Pelagic ciliate (Ciliophora) communities in the Southern Ocean: Bioindicator to water mass, habitat suitability classification and potential response to global warming

Although the pelagic ciliate (microzooplankton) play important role in carbon flux of marine ecosystems, their ecological function, habitat suitability classification and response to global warming were poorly documented in the Southern Ocean (SO). During summer of 2020/2021, ciliate (including aloricate ciliates and tintinnids) communities in waters from surface to 1000 m were investigated in the SO. Rapid attenuation of ciliate abun-dance and biomass occurred at 100 m depth in most stations. According to distinctive hydrographic features, five water masses were identified in the SO with unique ciliate community structure. Aloricate ciliate small size-fraction (10-20 & mu;m) was dominant in water masses deeper than 200 m. Tintinnids showed distinctive regional and vertical distribution characteristics, and their habitat suitability characterized by the Bio-index exhibiting different optimum survival zones. Variations in tintinnid habitat can be regarded as indicators for future SO climate change. Correlation between environment factors and biota revealed that depth, Chl a and dissolved oxygen were main factors determining ciliate composition. Combining tendency of phytoplankton and meso-/ macro-zooplankton to rapid global warming, ciliates may become smaller and more prominent in the future SO marine pelagic ecosystem. Our study has laid a foundation for recognizing the ciliate community and reinforcing the ideas about tintinnid indication for the change of future SO marine ecosystem.