Zooplankton Fecal Pellet Flux and Carbon Export in the Deep South China Sea

Zooplankton fecal pellets constitute a major component of passively sinking particles in the ocean. The sinking of zooplankton fecal pellets provides an efficient vehicle for the transfer and sequestration of particulate organic carbon in the deep sea, which has been widely reported in different ocean regions. However, most existing studies focus on the sinking flux of fecal pellets within the upper ocean, while the lower mesopelagic and bathypelagic zones are rarely investigated. Here, we report the spatiotemporal flux variation of zooplankton fecal pellets collected by two sediment traps deployed in mesopelagic and bathypelagic zones (500 m and 2190 m, respectively) of the southern South China Sea from June 2020 to May 2023. The average fecal pellet numerical flux is 3.21*10⁴and 4.64*10⁴ pellets m^-2 d^-1 at 500 m and 2190 m, respectively, corresponding to an average fecal pellet carbon flux from 0.43 to 0.84 mg C m^-2 d^-1 at these two depths. Fecal pellet fluxes display distinct seasonal patterns due to the control of the East Asian monsoon system, with higher fluxes in winter and spring, and lower fluxes in summer and autumn. Higher fecal pellet fluxes combining with the presence of extra-large pellets are found in bathypelagic zone, which is attributed primarily to in-situ reworking and repackaging of sinking particulate matter by deep-dwelling zooplankton communities, as well as lateral inputs from adjacent high productive continental coasts and shelves. We compare our results with global deep-sea (>500 m) fecal pellet flux data reported from different sediment-trap stations with distinct marine primary productivity and zooplankton biomass. Furthermore, we will report on the state of our progress on carbon isotope analysis (¹³C, ¹⁴C) for disentangling the source-to-sink dynamics of fecal pellets and its role in the deep-sea carbon export and sequestration.