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 (δ¹⁵N_DON) can provide insights into marine DON cycling but analytical challenges in δ¹⁵N_DON measurements have resulted in spatially and temporally sparse δ¹⁵N_DON 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 δ¹⁵Nvaluein 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.