Under-ice mesozooplankton samples were collected with a custom-made net (ROVnet, Wollenburg et al. 2020) attached to the ROV (Katlein et al. 2017) during the MOSAiC expedition (PS122) from 2019 to 2020. The net has an aperture of 0.24 m2 and a mesh size of 150 µm. The net was towed horizontally along linear or triangular transects at three depth horizons: directly under the sea ice (0 m), 10 m and occasionally as oblique tows at a deeper horizon between 20 and 100 m depth below the sea ice. The tow distance was calculated from the recorded position of the ROV (as given in Anhaus et al. 2023). All ROV net samples were preserved with a buffered 4% formaldehyde seawater solution and stored at room temperature until processed in the lab. The samples were analyzed with imaged-based ZooScan analysis (Gorsky et al. 2010) and all resulting images were taxonomically classified in the EcoTaxa dataset (https://ecotaxa.obs-vlfr.fr/prj/9968). This data collection includes individual datasets on abundance, and biovolume, biomass and respiration values derived from allometric relationships of the ZooScan image parameters.