Distribution and fluxes of marine particles in the South China Sea continental slope: implications for carbon export

Marine particles are key vectors in the ocean’s biological carbon pump, yet their distribution, size structure, contributions to particulate organic carbon (POC) flux, and the mechanisms controlling these processes remain poorly understood in marginal seas. In this study, we investigated the spatial distribution and carbon flux of marine particles along the continental slope of the South China Sea (SCS), using in situ imaging data collected by an Underwater Vision Profiler during a June 2015 cruise. We also examined how these particle-related processes respond to mesoscale eddy activity. Particle abundance and volume concentration (PVC) ranged from 0 to 783 particles L⁻¹ (mean ± SD: 68 ± 69 particles L⁻¹) and from 0 to 6.7 mm³ L⁻¹ (mean ± SD: 0.3 ± 0.4 mm³ L⁻¹), respectively. Small particles, which were defined as those with an equivalent spherical diameter less than 500 µm, overwhelmingly dominated in terms of abundance, accounting for more than 97 % of total counts. However, in terms of PVC, large particles contributed a greater share, averaging 61 % ± 12 %. PVC was significantly higher in the epipelagic layer (mean ± SD: 0.4 ± 0.7 mm³ L⁻¹) than in the mesopelagic layer (mean ± SD: 0.2 ± 0.1 mm³ L⁻¹, p < 0.01), indicating enhanced particle production in surface waters. Under the influence of mesoscale eddies, distinct differences in particle characteristics and carbon export were observed. Cyclonic eddies enhanced particle concentrations and favored the formation of large particles, while anticyclonic eddies were associated with a higher proportion of small particles. These patterns were linked to eddy-induced changes in nutrient availability and phytoplankton production. Consequently, POC fluxes in cyclonic eddy–influenced regions were consistently higher than those in anticyclonic regions throughout the water column, with POC fluxes reaching over twice the magnitude observed in anticyclonic eddy regions. This suggests that mesoscale eddies can influence carbon export by altering both the concentration and size composition of marine particles. Our study clarifies the distribution and size structure of marine particles along the SCS slope and highlights the importance of mesoscale physical processes in regulating particle–mediated carbon export, thereby enhancing our understanding of carbon cycling processes in dynamic marginal seas.