The gravitational sinking of organic particles transfers organic carbon from the surface to the deep ocean, a fundamental mechanism of the biological carbon pump, crucial for regulating climate change and supplying energy to deep-sea biota. However, in subtropical marginal seas, monsoons and plumes significantly influence particle organic carbon (POC) production, while sinking processes are impacted by complex physical-biological interactions, complicating the resolution of POC fate through limited sampling. In this study, we utilized the Underwater Vision Profiler 5 along with high-resolution remote sensing and reanalysis products to reconstruct the monthly climatology of particle size distribution (PSD) parameters and POC flux. Basin-scale reconstructions reveal that in oligotrophic basin regions, biovolume and PSD slope exhibit inverse trends, with slope decreasing and biovolume increasing in spring, indicating the production of larger particles and a higher potential for POC export. Conversely, nutrient supply from high-productivity plumes stimulates pico-phytoplankton blooms in summer, leading to a concurrent increase in biovolume and PSD slope, thereby enhancing carbon export. The reconstructed POC export from the euphotic zone in the South China Sea is approximately 116.4 Tg C y^-1, comparable to estimates from food web models, accounting for about 18% of net primary production. Our study elucidates the complex seasonal dynamics of particle properties and carbon export in various ecosystems of low-latitude marginal seas, providing new insights into the production and transformation of marine particles.

Particulate Organic Carbon Export,Particle size distribution,In situ opticle observation,Machine learning,Subtropical marginal sea