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

The western boundary currents are characterized by abundant diazotrophs including Trichodesmium, which may fuel N-2 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 N-2 fixation in the ECS and SYS. N-2 fixation rates (NFRs) were measured using a N-15(2) 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 mu mol N m(-2) d(-1) on average, respectively, with the highest values of 13.84 nmol N L-1 d(-1) and 511.8 mu mol 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 N-2 fixation in the ECS through promoting growth of filamentous diazotrophs and providing appropriate nutrient environment.

Automated summary

Western boundary currents, such as the Kuroshio in the North Pacific, bring abundant diazotrophs into marginal seas, which enhance N-2 fixation. The intrusion of Kuroshio greatly increased N-2 fixation in the East China Sea (ECS) and southern Yellow Sea (SYS). N-2 fixation rates (NFRs) were higher in the ECS oceanic and mesohaline regions compared to the SYS and ECS low-salinity and coastal upwelling regions.