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- Title
Enhancement of Summer Nitrogen Fixation by the Kuroshio Intrusion in the East China Sea and Southern Yellow Sea.
- Authors
Jiang, Zhibing; Zhu, Yuanli; Sun, Zhenhao; Zhai, Hongchang; Zhou, Feng; Yan, Xiaojun; Zeng, Jiangning; Chen, Jianfang; Chen, Quanzhen
- Abstract
The western boundary currents are characterized by abundant diazotrophs including Trichodesmium, which may fuel N2 fixation when they intrude into marginal seas. The Kuroshio, a western boundary current in the North Pacific, flows into the East China Sea (ECS) and southern Yellow Sea (SYS), which transports abundant Trichodesmium and diatom‐diazotroph associations (DDAs). Additionally, low nitrogen:phosphorus (N:P) ratio and relatively abundant dissolved iron have been observed in the offshore ECS because of the Kuroshio intrusion as well as riverine/atmospheric inputs of P and iron. We hypothesized that the intrusion of Kuroshio greatly enhanced N2 fixation in the ECS and SYS. N2 fixation rates (NFRs) were measured using a 15N2 bubble method during summer 2013. The surface and depth‐integrated NFRs in the ECS and SYS were 1.45 nmol N L−1 d−1 and 81.7 μmol N m−2 d−1 on average, respectively, with the highest values of 13.84 nmol N L−1 d−1 and 511.8 μmol N m−2 d−1. We found that NFRs were significantly higher in the ECS oceanic (Kuroshio water) and mesohaline regions (Kuroshio‐affected water) than in the SYS and the ECS low‐salinity and coastal upwelling regions. NFR was significantly positively correlated with the densities of Trichodesmium and DDAs, salinity, and temperature but was negatively with NO3− and N:P ratio. Generalized additive models confirmed that spatial variation in NFR was overwhelmingly contributed by Trichodesmium density. These findings suggested that the Kuroshio intrusion significantly enhanced N2 fixation in the ECS through promoting growth of filamentous diazotrophs and providing appropriate nutrient environment. Plain Language Summary: Marine diazotrophs convert unbioavailable nitrogen (N2) into bioavailable nitrogen (NH3) through N2 fixation, which relieves the restriction of nitrogen limitation to phytoplankton primary production and enhances oceanic carbon fixation, resulting in greater net sequestration of CO2 and carbon sink. The western boundary currents (e.g., Kuroshio) are characterized by abundant diazotrophs including Trichodesmium, which may fuel N2 fixation when they intrude into marginal seas. In order to test this hypothesis, N2 fixation rates were measured during summer in the East China Sea (ECS) and southern Yellow Sea (SYS) influenced by the Kuroshio intrusion. We observed active N2 fixation in the Kuroshio mainstream and affected waters in the ECS, which were characterized by abundant Trichodesmium and diatom‐diazotroph associations, severe deficient nitrogen, and available iron and phosphorus. Nevertheless, relatively low N2 fixation rates were detected in the SYS and the ECS low‐salinity and coastal upwelling regions because of low‐density filamentous diazotrophs and sufficient nitrogen. Our results revealed great enhancement of N2 fixation by the intrusion of Kuroshio in the marginal seas. This study provided high spatial resolution data sets of N2 fixation rate in the ECS and SYS during summer, which will be useful for understanding nitrogen and carbon biogeochemical processes. Key Points: High spatial resolution data sets of N2 fixation rate were measured in the East China Sea and southern Yellow Sea during summerSummer N2 fixation in the East China Sea was significantly enhanced by the Kuroshio intrusion, which was estimated to be 0.13–0.22 Tg NTrichodesmium and diatom‐diazotroph associations were estimated to contribute largely to summer N2 fixation in the East China Sea
- Subjects
KUROSHIO; UPWELLING (Oceanography); CARBON fixation; NITROGEN fixation; CARBON cycle; IRON; SPATIAL variation
- Publication
Journal of Geophysical Research. Biogeosciences, 2023, Vol 128, Issue 3, p1
- ISSN
2169-8953
- Publication type
Article
- DOI
10.1029/2022JG007287