Physical-biological coupling of N2 fixation in the northwestern South China Sea coastal upwelling during summer

Here, we present the first combined results of N-2 fixation rates (N-15(2) assay), dissolved iron (dFe,<0.2 m), and primary production (PP) (C-14 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 approximate to 3 orders of magnitude higher. Depth-integrated N-2 fixation rate ranged between 7.5 mol N m(-2) d(-1)and 163.1 mol N m(-2) d(-1) (average 46.4 mol 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.