Distributions of eight targeted current‐use pesticides (CUPs) chloroneb, simazine, atrazine, alachlor, dacthal, chlorobenzilate, methoxychlor, and permethrin were investigated in seawater and the atmosphere in a region covering the North Pacific to the Arctic Oceans during the 7th and 8th Chinese National Arctic Research Expedition (2016 and 2017) voyages of the research vessel R/V Xuelong (Snow Dragon in English). Total CUP concentrations in seawater have prominent seasonal and latitudinal trends, with higher concentrations occurring at lower latitudes in early summer. The major contributors of the CUPs (∑₈CUP) did not alter over different seasons with dominant chloroneb, alachlor, and atrazine accounting for more than 90%, but their concentrations did have marked seasonal changes. However, the compositions of the eight analyzed CUPs varied indistinctly between seasons indicating a possible combined environmental impact on these compounds rather than the effects of individual chemical properties. Three‐day backward air mass trajectories indicate that atmospheric masses from northeastern China are responsible for the high concentrations of CUPs in East China and Japan Seas, whereas those from the North Atlantic Ocean contribute to the low levels in local area. Fugacity ratios indicate potential volatilization and equilibrium of chloroneb in the Canada Basin of the Arctic Ocean and Japan Sea, respectively, and deposition of other CUPs in both regions. However, atmospheric concentrations are decoupled from those in seawater, which indicates a low exchange rate.

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.

Dissolved organic nitrogen (DON) is the dominant form of fixed nitrogen in most low and middle latitude ocean surface waters. Here, we report measurements of DON isotopic composition (δ¹⁵N) from the west South China Sea (SCS), with the goal of providing new insight into DON cycling. The concentration of DON in the surface ocean is correlated (r = 0.70, p < 0.0001) with chlorophyll a concentration, indicating DON production in these surface waters. The concentration and δ¹⁵N of DON fall in a relatively narrow range in the surface ocean (4.6 ± 0.1 μM and 4.3 ± 0.2‰ vs. air, respectively; ±SD), similar to other ocean regions. The mean DON δ¹⁵N above 50 m (4.5 ± 0.3‰) is similar to the δ¹⁵N of nitrate in the “shallow subsurface” (i.e., immediately below the euphotic zone; 4.6 ± 0.2‰) but is higher than the δ¹⁵N of suspended particles in the surface ocean (~2.3‰). This set of isotopic relationships has been observed previously (e.g., in the oligotrophic North Atlantic and North Pacific) and can be explained by the cycling of N between particulate organic nitrogen (PON), DON, and ammonium, in which an isotope effect associated with DON degradation preferentially concentrates ¹⁵N in DON. Consistent with this view, a negative correlation (r = 0.70) between the concentration and the δ¹⁵N of DON is observed in the upper 75 m, suggesting an isotope effect of ~4.9 ± 0.4‰ for DON degradation. Comparing the DON δ¹⁵N data from the SCS with other regions, we find that the δ¹⁵N difference between euphotic zone DON and shallow subsurface nitrate δ¹⁵N (Δδ¹⁵N_(DON‐NO3)) rises from ocean regions of inferred net DON production to regions of net DON consumption, with the SCS representing an intermediate case.