Zooplankton Fecal Pellet Characteristics and Contribution to the Deep‐Sea Carbon Export in the Southern South China Sea

Zooplankton fecal pellets play a significant role in particulate organic carbon (POC) export, which has been widely reported in different ocean regions. However, zooplankton fecal pellet in the tropical marginal sea and its contribution to the total POC flux has not been sufficiently investigated yet. Here, for the first time we report zooplankton fecal pellet characteristics and fluxes in the South China Sea from June 2020 to May 2021 via one mooring with two sediment traps deployed at 500 and 2,190 m depths. Average fecal pellet numerical flux was 2.30 x 10(4) and 4.22 x 10(4) pellets m(-2) d(-1) at 500 and 2,190 m, respectively, corresponding to average fecal pellet carbon (FPC) flux from 0.31 to 0.71 mg C m(-2) d(-1). Small ellipsoidal and spherical pellets accounted for more than 90% of the total numerical flux, while large cylindrical pellets, although relatively rare (6%), accounted for almost 15% of the FPC flux. Both fecal pellet numerical and carbon fluxes were higher in winter, when the strong East Asian winter monsoon winds and heavy rainfall promoted the marine primary production. Higher fecal pellet fluxes combining with the presence of extra-large pellets at 2,190 m compared to those at 500 m suggested the in-situ repackaging of deep-dwelling zooplankton communities in the mesopelagic and bathypelagic zones. Contribution of fecal pellets to the total POC flux ranged from 0.4% to 30.0% (averaging 9.0%), with higher values occurring during winter monsoon, at which time fecal pellets became a critical conveyor of carbon export to the deep sea. Plain Language Summary Zooplankton fecal pellets sinking from the sea surface and water column constitute an important component of the marine biological pump. Time series study of fecal pellet flux is therefore of great importance to investigate the deep-sea particulate organic carbon export. In this study, we examined the temporal and spatial variation of fecal pellet flux in the tropical marginal sea for the first time. Fecal pellet numerical flux and carbon flux at 500 and 2,190 m water depths in the southern South China Sea were quantified, which exhibited great seasonal signals and vertical changes. Strong East Asian winter monsoon contributed to higher fecal pellet fluxes through strengthening the water column mixing and enhancing the marine primary production. In-situ repackaging of deep-dwelling zooplankton communities were responsible for the increased fecal pellet flux in the deep sea. Our result of zooplankton fecal pellet flux in the southern South China Sea reflects a dynamic balance between the marine primary production, zooplankton consumption, recycling process, and in-situ production. Therefore, this study confirms the significant role that zooplankton fecal pellet plays in the local carbon export and provides new insight into the biogeochemical cycling in the tropical marginal sea.