Dissolved Organic Nitrogen Cycling in the South China Sea From an Isotopic Perspective

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.