The role of mixotrophic nanoflagellates (MNF) as bacterivores in the Arctic Ocean is well recognized, but their importance as grazers in the mesopelagic layer is less understood. To investigate the vertical variations in pigmented nanoflagellates (PNF), heterotrophic nanoflagellates (HNF), and MNF abundances, as well as their grazing activities and relationships, an Arctic expedition (ARA14B) was conducted in April 2023. The findings indicate that the abundance of HNF decreases with increasing water depth, while MNF abundance remains stable. The abundance of bacteria was found to significantly influence the abundance and grazing activities of HNF and MNF. In the nutrient-depleted euphotic zone, a few PNFs may resort to mixotrophy as a survival strategy, but their abundance and bacterivory are still considerably lower than HNF. However, in the mesopelagic layer, where light and prey abundance are the primary limiting factors for nanoflagellates (NF), mixotrophy appears to be the main trophic mode. This study is the first to examine NF bacterivory in the mesopelagic zone of the Arctic Ocean while considering all functional groups.

Arctic Ocean, Mesopelagic zone, Mixotrophy, Bacterivory, Abundance