Nutrient supply to the upper ocean is fundamental to sustaining new and export production. Quantifying these nutrient fluxes remains subject to considerable uncertainties and debates, largely due to the complex interplay of physical and biogeochemical processes. Based on field measurements, we reveal pronounced variations in hydrography, water masses, nutrient concentrations, and nutrient stoichiometry across the western North Pacific. By combining nutrient profiles and concurrent turbulence microstructure measurements, we quantified the diapycnal fluxes of nutrients. At the base of the euphotic zone, the effective diapycnal nitrate flux exhibited distinct regional contrasts: 13.4 ± 12.1  μmol m⁻² d⁻¹ in the western North Pacific Subtropical Gyre (NPSG), 58.1 ± 88.2  μmol m⁻² d⁻¹ in the transition zone, and 46.5 ± 57.5  μmol m⁻² d⁻¹ in the Western Subarctic Gyre (WSG). Compared with the transition zone and WSG, the lower flux in the NPSG is primarily attributed to stronger stratification, which suppresses diapycnal diffusivity. A seasonal nutrient budget for the central WSG reveals that the nitrogen required by net community production (NCP) in summer is mainly supplied by the drawdown of the upper-ocean nitrate inventory, which peaks at ~4010  μmol m⁻² d⁻¹ in June. This nitrate inventory is primarily replenished by vertical entrainment during wintertime, which contributes 52.4% of the annual nitrogen requirement for NCP. By contrast, in the oligotrophic western NPSG, N₂ fixation constitutes the dominant nitrogen source. This fundamental contrast in nutrient supply mechanisms underlies the distinct biogeochemical functioning and carbon export potential of these two key oceanic provinces.