Decoding pelagic ciliate (Ciliophora) community divergences in size spectrum, biodiversity and driving factors globally spanning five temperature zones

Abstract. The community structure of microzooplanktonic ciliates – encompassing size spectrum, biodiversity and biotic–abiotic interplay – is critical for unravelling their ecological role in marine ecosystems, yet it remains challenging to elucidate on a global scale. To address this knowledge gap, we conducted field observational studies across five temperature zones (North Frigid Zone, NFZ; Sub-Arctic Zone, SAZ; North Temperate Zone, NTZ; Torrid Zone, TZ; South Frigid Zone, SFZ). Our analysis demonstrates a sharp decline in ciliate abundance and biomass below the 100 m layer, with distinct vertical distribution patterns observed in each climate region. Moreover, although abundance of ciliate size spectra exhibited a decrease trend from small to large size spectra globally, there were steeper slope lines observed in both polar zones (NFZ and SFZ) compared to the other temperature zones. Latitudinally, ciliate abundance and tintinnid biodiversity exhibited an anti-phase relationship, where the TZ hosted peak biodiversity, while the polar seas showed the highest abundance. Furthermore, a multivariate biota–environment analysis indicated that temperature has a primary influence on ciliate community constitution in the global marine ecosystem, and the bottom-up control plays a key role in shaping assemblages. In conclusion, these results underscore the unprecedented divergences in ciliate trait structure among five temperature zones and can be taken as a guideline for assessing the potential effects of climate change on pelagic ciliates in future marine realms.