Nitrogen fixation driven by mesoscale eddies and the Kuroshio Current in the northern South China Sea and the East China Sea

N2fixation rates(NFR, in terms of N) in the northern South China Sea(nSCS) and the East China Sea(ECS) were measured using the acetylene reduction assay in summer and winter, 2009. NFR of the surface water ranged from 1.14 nmol/(L·d) to 10.40 nmol/(L·d)(average at(4.89±3.46) nmol/(L·d), n=11) in summer and 0.74 nmol/(L·d) to 29.45 nmol/(L·d)(average at(7.81±8.50) nmol/(L·d), n=15) in winter. Significant spatio-temporal heterogeneity emerged in our study: the anticyclonic eddies(AE)(P<0.01) and the Kuroshio Current(KC)(P<0.05) performed significantly higher NFR than that in the cyclonic eddies or no-eddy area(CEONE), indicating NFR was profoundly influenced by the physical process of the Kuroshio and mesoscale eddies. The depth-integrated N2fixation rates(INF, in terms of N) ranged from 52.4 μmol/(m2·d) to 905.2 μmol/(m2·d)(average at(428.9±305.5)μmol/(m2·d), n=15) in the winter. The contribution of surface NFR to primary production(PP) ranged from 1.7%to 18.5% in the summer, and the mean contribution of INF to new primary production(NPP) in the nSCS and ECS were estimated to be 11.0% and 36.7% in the winter. The contribution of INF to NPP(3.0%–93.9%) also decreased from oligotrophic sea toward the eutrophic waters affected by runoffs or the CEONE. Furthermore, we observed higher contributions compared to previous studies, revealing the vital roles of nitrogen fixation in sustaining the carbon pump of the nSCS and ECS.