Physical-biological coupling of n2 fixation in the northwestern south china sea coastal upwelling during summer

Abstract

Here, we present the first combined results of N2fixation rates (15N2assay), 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 N2fixation rate ranged between 0.1 nmol N L−1d−1and 5.6 nmol N L−1d−1(average 1.0 nmol N L−1d−1,n = 50) under nonbloom conditions. At aTrichodesmiumbloom station, N2fixation rate was ∼ 3 orders of magnitude higher. Depth-integrated N2fixation rate ranged between 7.5 μmol N m−2d−1and 163.1 μmol N m−2d−1(average 46.4 μmol N m−2d−1). Our results indicate that N2fixation 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 N2fixation was hindered due to relative deficiency of phosphate caused by massive phytoplankton utilization in the coastal upwelling. By comparison, N2fixation 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 N2fixation to PP (0.01–2.52%) also increased toward the open waters. As a significant external N source, summertime N2fixation is estimated to contribute a flux of 1.4 Gmol N to this area under nonbloom conditions. This study adds to the knowledge of N2fixation in the rarely studied subtropical coastal upwellings, and highlights the necessity of future comprehensive studies in such highly dynamic environments.