Characteristics and Fluxes of Vertical and Lateral Particles in the Mesopelagic Northern South China Sea

The lateral transport of sediments from the shelf to the ocean interior is ubiquitous on continental margins, exerting great impacts on the characteristics and flux of sinking particles in the mesopelagic ocean. In this study, we determined the biogenic components and stable carbon isotopes in sinking particles collected from the slope of the northern South China Sea using a time series sediment trap. Laterally derived particulate organic carbon (POC_lateral), which originated primarily from marine sources, as indicated by the ratio of POC to particulate nitrogen ratio and POC-δ¹³C, accounted for 31 ± 3% of the total POC flux during the one-and-a-half-year deployment. High fluxes of POC_lateral occurred in April and May 2021, comprising 69 ± 12% and 53 ± 9% of the total POC flux, respectively. This was likely due to the lateral input of shelf sediments entrained by a westward anticyclonic eddy. These allochthonous particles were mainly composed of small (<53 μm) sediments and were depleted in biogenic components, such as POC and biogenic silica (bSi). In contrast, the flux of vertically derived POC (POC_vertical), that is, local biogenic POC, was strongly correlated with the surface Chl-a concentration (R² = 0.62, p < 0.001) and bSi flux (R² = 0.83, p < 0.001), indicating that the POC_vertical flux in the mesopelagic zone was governed by both primary productivity and the growth of diatoms, with the latter being the main controlling factor. Our results highlight that the POC_vertical flux was controlled by the seasonal variations in primary productivity and the phytoplankton community structure, whereas the POC_lateral flux was closely linked to hydrodynamic processes, such as mesoscale eddies.