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Physical‐biological coupling of N 2 fixation in the northwestern S outh C hina S ea coastal upwelling during summer
Author(s) -
Zhang Run,
Chen Min,
Yang Qing,
Lin Yuanshao,
Mao Huabin,
Qiu Yusheng,
Tong Jinlu,
Lv E.,
Yang Zhi,
Yang Weifeng,
Cao Jianping
Publication year - 2015
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.1002/lno.10111
Subject(s) - upwelling , phytoplankton , trichodesmium , oceanography , nutrient , bloom , new production , algae , environmental science , biology , geology , chemistry , nitrogen fixation , ecology , nitrogen , organic chemistry , diazotroph
Here, we present the first combined results of N 2 fixation rates ( 15 N 2 assay), dissolved iron (dFe, < 0.2 μm), and primary production (PP) ( 14 C assay) in the northwestern South China Sea (NWSCS) coastal upwelling region during summer. Surface N 2 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, N 2 fixation rate was ∼ 3 orders of magnitude higher. Depth‐integrated N 2 fixation rate ranged between 7.5 μmol N m −2 d −1 and 163.1 μmol N m −2 d −1 (average 46.4 μmol N m −2 d −1 ). Our results indicate that N 2 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 N 2 fixation was hindered due to relative deficiency of phosphate caused by massive phytoplankton utilization in the coastal upwelling. By comparison, N 2 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 N 2 fixation to PP (0.01–2.52%) also increased toward the open waters. As a significant external N source, summertime N 2 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 N 2 fixation in the rarely studied subtropical coastal upwellings, and highlights the necessity of future comprehensive studies in such highly dynamic environments.