Description: We performed a temperature incubation experiment on board the RV Polarstern with a unicellular microbial community sampled from the Hausgarten station IV in Fram Strait during the campagin PS126 on June 1st, 2021 (Soltwedel et al., 2021). The community was sampled with CTD-bound niskin bottles (SBE 32 Carousel Water Sampler attached to a Seabird SBE911+ CTD-system; Seabird Scientific, Bellevue, WA, USA) from a depth of 15 m (Hoppmann et al., in review) and, after filtering the seawater through a 150 µm net, incubated in triplicate on plankton wheels in three temperature-controlled containers for ten days. To mimick todays and potential future temperature conditions of the Arctic ocean, we chose a control temperature of 2 °C, an intermediate warming scenario of 6 °C, and an extreme warming scenario of 9 °C. The goal was to investigate the effects of concurrent warming and Atlantification and therefore we chose an Arctic-Atlantic mixed water mass as community origin. This dataset comprises the chlorophyll, particulate nutrients, dissolved nutrients, carbonate chemistry, and flow cytometric measurements of the starting as well as the final communities. A total 300 mL of sample water for chlorophyll a, and 200 mL for particulate organic carbon and nitrogen (and the same volumes of ultrapure water for blank corrections), were vacuum-filtered (〈−200 mbar) onto pre-combusted glass-fiber filters (GF/F Whatman, Maidstone, UK). These were put into 2 mL cryovials (Sarstedt, Nümbrecht, Germany) and kept at −80 °C until processing. Filters for chlorophyll a were manually shredded in 6 mL of 90% acetone and extracted for 20 h at 8 °C according to the EPA method 445.0 (Arar et al., 1997). The extract was centrifuged to remove residual filter snips, and Chlorophyll a was determined on a Trilogy fluorometer (Turner Designs, San Jose, CA, USA) after correcting for phaeopigments via acidification (1 M HCl). Filters for particulate nutrients were also acidified (0.5 M HCl) and dried for 12 h at 60 °C. Analysis was performed using a gas chromatograph CHNS-O elemental analyzer (EURO EA 3000, HEKAtech, Wegberg, Germany). pH was measured with a pH meter (EcoScan pH 5, ThermoFisher Scientific, Waltham, MA, USA) including a glass electrode (Sentix 62, Mettler Toledo, Columbus, OH, USA) that was one-point calibrated with a technical buffer solution (pH 7, Mettler Toledo, Columbus, OH, USA). Samples for total alkalinity and dissolved nutrients were filtered through a 0.22 µm cellulose-acetate syringe filter (Nalgene, Rochester, NY, USA) and stored at 4 °C in 125 mL borosilicate bottles and 15 mL polycarbonate tubes. Total alkalinity was measured by duplicate potentiometric titration using a TitroLine alphaplus autosampler (Schott Instruments, Mainz, Germany) and corrected with certified reference materials from A. Dickson (Scripps Institution of Oceanography, San Diego, CA, USA). The full carbonate system was calculated for tfin using the software CO2sys (Pierrot et al., 2011) with dissociation constants of carbonic acid by Mehrbach et al. (1973), refitted by Dickson and Millero (1987). Dissolved nutrients were measured colorimetrically at on a continuous-flow autoanalyzer (Evolution III, Alliance Instruments, Freilassing, Germany) following standard seawater analytical methods for nitrate and nitrite (Armstrong et al., 1967), phosphate (Eberlein et al., 1987), silicate (Grasshoff et al., 2009), and ammonium (Koroleff et al. 1970). For flow cytometric measurements, 3.5 mL of the sample were preserved with hexamine-buffered formalin (0.5% final concentration) and stored at −80 °C after dark incubation for 15 min. For analysis, samples were thawed at room temperature, vortexed, and measured at a fast speed for three minutes using an Accuri C6 flow cytometer (BD Sciences, Franklin Lakes, NJ, USA) after setting the threshold of the FL-3 channel to 900. Phenotypic diversity (D2) was calculated for each sample based on the flow cytometric fingerprint according to Props et al. (2016), using the values of FSC-H, SSC-H, FL-2, FL-3, and FL-4. Parts of the metadata as well as calculations from it were used in the publication of Ahme et al. (2023). All scripts can be found on GitHub (https://github.com/AntoniaAhme/PS126CommunityExperiment). The sequence data are available at the European Nucleotide Archive (ENA).