Understanding the mechanisms that sustain high biological productivity in the Kuroshio region, despite its oligotrophic surface conditions, remains a fundamental issue in marine science. In particular, the shelf–slope region of the northern East China Sea, where the Kuroshio interacts with coastal waters, is expected to play a key role in supporting lower trophic ecosystem dynamics; however, its physical–biogeochemical coupling has not been fully resolved.
In this study, we investigated the spatial structure of water masses, nutrient distributions, vertical nitrate (NO₃⁻) fluxes, and phytoplankton community composition based on high-resolution hydrographic observations conducted in 2023 and 2024, with particular focus on the subsurface chlorophyll maximum (SCM) during the stratified summer period.
The results revealed a distinct stratified structure characterized by nitrate depletion in the surface layer and a sharp vertical gradient below. Analysis of vertical NO₃ fluxes, based on turbulence-derived diffusivity, showed intermittent but persistent positive flux convergence below and around the SCM, indicating localized nutrient supply from deeper layers. Notably, the maximum flux occurred below the SCM, suggesting that the SCM forms where sufficient light availability overlaps with subsurface nutrient supply rather than at the depth of maximum flux.
Size-fractionated chlorophyll a analysis demonstrated that nano-sized phytoplankton consistently dominated the community (approximately 60–70%) across seasons and years. This dominance is likely maintained by intermittent nutrient supply under stratified conditions, which favors phytoplankton adapted to low-to-moderate nutrient environments. Occasional increases in larger phytoplankton were observed in association with enhanced nutrient fluxes.
These results provide observational evidence that the vertical structure of nutrient supply regulates phytoplankton size structure in the shelf–slope region. This physical–biological coupling offers new insights into the mechanisms sustaining primary production and supporting fisheries productivity in the Kuroshio-influenced ecosystem.

Nutient,northern East China Sea,Shelf-slope,Phytoplankton