Enhancement of Summer Nitrogen Fixation by the Kuroshio Intrusion in the East China Sea and Southern Yellow Sea

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.

Diazotroph connectomics in Gulf Stream eddies supports N2 fixation in the North Atlantic

“…Indeed, we found similar N 2 xation rates within the cyclonic and anticyclonic eddies, with a trend to even higher rates, particularly with depth for the anticyclonic core (Table S1). This shows the overall importance of trapping eddies in supporting N 2 xation in one of the world's most mesoscale active regions, raising questions about their impact on other western boundary currents with similar observations 20,21 .…”

Nitrogen Fixation at the Mid‐Atlantic Bight Shelfbreak and Transport of Newly Fixed Nitrogen to the Slope Sea

“…Here, we show that not only was the absolute flux of newly fixed N greater in shelf waters, but the relative contribution of diazotrophs to community N turnover was higher as well (Table S2 and Figure S8 in Supporting Information ), suggesting that diazotrophic contributions are important in supporting productivity on the shelf. Assessing diazotrophic contributions and their physicochemical drivers in this system has bearing not only on the regional N budget, but on our understanding of N 2 fixation in continental shelf systems globally, particularly those impacted by western boundary currents (e.g., Jiang et al., 2023; Marshall et al., 2023) where diazotrophy has been observed for decades (e.g., Prufert‐Bebout et al., 1993; Saino & Hattori, 1978). Our findings call for greater focus on shelf environments as regions of N 2 fixation and contributors to the global N budget, and for a paradigm shift regarding the relative importance of N 2 fixation in fueling productivity near the coast.…”

Size-fractionated N2 fixation off the Changjiang Estuary during summer

“…The Changjiang Estuary (CE) and adjacent East China Sea (ECS) and southern Yellow Sea are primarily characterized by eutrophic, low-salinity Changjiang Diluted Water (CDW) and oligotrophic, warm, saline nearshore Kuroshio Branch Current (NKBC; modified Kuroshio subsurface water) and Taiwan Warm Current (TWC; a mixture of the intruded Kuroshio water and the Taiwan Strait water) ( Figure 1A ). Although nitrogen repletion and low temperature (<20°C) in the CE restrain the growth of Trichodesmium and N 2 fixation ( Zhang et al, 2012 ; Jiang et al, 2018 , 2023 ), relatively high NFR (3.37 nmol N L −1 d −1 ) was observed during spring ( Lin et al, 2013 ), likely contributed by unicellular diazotrophs, particularly heterotrophic diazotrophs. Conversely, abundant Trichodesmium ( Jiang et al, 2018 , 2019 ) and extremely high NFR (up to 62 nmol N L −1 d −1 ; Zhang et al, 2012 ; Shiozaki et al, 2015 ; Wu et al, 2018 ) occurred in the Kuroshio, which largely enhanced N 2 fixation in the ECS ( Jiang et al, 2023 ).…”

“…Although nitrogen repletion and low temperature (<20°C) in the CE restrain the growth of Trichodesmium and N 2 fixation ( Zhang et al, 2012 ; Jiang et al, 2018 , 2023 ), relatively high NFR (3.37 nmol N L −1 d −1 ) was observed during spring ( Lin et al, 2013 ), likely contributed by unicellular diazotrophs, particularly heterotrophic diazotrophs. Conversely, abundant Trichodesmium ( Jiang et al, 2018 , 2019 ) and extremely high NFR (up to 62 nmol N L −1 d −1 ; Zhang et al, 2012 ; Shiozaki et al, 2015 ; Wu et al, 2018 ) occurred in the Kuroshio, which largely enhanced N 2 fixation in the ECS ( Jiang et al, 2023 ). These results showed that NFR in the ECS was highly associated with Trichodesmium , probably indicating a high contribution of Trichodesmium to N 2 fixation.…”

“…These results showed that NFR in the ECS was highly associated with Trichodesmium , probably indicating a high contribution of Trichodesmium to N 2 fixation. The contribution of filamentous cyanobacteria to N 2 fixation in the ECS during summer was conservatively estimated to be >60% according to their densities in euphotic zone ( Jiang et al, 2023 ). However, Wu et al (2018) found that unicellular diazotrophs contributed more N 2 fixation than Trichodesmium at two stations near the ECS Kuroshio using the acetylene reduction assay.…”