5194/bg-11-1297-2014

Abstract

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. Although isotopic fractionation during NO3 uptake by phytoplankton seemed to drastically increase δ15NNO3 and δ18ONO3 in summer, relatively light nitrate with δ15NNO3 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.

Highlights

Surrounded by China, Korea, Japan, and Taiwan, the East China Sea (ECS) is a very productive marginal sea that is used as a spawning and nursery ground by many species of fish and squid (Sakurai et al, 2000; Hiyama et al, 2002)
Taiwan Warm Current Water (TWCW) and Kuroshio water, which contribute most to the water budget of the region, have been estimated to be major sources of NO3 based on nutrient budget models (Chen and Wang, 1999; Zhang et al, 2007a), our results show that Changjiang Diluted Water (CDW), Yellow Sea Cold Water Mass (YSCWM) and Kuroshio Subsurface Water (KSSW) are important nutrient sources for primary production at different locations on the northern continental shelf of the ECS
Based on a survey of dual isotopes of NO3, δ15NNO3 – ln[NO3] dynamics, and T –S diagrams, different sources of NO3 and their actual use by phytoplankton were successfully identified in the northern ECS

Summary

Introduction

Surrounded by China, Korea, Japan, and Taiwan, the East China Sea (ECS) is a very productive marginal sea that is used as a spawning and nursery ground by many species of fish and squid (Sakurai et al, 2000; Hiyama et al, 2002). The jack mackerel, one of the most commercially important fish resources, hatches during winter in the southern ECS, and juveniles migrate in early summer to northern areas along the shelf-break region of the central ECS and move farther north in late summer (Sassa et al, 2006, 2009). Primary production and subsequent zooplankton production in the ECS during each season, as well as the corresponding nutrient supplies supporting phytoplankton growth, need better understanding because of their importance as food sources for juvenile fishes. Umezawa et al.: Seasonal shift of the nitrate sources in the northern East China Sea

Methods

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Discussion

Conclusion