Physical‐biological coupling of N2 fixation in the northwestern South China Sea coastal upwelling during summer
Physical‐biological coupling of N2 fixation in the northwestern South China Sea coastal upwelling during summer
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DOI：
10.1002/lno.10111
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发表时间：
2015-07
影响因子：
4.5
通讯作者：
Run Zhang;Min Chen;Qing Yang;Y. Lin;H. Mao;Y. Qiu;J. Tong;E. Lv;Zhi Yang;Weifeng Yang;Jianping Cao
中科院分区：
文献类型：
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作者：
Run Zhang;Min Chen;Qing Yang;Y. Lin;H. Mao;Y. Qiu;J. Tong;E. Lv;Zhi Yang;Weifeng Yang;Jianping Cao
本文首次报道了南海西北部沿岸上升流区夏季N2固定率（~（15）N2测定）、溶解铁（dFe，< 0.2 μm）和初级生产力（~（14）C测定）的综合结果。在非水华条件下，地表固氮速率介于0.1 nmol N L−1 d−1和5.6 nmol N L−1 d−1之间（平均值为1.0 nmol N L−1 d−1，n = 50）。在束毛藻水华站点，固氮率高出103个数量级。深度积分N2固定速率介于7.5 μmol N m−2 d− 1和163.1 μmol N m−2 d−1之间（平均值为46.4 μmol N m−2 d−1）。我们的研究结果表明，N2固定是不太可能限制在西北南海大陆沃茨中的Fe的可用性，相反，沿海上升流引起的物理和生物过程的综合效应可能发挥了决定性的作用。随着涌升冷，dFe-丰富，营养丰富的沃茨，非重氮营养型浮游植物的生长将优先增强，而N2固定由于在沿海涌升中大量浮游植物利用造成的磷酸盐相对不足而受到阻碍。相比之下，在近海沃茨，随着PP的降低，N2固定显著沿着升高，这可能是由于从缺磷到缺氮的转变。与此相一致，氮固定对PP的贡献（0.01-2.52%）也朝着开放的沃茨增加。作为一个显着的外部N源，夏季固定N2的估计贡献通量为1.4 Gmol N在nonbloom条件下，这一地区。这项研究增加了N2固定在很少研究的亚热带沿海隆起的知识，并强调了在这种高度动态的环境中进行全面研究的必要性。
Here, we present the first combined results of N2 fixation rates (15N2 assay), dissolved iron (dFe, < 0.2 μm), and primary production (PP) (14C assay) in the northwestern South China Sea (NWSCS) coastal upwelling region during summer. Surface N2 fixation rate ranged between 0.1 nmol N L−1 d−1 and 5.6 nmol N L−1 d−1 (average 1.0 nmol N L−1 d−1, n = 50) under nonbloom conditions. At a Trichodesmium bloom station, N2 fixation rate was ∼ 3 orders of magnitude higher. Depth‐integrated N2 fixation rate ranged between 7.5 μmol N m−2 d−1and 163.1 μmol N m−2 d−1 (average 46.4 μmol N m−2 d−1). Our results indicate that N2 fixation is unlikely limited by Fe availability in the NWSCS continental waters, instead, the coastal upwelling‐induced combined effects of physical and biological processes may have played a decisive role. With the upwelled cold, dFe‐rich, nutrient‐replete waters, nondiazotrophic phytoplankton growth would be preferentially enhanced while N2 fixation was hindered due to relative deficiency of phosphate caused by massive phytoplankton utilization in the coastal upwelling. By comparison, N2 fixation was notably elevated along with decreased PP in the offshore waters, probably due to a shift from P‐deficiency to N‐deficiency. Consistently, the contribution of N2 fixation to PP (0.01–2.52%) also increased toward the open waters. As a significant external N source, summertime N2 fixation is estimated to contribute a flux of 1.4 Gmol N to this area under nonbloom conditions. This study adds to the knowledge of N2 fixation in the rarely studied subtropical coastal upwellings, and highlights the necessity of future comprehensive studies in such highly dynamic environments.