Kuroshio Shape Composition and Distribution of Filamentous Diazotrophs in the East China Sea and Southern Yellow Sea

The filamentous Trichodesmium and Richelia/Calothrix appear to be crucial N2‐fixing cyanobacteria throughout the warm oligotrophic ocean, particularly in western boundary currents. The Kuroshio, the hotspot of N2 fixation in the Pacific Ocean, intrudes into the East China Sea (ECS) and southern Yellow Sea (SYS), which may profoundly influence composition and distribution of filamentous diazotrophs. We provide high spatial resolution dataset of filamentous diazotrophs in the ECS and SYS in July and August 2013. Results showed that Trichodesmium colonies and Richelia/Calothrix were strictly restricted to warm, saline, nitrogen‐limited waters of the ECS influenced by Kuroshio and were not detected in the SYS. The density of Trichodesmium in the ECS and SYS was 8.48 × 106 trichomes/m2, and colonial trichomes accounted for 40%. In addition to free filaments, Richelia/Calothrix were found to be symbiotic with diatoms Hemiaulus, Rhizosolenia/Guinardia, Chaetoceros, and Bacteriastrum with a total density at 2.13 × 106 heterocysts/m2 in the ECS. The densities of filamentous diazotrophs were significantly higher in high‐salinity region of the ECS (mainly controlled by Kuroshio) than in low‐salinity region of the ECS and in the SYS. The densities of Trichodesmium and Richelia/Calothrix were significantly and positively correlated with temperature, salinity, and mixed‐layer depth but were negatively correlated with NO3, turbidity, and chlorophyll‐a. These results suggested that composition and distribution of filamentous diazotrophs in the ECS and SYS were largely shaped by Kuroshio and associated physicochemical properties. We hypothesize that active N2 fixation in Kuroshio may be substantially supported by Richelia/Calothrix in addition to Trichodesmium. Plain Language Summary The filamentous diazotrophs Trichodesmium and Richelia/Calothrix are abundantly distributed and they frequently form blooms in western boundary currents. Their N2‐fixing activity effectively alleviated nitrogen limitation to primary production, which are crucially involved in nitrogen budget and biological pump. The Kuroshio, a western boundary current, is recognized as the hotspot of N2 fixation in the Pacific and strongly intrudes into China's marginal seas. The incursion of Kuroshio may profoundly influence composition and distribution of filamentous diazotrophs. We first provide high spatial resolution dataset of filamentous diazotrophs throughout the East China Sea and southern Yellow Sea