Transparent Exopolymer Particle (TEP) abundance and carbon from CTD niskin bottle samples collected depth profiles from the Barents Sea and Fram Strait regions on cruises JR18006 and JR18007, summer 2019.

‘Atlantification’ is pushing warmer Atlantic waters into the Arctic, changing microbial plankton communities with unclear impacts on organic matter cycling. Polysaccharide-rich microgels, such as transparent exopolymer particles (TEP), are an important part of the organic matter pool with a major role in carbon export. During two consecutive cruises in the Fram Strait (JR18007, August 2019) and Barents Sea (JR18006, July 2019), TEP abundance and carbon were analysed throughout the water column. TEP abundance and area were measured microscopically. The sample volume (10-250 ml) was chosen according to the prevailing TEP concentrations determined under the microscope onboard. Samples were filtered onto 0.4 µm nucleopore membranes (Whatman) and stained with 1 ml Alcian Blue solution (0.2 g l-1 w/v) for 3 seconds. Back at the lab, filters were mounted on microscopy slides that had been sprayed with white paint on the bottom side to avoid the filter pores becoming visible. Mounted filters were stored at -20°C. 30 images per sample were analysed using a DM 1,000 LED (Leica) equipped with a MC190 (Leica). The pictures with a resolution of 1388 x 1040 pixels were saved using Leica Application Suite Version 3.4.0 (Leica). All particles larger than 0.2 µm2 were analysed with ImageJ. A filter prepared with 10 ml MilliQ water served as a blank. The TEP-carbon content was calculated using the formula with D=-2.55 and r being the radius of the examined TEP: TEP-Cmicro [µg L-1] = 0.25x 10-6x r-D. These data were collected as part of the Micro-ARC project.