Diatom bloom trigger notable variations in microzooplanktonic ciliate composition, body-size spectrum and biotic-abiotic interaction in the Arctic Ocean

How microzooplanktonic ciliate adaptative strategies differ across diatom bloom and non-diatom bloom areas in the Arctic Ocean remains poorly documented. To address this gap, two different situations were categorized in the Arctic Ocean at summer 2023: diatom bloom stations (DBS) (genus Thalassiosira, chain-like) and non-diatom bloom stations (nDBS). Total abundance of ciliate at 3 m and 25 m in DBS was 2.8 and 1.8 folds higher than in nDBS, respectively. Aloricate ciliates were singled out in both DBS and nDBS, whilst their average abundance and biomass of large size-fraction (>50 μm) in former were 4.5-5.6 folds higher than in latter. Regarding tintinnids, high abundance of Ptychocylis acuta (Bering Strait species) mainly occurred at DBS, coupled with distribution of co-occurring Pacific-origin species Salpingella sp.1, collectively suggested a strong intrusion of Pacific Inflow during summer 2023. Additionally, presence of high abundance of Acanthostomella norvegica and genus Parafavella in nDBS might indicate the trajectory of the Transpolar Drift. Alternatively, tintinnids can serve as credible bioindicators for either monitoring currents or evaluating microzooplankton Borealization. Average abundance of total ciliate within 15-135 μm body-size spectrum in DBS was higher than nDBS. Moreover, spearman's rank correlation between biotic and abiotic analysis revealed that temperature and dissolved oxygen at DBS determined tintinnid species richness and ciliate total abundance, respectively. The results clearly demonstrate that remarkable divergences in large size-fraction of ciliate abundance between DBS and nDBS validate their irreplaceable role in controlling phytoplankton outbreak and associated biological processes in polar seas.

Full-depth vertical distribution of planktonic ciliates (Ciliophora) and a novel bio-index for indicating habitat suitability of tintinnid in the Arctic Ocean

Despite the planktonic ciliate importance in the microzooplankton compartment, their full-depth vertical distribution in the Arctic Ocean was poorly documented as well as the related variations in different water masses. The full-depth community structure of planktonic ciliates was investigated in the Arctic Ocean during summer 2021. The ciliate abundance and biomass decreased rapidly from 200 m to bottom. Five water masses were identified throughout the water column and each one exhibited a unique ciliate community structure. Aloricate ciliates were singled out as the dominant group with average abundance proportion to total ciliates at each depth >95%. Large (>30 μm) and small (10-20 μm) size-fractions of aloricate ciliates were abundant in shallow and deep waters, respectively, which revealed an anti-phase relationship in vertical distribution. Three new record tintinnid species were found during this survey. Pacific-origin species Salpingella sp.1 and Arctic endemic species Ptychocylis urnula occupied the top abundance proportion in the Pacific Summer Water (44.7%) and three water masses (≥38.7%, Mixed Layer Water, Remnant Winter Water, Atlantic-origin Water), respectively. The habitat suitability of tintinnid abundant species was characterised by the Bio-index revealing a distinct death-zone for each species. Variations in survival habitat of abundant tintinnids can be regarded as indicators for the future Arctic climate change. These results provide fundamental data on the microzooplankton response to the intrusion of Pacific waters into the Arctic Ocean upon its rapid warming.

Hydrographic Feature Variation Caused Pronounced Differences in Planktonic Ciliate Community in the Pacific Arctic Region in the Summer of 2016 and 2019

Planktonic ciliates are an important component of microzooplankton, but there is limited understanding of their responses to changing environmental conditions in the Pacific Arctic Region. We investigated the variations of ciliate community structure and their relationships with environmental features in the Pacific Arctic Region in the summer of 2016 and 2019. The Pacific water was warmer and more saline in 2019 than in 2016. The abundance and biomass of total ciliate and aloricate ciliate were significantly higher in 2019 than those in 2016, while those of tintinnid were significantly lower. The dominant aloricate ciliate changed from large size-fraction (> 30 μm) in 2016 to small size-fraction (10-20 μm) in 2019. More tintinnid species belonging to cosmopolitan genera were found in 2019 than in 2016, and the distribution of tintinnid species (Codonellopsis frigida, Ptychocylis obtusa, and Salpingella sp.1) in 2019 expanded by 5.9, 5.2, and 8.8 degrees further north of where they occurred in 2016. The environmental variables that best-matched tintinnid distributions were temperature and salinity, while the best match for aloricate ciliate distributions was temperature. Therefore, the temperature might play a key role in ciliate distribution. These results provide basic data on the response of the planktonic ciliate community to hydrographic variations and implicate the potential response of microzooplankton to Pacification as rapid warming progresses in the Pacific Arctic Region.