AUTHOR=Jiang Zhibing , Zhu Yuanli , Sun Zhenhao , Zhai Hongchang , Zhou Feng , Yan Xiaojun , Chen Quanzhen , Chen Jianfang , Zeng Jiangning 

TITLE=Size-fractionated N2 fixation off the Changjiang Estuary during summer

JOURNAL=Frontiers in Microbiology

VOLUME=14

YEAR=2023

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2023.1189410

DOI=10.3389/fmicb.2023.1189410

ISSN=1664-302X

ABSTRACT=<p>Recent evidence has shown active N<sub>2</sub> 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<sub>2</sub> fixation may be contributed by different groups (i.e., <italic>Trichodesmium</italic> and heterotrophic diazotrophs). Here, for the first time, we provide direct measurement of size-fractionated N<sub>2</sub> fixation rates (NFRs) off the CE during summer 2014 using the <sup>15</sup>N<sub>2</sub> bubble tracer method. The results demonstrated considerable spatial variations (southern &gt; northern; offshore &gt; inshore) in surface and depth-integrated NFRs, averaging 0.83 nmol N L<sup>−1</sup> d<sup>−1</sup> and 24.3 μmol N m<sup>−2</sup> d<sup>−1</sup>, respectively. The highest bulk NFR (99.9 μmol N m<sup>−2</sup> d<sup>−1</sup>; mostly contributed by &gt;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 (&lt;10) and abundant <italic>Trichodesmium</italic> (up to 10.23 × 10<sup>6</sup> trichomes m<sup>−2</sup>). However, low NFR (mostly contributed by &lt;10 μm fraction) was detected in the CE controlled by the CDW, where NO<sub>x</sub> concentration (up to 80 μmol L<sup>−1</sup>) and N/P ratio (&gt;100) were high and <italic>Trichodesmium</italic> abundance was low. The &gt;10 μm fraction accounted for 60% of depth-integrated bulk NFR over the CE and adjacent shelf. We speculated that the present NFR of &gt;10 μm fraction was mostly supported by <italic>Trichodesmium</italic>. Spearman rank correlation indicated that the NFR was significantly positively correlated with <italic>Trichodesmium</italic> abundance, salinity, temperature and Secchi depth, but was negatively with turbidity, N/P ratio, NO<sub>x</sub>, and chlorophyll <italic>a</italic> concentration. Our study suggests that distribution and size structure of N<sub>2</sub> fixation off the CE are largely regulated by water mass (intruded Kuroshio water and CDW) movement and associated diazotrophs (particularly <italic>Trichodesmium</italic>) and nutrient conditions.</p>