Description: Discrete seawater samples for dissolved inorganic carbon (DIC) and total alkalinity (TA) were collected from the Polarstern (https://sensor.awi.de/?site=search&q=vessel:polarstern:ctd_sbe9plus_321) and Ocean City (https://sensor.awi.de/?site=search&q=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. Following Dickson et al. (2007), seawater samples were collected between 30 October 2019 and 28 September 2020 in borosilicate (3.3) bottles (250 ml), fixed with saturated mercuric chloride (HgCl₂) solution (100 µl), capped with greased (Apiezon® L) ground glass stoppers secured by insulating tape, and stored dark +4°C until post-cruise analysis at the Institute of Marine Research (IMR, Norway), University of East Anglia (UEA, UK), and Hokkaido University (Japan). DIC was determined by coulometric titration (Johnson et al., 1985) using a Versatile INstrument for the Determination of Total inorganic carbon and titration Alkalinity (VINDTA 3C, Marianda, Germany) at UEA, a VINDTA 3D at IMR, and a custom-built extraction system at Hokkaido University (Ono et al., 1998). TA was determined by potentiometric titration using a VINDTA 3C at UEA, a Versatile Instrument for the Determination of Titration Alkalinity (VINDTA 3S, Marianda, Germany) at IMR, and a TA analyzer ATT-05 (Kimoto Electric Co., Ltd., Japan) at Hokkaido University. The accuracy for both DIC and TA was set at IMR and UEA by routine analysis of certified reference material (CRM batch #182, #191) distributed by A. G. Dickson (Scripps Institution of Oceanography, La Jolla, CA, USA). Reference materials (Batch AR and AU; KANSO Technos Co., Ltd., Osaka, Japan), traceable to the Scripps CO₂ CRM, were used at Hokkaido University. Analytical precision for DIC and TA content is 〈 ±2 μmol/kg and 〈 ±2 μmol/kg, respectively, based on CRM replicates. Based on secondary quality control (Jutterström et al., 2010; Tanhua et al., 2010), DIC and TA were adjusted by -8 µmol/kg and -23 µmol/kg, respectively, for events PS122/1_7-49, PS122/1_8-46, PS122/1_9-50, and PS122/1_10-45. Data quality is indicated by flags following the consolidated WOCE system of Jiang et al. (2022).

Description: Samples for the analysis of dissolved nutrients were collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) from the water column, sea ice cores and from special events/locations (e.g., leads, melt ponds, brine, incubation experiments). Samples for dissolved inorganic nutrients (NO3 +NO2 , NO2 , PO4 , Si(OH)4, NH4 ) were analysed onboard during PS122 legs 1 to 3, with duplicate samples collected from CTD casts for later analysis of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP). From leg 4, all samples collected were stored frozen at -20°C for later analysis. Analyses of stored samples were carried out at the AWI Nutrient Facility between January and March 2021. Nutrient analyses onboard and on land were carried out using a Seal Analytical AA3 continuous flow autoanalyser, controlled by the AACE software version 7.09. Best practice procedures for the measurement of nutrients were adopted following GO-SHIP recommendations (Hydes et al., 2010; Becker et al., 2019). Descriptions of sample collection and handling can be found in the various cruise reports (Haas & Rabe, 2023; Kanzow & Damm, 2023; Rex & Metfies, 2023; Rex & Nicolaus, 2023; Rex & Shupe, 2023). Here we provide data from the water column, obtained from the analysis of discrete samples collected from CTD-Rosette casts from Polarstern (https://sensor.awi.de/?site=search&q=vessel:polarstern:ctd_sbe9plus_321) and Ocean City (https://sensor.awi.de/?site=search&q=vessel:polarstern:ctd_sbe9plus_935). Data from sea ice cores and special events are presented elsewhere. Data from sea ice cores and special events are presented elsewhere. For reference, here we included data from CTD-BTL files associated with nutrient samples. These data are presented by Tippenhauer et al. (2023) Polarstern CTD and Tippenhauer et al. (2023) Ocean City CTD.