Phytoplankton play a central role in the Earth’s climate system by regulating marine carbon cycling. Subsurface enhancement of phytoplankton is widely observed across the North Pacific. However, its spatiotemporal variability and contribution to primary production remain insufficiently constrained. Here, we integrate BGC-Argo profile observations, random forest-based three-dimensional reconstruction, and the Carbon-based Production Model (CbPM) to investigate subsurface enhancements of chlorophyll concentration (Chl) and phytoplankton carbon biomass (C_phy) across five ecological regions of the North Pacific, as well as their implications for primary production.

Our results reveal pronounced regional differences in subsurface Chl and C_phy enhancement. Subsurface enhancement of C_phy is primarily controlled by surface nutrient limitation, whereas subsurface Chl enhancement is more strongly associated with light availability below the mixed layer depth. In addition, subsurface phytoplankton structures enhance primary production mainly through increased physiological photoacclimation, as reflected by elevated Chl:C_phy ratios, while subsurface peaks in C_phy biomass provide an additional contribution. This study improves our understanding of subsurface phytoplankton structure and its role in the North Pacific carbon cycle, and provides new observational and modeling constraints for assessing marine carbon transport processes.