In the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO 3 À ) have been used to investigate NO 3 À assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past.

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.

The high sedimentation rate but low primary production in surface ocean raised a question whether particles from local upper water column could support high deposition in the Aleutian Basin. Here we first present large 210Po‐210Pb disequilibria in deep water of the Aleutian Basin. Dissolved 210Po and 210Pb were depleted relative to 210Pb and 226Ra, respectively, in deep water below 1000 m, as well as decreased with depth, suggesting enhanced particle scavenging in the deep water. The 210Po residence times (1–2 a) in deep water were comparable to those in the upper water column, indicating that 210Po scavenging rates were high in deep water of the Aleutian Basin. The export fluxes of 210Po from the upper 100 m were estimated to be 0.2–0.8 Bq/m2/d, much lower than those in the deep water (7–8 Bq/m2/d). Similarly, POC export fluxes in deep water (24–80 mmolC/m2/d) were higher than those in the upper 100 m (∼1 mmolC/m2/d). Such a large discrepancy between the upper and deep water suggested that particles from local upper water column could not totally meet the enhanced scavenging in the deep water. Based on mass balance calculations, the extra fluxes of 210Pb and POC imported to deep water were estimated to be 8–12 Bq/m2/d and 22–79 mmolC/m2/d, respectively. The ratio of POC to particulate 210Pb (i.e., POC/PPb) in the extra source was estimated to be 6.5 mmol/Bq, which was lower than that in the Bering Shelf with a mean POC/PPb ratio of 10.9 mmol/Bq, implying that particles in the Bering Shelf could be a potential source for the enhanced particle scavenging in deep water of the Aleutian Basin. However, quantitative and detailed role of ridges and manganese from sediments in particle scavenging in the deep water was unclear, and further studies are necessary.

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