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 ¹⁵ N/ ¹⁴ N ratio (δ ¹⁵ N) 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 δ ¹⁵ N 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 δ ¹⁵ N, 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 δ ¹⁵ N difference. For example, N ₂ fixation tends to lower the δ ¹⁵ N of surface DON relative to subsurface nitrate mostly through encouraging DON accumulation rather than by the specific δ ¹⁵ N of the nitrogen introduced by N ₂ 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 δ ¹⁵ N suggest net DON degradation. In this phosphorus-rich, nitrogen-poor upwelling environment where low iron may restrict N ₂ fixation, net decomposition of DON may supply needed recycled nitrogen to the ecosystem.