Beschreibung: Discrete seawater samples for total dissolved oxygen were collected from the Polarstern (vessel:polarstern:ctd_sbe9plus_321) and Ocean City (vessel:polarstern:ctd_sbe9plus_935) CTD/Rosette systems during the Multidisciplinary drifting observatory for the study of Arctic Climate (MOSAiC) expedition, 20 September 2019 – 14 October 2020. Dissolved oxygen was analyzed on board RV Polarstern during legs 3, 4, and 5 between 6 March 2020 and 17 September 2020. Seawater was drawn from Niskin bottles into Winkler flasks using silicon tubing, filling from the bottom and allowing seawater to overflow by at least two flask volumes (approximately 120 mL). The samples were directly pickled by the addition of 1 mL each of concentrated manganese chloride (3 M) and alkaline (NaOH 8 M) sodium iodide (4 M) solutions to form manganese oxyhydrodoxide precipitate. Samples were stored in the dark and underwater at room temperature until analysis. Dissolved oxygen was determined using an automatic Winkler titration setup with potentiometric detection (Titrino plus 848, Metrohm, Switzerland). Reagents were prepared according to Langdon (2010). Blanks were determined before each run by addition of reagents to MilliQ water. The titrations were performed directly in the flasks used for sampling after the manganese oxyhydroxide precipitate was dissolved by addition of sulphuric acid (5 M). Sodium thiosulphate (~0.2 N) solutions were prepared onboard for the titration and the concentration was determined before each run using an iodate standard (0.01 N, OSIL, United Kingdom). The analytical precision based on duplicate samples analysis was 0.4 ± 2.6 µmol/kg (n = 60).