Time-series variations of zooplankton fecal pellet flux in the western South China Sea and their implications for marine biological pump

The settling of zooplankton fecal pellets plays a vital role in transporting particulate organic carbon to the deep ocean, serving as an essential component of the marine biological pump. Here, we present four-year (June 2020 to July 2024) time-series variations of zooplankton fecal pellet flux from two sediment traps respectively at 500 m and 1170 m water depth in the western South China Sea. Of all samples, the numerical flux of fecal pellets was averagely 2.94 and 1.14*10⁴ m^-2 d^-1 at 500 m and 1170 m, respectively, and the carbon flux of fecal pellets was averagely 0.20 and 0.14 mg Cm^-2 d^-1 at 500 m and 1170 m, respectively. Both numerical and carbon flux of fecal pellet decreased with water depth. Three morphological types (spherical, cylindrical, and ellipsoidal) of fecal pellets were identified in this study. Distinct morphological types of fecal pellets contribute differently to the numerical and carbon fluxes. At both water depths, ellipsoidal and spherical pellets accounted for 95% of the numerical flux. Cylindrical pellets were rare in abundance (5%), but accounted for more than 20% of the total fecal pellet carbon flux. During the entire sampling period, the proportion of particulate organic carbon flux attributed to zooplankton fecal pellets ranged from 0.5 to 14.0%, with the average value of 5.1%. From the long-term perspective, both numerical and carbon fluxes varied greatly through time, exhibiting clear seasonal and interannual variations. During 2022 to 2023, the fecal pellet numerical flux was 3 times higher than the other three years. Within each year, fluxes of fecal pellet showed increases when the East Asian monsoon system converted. The contribution of fecal pellet carbon to overall organic carbon indicated that fecal pellets play a quite significant role in transporting organic carbon to the deep sea. The changes of fecal pellet flux were most likely influenced by El Niño and La Niña events in long term and regulated by East Asian monsoon system seasonally. As a result, multiple mechanisms, such as long-term climate event, seasonal monsoon system, zooplankton community structure, and other environmental conditions could be responsible for the production and fate of fecal pellets as well as their contribution to the overall particulate organic carbon. Thus, the settling of fecal pellets could give enlightening insights on the cycle of organic carbon and reveal how climate change influenced the marine biological pump.