Tracing Water Masses and Particle Dispersion on the Northern South China Sea Shelf Using POM Signatures under Contrasting Wind Conditions: A New Perspective

River-dominated shelf systems are shaped by complex hydrodynamic and biogeochemical interactions, making source-to-sink (S2S) tracking a challenge. To explore the influence of physical processes on particle distribution in the northern South China Sea, two research cruises were conducted along the Guangdong coast in the summers of 2018 and 2020. Hydrographic profiles and particle volume concentrations were measured using a CTD and a laser in-situ scattering transmissometer (LISST). Seawater samples from the surface, pycnocline, and bottom layers were analyzed for chlorophyll-a (Chl-a), particulate organic matter (POM, including POC, PN, and δ¹³CPOC), and nutrients. Empirical orthogonal function (EOF) analysis revealed distinct patterns of hydrodynamic control in both years. In 2018, stratification was intensified by local rainfall from a cyclonic system and the subsurface seawater, leading to the accumulation of larger particles at the pycnocline. In 2020, however, the Zhujiang River plume (ZRP) played a dominant role in stabilizing the water column under the southwesterly monsoon, entraining bio-particles enriched in δ¹³CPOC. This process facilitated the northeastward transport of biogenic material into the Taiwan Strait, possibly influencing sediment composition along the plume pathway. Although the ZRP and subsurface chlorophyll maximum shared similar biogeochemical signatures, the ZRP exhibited higher POC-to-Chl-a ratios and greater particle bulk densities, indicating more advanced POM degradation. Notably, the significantly negative correlation between δ¹³CPOC and salinity along the ZRP highlights δ¹³CPOC serves as a robust tracer of riverine water, more effective than the traditional N/P ratio in this river-dominated shelf system.