291 / 2024-09-13 15:27:41

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

N2 fixation,western North Pacific,observations,machine learning

Biological nitrogen (N₂) fixation is a process that provides new bioavailable nitrogen, supporting ocean production and carbon sequestration. The North Pacific Subtropical Gyre (NPSG) is recognized as one of the largest areas with carbon dioxide sink flux in the global ocean, although it has low nitrate concentrations in the surface water, suggesting the potential important role of N₂ fixation in fueling primary production. High N₂ fixation rates have been observed in the (sub)tropical western North Pacific, which occupies approximately one-third of the NPSG area. However, due to the limited number of observations, the spatio-seasonal patterns and environmental controls of N₂ fixation rates in this region remain to be fully understood, which hinders the estimation of N₂ fixation flux. To tackle this issue, we conducted high-density observations from 2016 to 2021 in the (sub)tropical western North Pacific and used random forest and support vector regression algorithms to simulate N₂ fixation rate. Our results show that the whole region contributes N₂ fixation flux of 5.72 to 6.45 Tg N yr^-1. N₂ fixation flux has a strong seasonal variation and peaks rates in summer. It also exhibits significant spatial pattern with the western NPSG and the Kuroshio contribute the most. The seasonal variation of N₂ fixation rate is affected by sea surface temperature and photosynthetically available radiation, while the spatial distribution of N₂ fixation rate is determined by the supply of nutrient iron, nitrogen, and phosphate. We also predicted the response of N₂ fixation to future climate change and found that it was greatly impacted by rising sea surface temperature. This study improved estimation of the regional N₂ fixation in the (sub)tropical western North Pacific and advances our understanding of its role in ocean productivity as well as its response to climate change.