Size-fractionated N2 fixation off the Changjiang Estuary during summer

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

doi:10.3389/fmicb.2023.1189410

Size-fractionated N₂ fixation off the Changjiang Estuary during summer

Front Microbiol. 2023 May 9:14:1189410. doi: 10.3389/fmicb.2023.1189410. eCollection 2023.

Authors

Zhibing Jiang^{1

2

3

4

5}, Yuanli Zhu^{1

2

5}, Zhenhao Sun¹, Hongchang Zhai¹, Feng Zhou^{4

5}, Xiaojun Yan⁶, Quanzhen Chen¹, Jianfang Chen^{1

4}, Jiangning Zeng^{1

2}

Affiliations

¹ Key Laboratory of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China.
² Key Laboratory of Ocean Space Resource Management Technology, Ministry of Natural Resource, Hangzhou, China.
³ Key Laboratory of Nearshore Engineering Environment and Ecological Security of Zhejiang Province, Hangzhou, China.
⁴ State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China.
⁵ Observation and Research Station of Marine Ecosystem in the Yangtze River Delta, Ministry of Natural Resources, Hangzhou, China.
⁶ Marine Science and Technology College, Zhejiang Ocean University, Zhoushan, China.

Abstract

Recent evidence has shown active N₂ 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 N₂ fixation may be contributed by different groups (i.e., Trichodesmium and heterotrophic diazotrophs). Here, for the first time, we provide direct measurement of size-fractionated N₂ fixation rates (NFRs) off the CE during summer 2014 using the ¹⁵N₂ 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^-1 d^-1 and 24.3 μmol N m^-2 d^-1, respectively. The highest bulk NFR (99.9 μmol N m^-2 d^-1; 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) and abundant Trichodesmium (up to 10.23 × 10⁶ trichomes m^-2). However, low NFR (mostly contributed by <10 μm fraction) was detected in the CE controlled by the CDW, where NO_x concentration (up to 80 μmol L^-1) and N/P ratio (>100) were high and Trichodesmium abundance was low. The >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 Trichodesmium. Spearman rank correlation indicated that the NFR was significantly positively correlated with Trichodesmium abundance, salinity, temperature and Secchi depth, but was negatively with turbidity, N/P ratio, NO_x, and chlorophyll a concentration. Our study suggests that distribution and size structure of N₂ fixation off the CE are largely regulated by water mass (intruded Kuroshio water and CDW) movement and associated diazotrophs (particularly Trichodesmium) and nutrient conditions.

Keywords: Changjiang Diluted Water; Changjiang Estuary; Kuroshio; N2 fixation; Trichodesmium; unicellular diazotrophs.