Isotopic evidence of dissolved organic nitrogen production and consumption in ocean surface waters

Unlike dissolved inorganic nitrogen, which is scarce in much of the tropical, subtropical, and temperate surface ocean, dissolved organic nitrogen (DON) has significant concentrations and gradients. Accordingly, the lateral transport of DON may fuel biological productivity in nutrient-depleted low-latitude surface waters. Here, we report measurements of the concentration and ¹⁵N/¹⁴N ratio (δ¹⁵N) of DON in surface waters over the Pacific and Atlantic basins. DON concentration is generally highest in subpolar and equatorial waters, with minima in the subtropical gyres. A δ¹⁵N correlation is observed between surface DON and the nitrate consumed in the surface waters, indicating in situ DON production and consumption. Samples with lower DON concentrations tend to have anomalously high DON δ¹⁵N, confirming net DON transport to, and consumption in, the subtropical gyres. Net local DON production or consumption appears to dominate the observed spatial variation in the DON-to-subsurface nitrate δ¹⁵N difference. For example, N₂ fixation tends to lower the δ¹⁵N of surface DON relative to subsurface nitrate mostly through encouraging DON accumulation rather than by the specific δ¹⁵N of the nitrogen introduced by N₂ fixation. Unique conditions appear to apply in the equatorial Atlantic, where despite high productivity, a local minimum in DON concentration and a slight elevation in DON δ¹⁵N suggest net DON degradation. In this phosphorus-rich, nitrogen-poor upwelling environment where low iron may restrict N₂ fixation, net decomposition of DON may supply needed recycled nitrogen to the ecosystem.