Trichodesmium is broadly distributed and occasionally blooms in the East China Sea (ECS) and southern Yellow Sea, where it contributes to local N and C budgets. However, its population structure, spatiotemporal distribution, controlling factors, and N2 fixation remain poorly documented. Here we provide high spatial resolution data sets of Trichodesmium during the four seasons of 2011–2012 using water‐ and net‐collected methods. The net‐collected method captures colonial trichomes of Trichodesmium effectively but results in an underestimation of free trichomes. Colonies are rarely observed and occur only on the ECS shelf, which are easily missed in water‐collected samples. Depth‐integrated densities of Trichodesmium were found to be significantly higher in warm seasons than in cold seasons. Maximum densities in the water column were generally found at depths of 10–50 m. Trichodesmium thrives on the oligotrophic, warm, offshore ECS shelf (controlled by the Kuroshio and Taiwan Warm Current), but restrains in the cold southern Yellow Sea and the eutrophic, inshore ECS. Seasonal and spatial variations in Trichodesmium are closely correlated with physicochemical properties (mainly temperature and P), which are primarily controlled by circulation alteration and water mass movement. The N2 fixation rates of Trichodesmium in the ECS in summer and autumn (>20°C) are roughly estimated at 17.1 and 41.7 μmol N m−2 d−1 under nonbloom conditions, which potentially contribute to 81% and 57% of biological N2 fixation, respectively. Compared with historical data since the 1970s, Trichodesmium densities have increased considerably in all seasons, and the distribution boundary has shifted northward under regional warming and hydrological changes.

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The filamentous Trichodesmium and Richelia/Calothrix appear to be crucial N 2 -fixing cyanobacteria throughout the warm oligotrophic ocean, particularly in western boundary currents. The Kuroshio, the hotspot of N 2 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 × 10 6 trichomes/m 2 , 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 × 10 6 heterocysts/m 2 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 NO 3 , 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 N 2 fixation in Kuroshio may be substantially supported by Richelia/Calothrix in addition to Trichodesmium.

The southern Yellow Sea (SYS) shows extensive spatial variations in water masses, particularly YS Warm Current Water (YSWCW) and YS Coastal Current Water in cold and YS Bottom Cold Water (YSBCW) and Changjiang Diluted Water in warm seasons. These hydrographic processes result in significant spatial changes in nutrients, stratification, and turbidity, which may highly regulate phytoplankton distribution in the SYS. Here, we provide high spatial resolution data sets of phytoplankton in the SYS in all seasons during 2011-2012. Diatoms thrived in eutrophic Changjiang Diluted Water and YS Coastal Current Water regions. Notably, benthic/meroplanktonic diatoms, including Paralia sulcata and Rhaphoneis spp., were dominant near the Changjiang Estuary and Subei Shoal. However, cryptophytes and haptophytes were dominant in oligotrophic offshore regions controlled by YSWCW and YSBCW. Phytoplankton abundance and chlorophyll a were considerably lower in the YSWCW and YSBCW than in other regions, and the community composition varied significantly among regions. Algal blooms were observed off the SYS coast in spring and near the Changjiang Estuary in summer, which were characterized by abundant nutrients, low turbidity, and a relatively stable water column. Canonical correspondence analysis demonstrated that stratification in cold and nutrients in warm seasons were most important variables correlated with phytoplankton community. Variation partitioning analysis confirmed that spatial changes in phytoplankton community composition were largely structured by nutrients rather than by physical factors in all seasons except winter. Our findings highlight the significance of water column stability and nutrients associated with water mass variation in shaping spatial changes of phytoplankton community in the SYS.