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 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.

Seasonal and Spatial Changes in Trichodesmium Associated With Physicochemical Properties in East China Sea and Southern Yellow Sea

“…Enhanced Trichodesmium blooms have been observed frequently in the tropical/subtropical Atlantic, western Pacific, and Indian Oceans (Parab & Matondkar, ; Villareal & Carpenter, ; Westberry & Siegel, ), which are located downwind of major global dust production areas (Saharan/Sahel, Gobi Deserts, and deserts bounding the Arabian Sea, respectively) (Jickells et al, ; Karl et al, ). Our present reported blooming density (5,445 trichomes L −1 ) are consistent with those previously reported in the SCS (2,797 trichomes L −1 ) (Zhang et al, ) and tropical central (15°N–7°S, 1 × 10 3 trichomes L −1 ) (Snow et al, ) and eastern North Atlantic (2,860 trichomes L −1 ) (Tyrrell et al, ), but are considerably lower than that observed in the Kuroshio (>2 × 10 4 trichomes L −1 ) (Shiozaki et al, ), central (>1 × 10 4 trichomes L −1 ) (Capone et al, ) and eastern (40.07 × 10 4 trichomes L −1 ) (Parab & Matondkar, ) Arabian Sea, western North Atlantic (20.62 × 10 3 trichomes L −1 ) (Carpenter et al, ), and Brazil Current in the South Atlantic (10.9 × 10 6 trichomes L −1 ) (Detoni et al, ). These findings are largely attributed to the higher terrestrial/riverine and atmospheric inputs of macronutrients and micronutrients in the ECS, Arabian Sea, and tropical/subtropical North Atlantic than in other seas, particularly with respect to the availability of Fe and P (Jickells et al, ; Karl et al, ; Rubin et al, ; Sañudo‐Wilhelmy et al, ; Snow et al, ; Sohm, Webb, & Capone, ).…”

Nitrogen Fixation by Trichodesmium and unicellular diazotrophs in the northern South China Sea and the Kuroshio in summer

“…The nSCS basin showed low N 2 fixation activity. These rates were in the same order of magnitude as those measured under non-bloom conditions in the East China Sea, the southern Yellow Sea and the nSCS coastal upwelling 27,28 . The high N 2 fixation rates in the nSCS shelf were probably attributed to diazotrophic blooms in sampling stations.…”

“…On the basis of a C: N = 6.6, N 2 fixation by the whole-water was calculated to contribute to 1.7% to 18.5% of the primary production in surface waters, suggesting that N 2 fixation plays a very important role in supporting surface phytoplankton N demand in some stations. Such values were exceeded the previous studies in the nSCS, and were comparable with N input by biological N 2 fixation in other oceans 27,28,56 . The variation of N 2 fixation rate by Trichodesmium was consistent with that of the standing crop of Trichodesmium .…”

“…Iron is a important cofactor of nitrogenase, and it plays an crucial role in the synthesis and expression of nitrogenase in diazotroiphs 50 . According to literatures, dissolved Fe (dFe) in surface waters ranged from 0.2–0.3 nmol L −1 in the SCS basin stations and 1.8–43.2 nmol L −1 in the northwest SCS coastal stations 28,37 . It was suggested that Fe availability may limit the rates of N 2 fixation in the SCS basin, while it was not happened in the nSCS coastal stations.…”

Kuroshio Shape Composition and Distribution of Filamentous Diazotrophs in the East China Sea and Southern Yellow Sea

“…Because colonies were easily missed during water‐sample collection due to the limited volume (~1 L) of sampled seawater and low colony densities (Jiang et al, ; Shiozaki, Takeda, et al, ), the present colonial trichomes might be underestimated. Nevertheless, this contribution was higher than those reported in the South China Sea (Zhang et al, ) and at station ALOHA (Letelier & Karl, ), but was lower than those (>70%) in the North Pacific subtropical gyre (Sohm, Subramaniam, et al, ) and North Atlantic (Carpenter et al, ). Particularly in the southeastern ECS, the colonial trichomes accounted for 68%, which was consistent with that (69%) reported in upstream Kuroshio during summer (Chen et al, ).…”