Distinct fates of dissolved organic nitrogen in the upper waters of Western Tropical North Pacific

Dissolved organic nitrogen (DON) is a prominent component of fixed nitrogen in the upper ocean, yet remains one of the least characterized marine nitrogen reservoirs. Stable nitrogen isotopes of DON (δ15NDON) can provide insights into marine DON cycling but analytical challenges in δ15NDON measurements have resulted in spatially and temporally sparse δ15NDON datasets for the upper ocean. In this study, we report δ15N measurements of both solid-phase-extracted DON (SPE-DON) and NO2- + NO3- in the upper 300 m of two sections (spanning 23º×15º) in the West Tropical North Pacific (WTNP). δ15N values imply that DON in the upper waters of the WTNP primarily originates primarily from N2 fixation and phytoplankton production. DON consumption occurred throughout the water column, with isotopic fractionation factors decreasing systematically with depth (ε = 6.0 ± 1.1‰ at surface to 1.5 ± 0.6‰ at 300 m). Based on a matrix of Rayleigh fractionation models, the initial DON concentration and its δ15N value in the upper WTNP was determined to be 5.0 ± 0.7 μmol/L and 1.5 ± 0.3‰, respectively. Water mass mixing combined with DON production and consumption govern its large-scale cycling in the WTNP. As such, projected surface ocean warming and intensified stratification may impact DON cycling across oligotrophic oceans.