Assessing N2 fixation flux and its controlling factors in the (sub)tropical western North Pacific through high-resolution observations

Abstract

The (sub)tropical western North Pacific is potentially an area of intense nitrogen (N₂) fixation in the global ocean, despite limited understanding of the flux and controlling factors. We conducted high-resolution observations from 2016 to 2021 in this region and used machine learning algorithms to simulate N₂ fixation flux. Models estimated an N₂ fixation flux from 5.72 to 6.45 Tg N yr⁻¹, with strong seasonal variation and peak rates in summer. The western North Pacific Subtropical Gyre and the Kuroshio Current contributed more to N₂ fixation flux than did the adjacent areas. Models suggested that sea surface temperature, photosynthetically available radiation, and nutrient supply were most strongly correlated with seasonal and spatial variations in N₂ fixation. This study provides an improved estimation of N₂ fixation in the western North Pacific and advances our understanding of its role in ocean productivity.