Atmospheric deposition and lateral ocean transport enhance nitrogen supply to the North Pacific Subtropical Gyre

Atmospheric nitrogen deposition is an important external nitrogen source to the ocean that can fuel export production, yet its origin and contribution remain uncertain in the nitrogen-limited North Pacific Subtropical Gyre (NPSG). We present aerosol nitrate and reduced nitrogen (RN) concentrations and nitrogen isotopic composition (δ15N), along with air-mass back trajectories, across the NPSG in summer and winter. High δ15N values (–0.4‰ to 3‰) of aerosol nitrate and RN suggest that natural sources dominate in both seasons, contributing only modestly to the local external nitrogen supply. A synthesis of historical observations reveals pronounced zonal gradients in aerosol nitrogen concentrations and δ15N between the NPSG and transition zone, indicating enhanced anthropogenic influence in the latter, where nitrogen limitation is weaker. We estimate that lateral ocean transport from the transition zone increases external nitrogen inputs to the NPSG by 18%, highlighting an indirect pathway linking human emissions to oligotrophic ocean productivity.