Insights into the pelagic ciliate community in the Bering Sea: Carbon stock, driving factors and indicator function for climate change

Microzooplankton carbon stock and its biotic-abiotic interactions are important linkage in marine carbon biogeochemistry, which remain poorly documented in the Bering Sea. In the summer of 2019, we investigated the community structure of pelagic ciliates from the surface to bottom (or up to 200 m) in the Bering Sea. High abundance and biomass of ciliate occurred mainly in waters shallower than 50 m. The average carbon stock of ciliates were 350.0 ± 96.4 mg C m⁻² and 218.4 ± 148.8 mg C m⁻² in oceanic and shelf regimes, respectively. Aloricate ciliates were the dominant group, with an average abundance proportion to total ciliates of over 80% at each depth. We identified 45 tintinnid species belonging to 15 genera, including 7 new record species in the Bering Sea. The abundance proportion of their lorica oral diameter (LOD) size-classes at each sampling depth was mainly within the fractions of 12–16 μm and 24–28 μm. Furthermore, both Codonellopsis frigida and Parafavella jorgenseni can act indicator species of Pacific inflows based on their distributions. Strong correlations were reflected between abiotic environmental factors and the distribution of these indicator species, for instance, the positive correlation between salinity and the cold water-adapted species C. frigida, confirming the dominant role of bottom-control on the ecosystem. Our study laid a foundation for deep understanding of ciliate community at summertime of the Bering Sea, and its role in driving biogeochemical fluxes.