Seasonal Shift of Dissolved Organic Matter Stoichiometry in the Surface North Pacific Subtropical Gyre: Regulation by Enhanced N 2 Fixation and DOP Utilization

Abstract Dissolved organic matter (DOM) pool acts as both nutrient source and sink in the oligotrophic surface ocean. However, the drivers of seasonal variations in DOM stoichiometry and their relationship to nutrient cycling remain poorly understood. Here, we present a seasonal synthesis of dissolved organic nitrogen (DON) and phosphorus (DOP) distributions across the surface North Pacific Subtropical Gyre (NPSG) with associate biogeochemical measurements. In the central NPSG, summer exhibited elevated DON and depleted DOP, coinciding with a fourfold increase in N 2 ‐fixation rates compared with winter. This seasonal shift reflects preferential DOP utilization and accumulation of DON via N 2 ‐fixation. Mass and isotopic balance models confirm that DOP can supply about 40%–100% of summer phosphorus demand while about 50% of fixed nitrogen partitions into DON. These findings demonstrate that seasonal DOM stoichiometric flexibility encode dynamic nutrient cycling, highlighting the fundamental role of DOM transformation in regulating ocean biogeochemical processes.