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.

Noctiluca scintillans feeds on a large number of phytoplankton, including diatoms and dinoflagellates, and frequently forms a red tide in the East China Sea (ECS) and southern Yellow Sea (SYS). However, the spatiotemporal distribution pattern, controlling factors, and long-term change of N. scintillans in the ECS and SYS remain unclear. In the present study, we collected N. scintillans samples from the ECS and SYS throughout the four seasons of 2011. We sampled phytoplankton and environmental parameters simultaneously. The depth-integrated abundance (DIA) of N. scintillans was the highest and lowest in summer and winter, respectively. N. scintillans is distributed abundantly in eutrophic coastal waters and the Changjiang Estuary, which are characterized by high concentrations of phytoplankton and chlorophyll-a. A Spearman correlation test demonstrated that its DIA in the upper 30-m water column was generally more significantly associated with phytoplankton abundance and chlorophyll-a concentration than with temperature and salinity. The results of the generalized additive models revealed that chlorophyll-a concentration explained more of the variation in N. scintillans abundance than temperature and salinity throughout the year, particularly in warm seasons. These findings indicate that the seasonal and spatial changes of N. scintillans are largely regulated by phytoplankton biomass. Compared with the historical data from 1959 and 2002, the abundance of N. scintillans in the Changjiang Estuary increased considerably in 2011 with increasing phytoplankton abundance resulting from accelerated eutrophication and warming. These results clarify the controlling factors, red-tide formation mechanism, and changing trends associated with the N. scintillans in the ECS and SYS.

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