Quantifying nutrient supply to the eddy-influenced subtropical North Pacific upper ocean: Modified optimum multiparameter analysis using rare earth elements from three GEOTRACES cruises

Horizontal/Vertical nutrient supply in the upper ocean of the North Pacific Subtropical Gyre (NPSG) plays a pivotal role in biogeochemical cycling and CO₂ uptake. However, research quantifying water/nutrient transport based on direct chemical observations and measurements is limited. Based on observations made during three GEOTRACES cruises in spring, summer, and winter, we identified horizontal and vertical water sources and quantified the water and nutrient supply, applying modified Optimum Multiparameter (OMP) analysis based on iterative calculation, in which rare earth elements (REEs) were used as quasi-conservative chemical tracers. The mean quantification results with a depth of ≤ 200 m show that Equator-derived water (Nutrient fraction: 51% ± 37%) and vertical supply (31% ± 33%) are the dominant nutrient sources; northern NPSG-derived water (0% ± 1%) has little influence; North Equatorial Current-derived water shows a higher contribution at 200–300 m (38% ± 26%) than the shallow layers (10% ± 19%); coast-derived water (7% ± 15%) contributes to NPSG in an inconsistent way. In addition, the enhanced vertical nutrient supply during the sampling period, which is more significant in spring, is likely to be attributed to the influence of what are considered different types of eddies based on the sea surface height. The vertical fluxes of dissolved inorganic nitrogen in the bottom euphotic layer at stations near warm, cold, and no eddies were estimated to be 0.10–0.76, 0.21–2.13, and 0.066–0.53 mmol m⁻² d⁻¹, respectively, which are 1–100 times the supply from nitrogen fixation. These nutrient fluxes could explain 5–169 mg C m⁻² d⁻¹ of the carbon fixation in the euphotic zone.