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 surface waters, indicating in situ DON production and consumption. Samples with anomalously high DON δ¹⁵N tend to have lower DON concentrations, confirming net DON transport to, and consumption in, the subtropical gyres. Net local DON production or consumption appears to drive the observed spatial variation in the DON-to-subsurface nitrate δ¹⁵N difference, with N₂ fixation influencing this difference only to the degree that it encourages DON accumulation. Unique conditions appear to apply in the equatorial Atlantic, where despite high productivity, a local minimum in DON concentration and an elevated DON δ¹⁵N indicate net DON degradation. In this phosphorus-rich, nitrogen-poor upwelling environment where low iron may restrict N₂ fixation, selective decomposition of DON may supply nitrogen.
 

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