With the exception of a single small-scale study that examined protists in several deep-ocean water masses 14, most studies of protists in the Southern Ocean focused on the photic layer 15, 16, 17, 18, 19, 20, 21. Although polar oceans belong to one of the most productive and rapidly changing ecosystems on Earth 13, the Southern Ocean remains poorly represented in these large surveys of marine protists. Marine protist communities are now actively mapped by large metabarcoding projects 4, 5, 6, 7, 8, 9, 10, 11, 12, but those are focused mostly on the tropical and temperate regions and on the sunlit ocean, where the bulk of the ocean’s productivity takes place. Their distribution is mainly influenced by a combination of abiotic factors (with temperature and oxygen concentration being the most important) and biotic interactions 5. Even in the photic layer, they are more diverse and abundant than primary eukaryotic producers 4. Heterotrophic protists are a vital component of the ocean’s plankton throughout the water column 1, 2, 3. The strongly correlated distribution of syndinians and diplonemids along the depth and oxygen gradients suggests their close ecological link and moves us closer to understanding the biological role of the latter group in the ocean ecosystem. At the beginning of this succession, in the photic zone, algae, bacteriovores, and predators of small eukaryotes dominate the community, while another community develops as the water sinks deeper, mostly composed of parasitoids (syndinians), mesoplankton predators (radiolarians), and diplonemids. We focus on factors affecting the distribution of planktonic pico-nano eukaryotes and observe an ecological succession of eukaryotic communities as the water masses move away from the surface and as oxygen becomes depleted with time. Here we present an analysis of a unique set of DNA metabarcoding samples from the western Weddell Sea sampled throughout the whole water column and across five water masses with different characteristics and different origin. Polar oceans belong to the most productive and rapidly changing environments, yet our understanding of this fragile ecosystem remains limited.
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