Downward vertical flux of protist cells and biomass in the northern Barents Sea during May 2021, Nansen Legacy cruise 2021704 Q2

Samples for protist cell and biomass flux were collected with short-term drifting sediment traps between 75° and 85°N in the northern Barents Sea with R/V Kronprins Haakon in May 2021 as part of the Nansen Legacy project (cruise 2021704, Q2). High-density bottom water was collected at the station and filtered (Whatman GF/F 0.7µm), and then filled into the trap cylinders (KC Denmark, aspect ratio >6) prior to their deployment. Afterwards, the sediment traps were deployed with 2 or 4 cylinders at each depth (30, 40, 60, 90, 120, 200m) for a duration between 12 to 27 hours. Depending on the sea ice conditions, the sediment traps were deployed in open water, anchored to a sea ice floe or within an ice lead. At selected stations, single trap cylinders were deployed by divers 1m below the sea ice. Upon retrieval, the content of the cylinders from each depth was pooled and stored at 4°C in dark until further analyses. For protist examination, a subsample of 100ml was transferred to a brown glass bottle and fixed with a mixture of glutaraldehyde-lugol. Protists were identified to the lowest possible taxonomic level in accordance with the World Register of Marine Species (WoRMS) and counted under an inverted light microscope (Nikon Eclipse TE-300 and Ti-S) using the Uthermöhl method (Edler & Elbrächter, 2010; Utermöhl, 1958). Protist carbon biomass flux was calculated from abundances using established geometric relationships for biovolume conversion (Hillebrand et al., 1999) and biovolume-to-carbon conversion factors (Menden-Deuer & Lessard, 2000).