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The 210Po‐210Pb pair is increasingly used as a proxy of quantifying organic carbon export from the euphotic zone. However, disequilibria between 210Po and 210Pb in mesopelagic water have been poorly studied. Here we present unusual deficiencies of 210Po with respect to 210Pb in mesopelagic water (200–1000 m) in the South China Sea (SCS). The total particulate matter (TPM) increased by up to 32% in the mesopelagic layer comparing with the euphotic zone. The total 210Po/210Pb ratio varied from 0.41 to 0.98 with an average of 0.72 ± 0.19, showing an enhanced removal of 210Po in mesopelagic water. On average, particulate 210Po and 210Pb increased by 23% and 32% at the slope stations, respectively. These results indicated that the 210Po deficits result from lateral transport, probably via benthic nepheloid layer. Based on the deficiency of 210Po, the residence times of particulate 210Po were estimated to range from 0.11 to 0.25 year (avg. 0.17 ± 0.07 year), allowing resuspended sediment to disperse over a long range. The export fluxes of 210Po varied from 68 to 121 dpm m−2 d−1 with an average of 96 ± 27 dpm m−2 d−1, which was 6 times that out of the euphotic zone. Using the 210Po deficits, the export fluxes of TPM out of the mesopelagic layer were quantified to vary from 4.19 to 10.20 g m−2 d−1, revealing a large amount of particles from the shelf to the SCS basin. This study suggests that 210Po‐210Pb could be an effective tracer of tracking particle cycling in mesopelagic water.

Nitrogen fixation in the subtropical East China Sea (ECS) and the southern Yellow Sea (YS) were measured using 15 N 2 tracer assay during June and July 2006. Depth-integrated nitrogen fixation (2 to 221 μmol N m −2 d −1 ) was highest in the oceanic main path of the Kuroshio Current in the northeastern ECS, and in the mesohaline (surface salinity 30 to 34) stations. Very little N 2 fixation was encountered in the low-salinity (surface salinity < 30) area, and we suggest that N 2 fixation was hindered by the nutrient conditions (lack of 'excess' phosphate relative to nitrate, xsPO 4 = [PO 4 ] − [NO 3 ]/16) near the Changjiang (Yangtze River) mouth. In the mesohaline waters, N 2 fixation was positively correlated with the vertical density (σ t ) gradient in the upper water column (30 m), indicating that N 2 fixation can also be controlled by physical regime, and enhanced water column stratification may promote N 2 fixation during summer. N 2 fixation met 0.01 to 4.6% of nitrogen demand by primary production, suggesting that N 2 fixation was not a major contributor to primary production in the study area. Estimated new N flux by N 2 fixation to the ECS continental shelf (13 Gg N) during summer is at the same order of magnitude as atmospheric deposition, but much lower than either the Kuroshio subsurface water upwelling or the Changjiang riverine input.

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.

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