The impact of the western boundary Kuroshio current on dissolved organic nitrogen cycling in the South China Sea

The dissolved organic nitrogen (DON) supplied horizontally from open ocean to marginal seas through western boundary current can influence upper regional nitrogen cycling. However, the observation on DON and its biogeochemical impacts on marginal seas are scarce. In this study, we investigated the distribution and degradation of DON transported by Kuroshio intrusion and its role in nitrogen cycling in the northern South China Sea (nSCS). The concentration and δ¹⁵N of DON fall in a relatively narrow range in the upper 100 m water column (4.3 ± 0.6 μM and 3.5 ± 1.6‰ vs. air, respectively; ± SD). The mean DON δ¹⁵N above 100 m is lower than the nitrate δ¹⁵N of in the “shallow subsurface” (below 100 m; 4.8 ± 1.2‰) but is higher than the δ¹⁵N of suspended particles in the surface ocean (~1.9 ± 0.5‰). This isotopic relationship between DON and suspended particles can be explained by the cycling of N between particulate organic nitrogen (PON), and DON, in which an isotope effect associated with DON degradation preferentially concentrates ¹⁵N in DON. Accordingly, a negative correlation (r = 0.72) between the concentration and the δ¹⁵N of DON is observed in the upper 100 m, suggesting an isotope effect of ~7.2 ± 1.0‰ for DON degradation. Furthermore, using an isopycnal mixing model, the exchange and biodegradation processes of Kuroshio-intruded DON were also quantified. We estimated the amount of DON carried by the Kuroshio intrusion was approximately 0.93 mmol m^-2 d^-1 in the upper 100 m. Concomitantly, the amount of nitrogen released from the enhanced DON degradation by Kuroshio intrusion is comparable to the supply deeper water and nitrogen fixation in surface waters. This study suggests that the enhanced biodegradation of DON during the western boundary currents intrusion could serve as an important nutrient source to marginal seas.