Summertime tintinnids in surface water of the Weddell and Cosmonaut seas: community structure and relationships with different water masses

Tintinnids (Ciliophora) are important microzooplankton grazers. In the Southern Ocean, they are found in the Antarctic Zone, Polar Front and Subantarctic Zone. The Antarctic Zone encompasses large gyres (Weddell Gyre and Ross Gyre) and the Antarctic Slope Current around the continent. The influence of these water masses on tintinnid communities has not been studied. This study investigated the tintinnid community structure in the Weddell and Cosmonaut seas in the summer of 2022. In the Weddell Gyre, tintinnid abundance was significantly lower in the interior than at the fronts. The dominant species differed between the east and west fronts: the proportion of Codonellopsis gaussi was high at the west front, whilst Laackmanniella naviculaefera, Salpingella sp. and Salpingella faurei showed high abundances at the east front. Tintinnid communities varied from inshore to offshore of the Cosmonaut Sea, possibly because of the influence from the Antarctic Slope Current and Antarctic Circumpolar Current. The Antarctic Slope Current was characterized by the occurrence of Cymatocylis drygalskii, whilst the Antarctic Circumpolar Current was characterized by Codonellopsis glacialis, Cymatocylis convallaria and Cy. calyciformis. We proposed that Cy. drygalskii can be used as an indicator of the Antarctic Slope Current. Moreover, we classify polymorphic C. gaussi into three types, in accordance with their loricae, and report their distribution character-istics in water masses. Our results contribute to a better understanding of tintinnid horizontal distribution in different parts of the Weddell Gyre and water masses and serve as a baseline for future studies of pelagic community responses to climate change in the Southern Ocean.