Distinct Fates of Dissolved Organic Nitrogen in the Upper Waters of Western Tropical North Pacific Ocean

This is the authors' abstract. We don't add key points for this paper.

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 ( δ 15 N DON ) can provide insights into marine DON cycling, but analytical challenges in δ 15 N DON measurements have resulted in spatially and temporally sparse δ 15 N DON data sets for the upper ocean. In this study, we report δ 15 N measurements of both solid‐phase‐extracted DON and NO 2 − + NO 3 − in the upper 300 m of two sections (spanning 23° × 15°) in the West Tropical North Pacific (WTNP). δ 15 N values imply that DON in the upper waters of the WTNP primarily originates from N 2 fixation and phytoplankton production. DON consumption occurred throughout the water column, with isotopic fractionation factors decreasing systematically with depth ( ε = 5.6‰ ± 1.3‰ at surface to 1.5‰ ± 0.6‰ at 300 m), likely reflecting a shift from labile to recalcitrant substrates. Based on a system of three Rayleigh equations, the initial DON concentration and its δ 15 N value in the upper WTNP were determined to be 4.8 ± 0.2 μmol/L and 1.5‰ ± 0.1‰, respectively. Water mass mixing combined with DON production and consumption governs its large‐scale cycling in the WTNP. As such, projected surface ocean warming and intensified stratification may critically impact DON cycling across oligotrophic oceans.