Dynamic upper-ocean processes enhance mesopelagic carbon export of zooplankton fecal pellets in the southern South China Sea

Abstract. Zooplankton are key contributors to the marine biological pump by converting phytoplankton-derived organic carbon into fast-sinking fecal pellets. Despite the established role of upper ocean dynamics in regulating epipelagic biogeochemistry and plankton communities, their impact on mesopelagic fecal pellet carbon export remains poorly constrained. Here, we present time-series sediment trap mooring observations of fecal pellet fluxes at 500 m from August 2022 to May 2023 in the southern South China Sea. Zooplankton fecal pellet fluxes display distinct seasonal patterns, with average numerical and carbon fluxes of 7.39 × 10⁴ pellets m^-2 d^-1 and 1.27 mg C m^-2 d^-1, respectively. Fecal pellets account for 10.0 to 42.6 % (average 21.6 %) of particulate organic carbon export, exceeding most oligotrophic regions. Mesopelagic fecal pellet fluxes are strongly correlated with upper-ocean dynamic processes, including winter mixing, tropical cyclones, and mesoscale eddies. Two tropical cyclones increase regional fecal pellet carbon export by more than 10 % of the annual carbon flux. One spring peak contributes more than 60 % of the total flux, likely driven by the combined effects of winter mixing, cold eddy activity, and spring zooplankton blooms. Our results highlight the critical role of upper-ocean dynamics in fecal pellet carbon export in deep water layers.