Size-fractionated N 2 fixation off the Changjiang Estuary during summer

Recent evidence has shown active N fixation in coastal eutrophic waters, yet the rate and controlling factors remain poorly understood, particularly in large estuaries. The Changjiang Estuary (CE) and adjacent shelf are characterized by fresh, nitrogen-replete Changjiang Diluted Water (CDW) and saline, nitrogen-depletion intruded Kuroshio water (Taiwan Warm Current and nearshore Kuroshio Branch Current), where N fixation may be contributed by different groups (i.e., and heterotrophic diazotrophs). Here, for the first time, we provide direct measurement of size-fractionated N fixation rates (NFRs) off the CE during summer 2014 using the N bubble tracer method. The results demonstrated considerable spatial variations (southern > northern; offshore > inshore) in surface and depth-integrated NFRs, averaging 0.83 nmol N L d and 24.3 μmol N m d , respectively. The highest bulk NFR (99.9 μmol N m d ; mostly contributed by >10 μm fraction) occurred in the southeastern East China Sea, where suffered from strong intrusion of the Kuroshio water characterized by low N/P ratio (10 μm fraction accounted for 60% of depth-integrated bulk NFR over the CE and adjacent shelf. We speculated that the present NFR of >10 μm fraction was mostly supported by . Spearman rank correlation indicated that the NFR was significantly positively correlated with abundance, salinity, temperature and Secchi depth, but was negatively with turbidity, N/P ratio, NO , and chlorophyll concentration. Our study suggests that distribution and size structure of N fixation off the CE are largely regulated by water mass (intruded Kuroshio water and CDW) movement and associated diazotrophs (particularly ) and nutrient conditions.