Impacts of ridge-induced upwelling on the biological carbon pump in the tropical Northwestern Pacific

Abstract. Submarine ridges are prominent topographic features in the global ocean, yet their role in regulating the biological carbon pump remains underexplored. In this study, we investigated the spatial variability of phytoplankton production, particle dynamics, and carbon export along a meridional transect in the tropical Northwestern Pacific, spanning a warm eddy (WE), a cold eddy (CE), and the Kyushu-Palau Ridge (KPR). A suite of in situ measurements, including ¹⁴C-based primary production, HPLC pigment analysis, and particle profiling via Underwater Vision Profiler (UVP5-HD), was used to assess regional differences in biological carbon pump processes. Marked thermocline shoaling and nutrient uplift were observed in both the CE and KPR regions, but the KPR-derived upwelling waters had higher nutrient concentrations, resulting in the highest nutrient inventories in the upper 200 m and supporting elevated primary production (142.05 mg C m^-2 d^-1) and phytoplankton biomass there. Correspondingly, the KPR region exhibited the highest particle volume concentrations in the upper 200 m, followed by the CE, with the WE and background regions showing the lowest values. The 0–2000 m water column in the KPR region was also characterized by a substantially greater contribution of large particles (ESD ≥ 500 μm), which in turn supported enhanced particulate organic carbon (POC) export in this area. POC fluxes in the KPR region reached 9.04 ± 6.74, 5.52 ± 0.10, and 3.09 ± 1.96 mg C m^-2 d^-1 at 200 m, 1000 m, and 2000 m, respectively, which were 2.8 to 5.7 times higher than those in the CE region and 5.9 to 11.4 times higher than in the background region. Consistently, export efficiency (e-ratio) peaked in the KPR region (10 %), exceeding those in the CE (3 %), WE (3 %), and background (5 %) regions. Using the KPR as a representative case, our results highlight the critical role of ridge-induced upwelling in regulating phytoplankton production and particle dynamics, as well as enhancing biological carbon export and surface–deep coupling in oligotrophic oceans. These findings underscore the importance of incorporating such topographic processes into global oceanic carbon cycle research.