Phytoplankton species counts from CTD Niskin samples collected in surface waters in the Barents Sea on ChAOS cruises c.

Phytoplankton species counts were conducted at six stations in the Barents Sea during the Changing Arctic Ocean cruises JR17007 (10th July - 5th August 2018) and JR18006 (29th June - 2nd August 2019) on board the RSS James Clark Ross. Samples were collected using a rosette sampler equipped with 24 Niskin bottles (20 L) to collect water at defined depths. For each station four surface depths were sampled for analysis. Two methods were used to quantify phytoplankton abundance, for larger species light microscopy was used and for smaller species flow cytometry. For light microscopy, duplicate samples were fixed per depth, one with acid Lugol's solution (2% final concentration) and one with neutral formalin (4% final concentration). Samples were stored in the dark prior to analysis at Plymouth Marine Laboratory. Samples were analysed using inverted settlement microscopy according to Widdicombe et al (2010). Species biovolumes were measured and calculated according to Olenina et al (2006). For flow cytometry, duplicate samples were collected and fixed with glutaraldehyde solution (0.25% final) and Pluronic F-68 solution (0.01% final). Samples were stored at 4°C for 12 hours before storage at -80°C until analysis at Plymouth Marine Laboratory. Samples were then analysed using Becton Dickinson FACSort™ according to Tarran et al (2006).