Description: Algal pigment concentrations were retrieved from meltponds and leads, during Leg 4 and 5 of the MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate) in 2020. The summer increase in meltpond area and open lead water in the Arctic associated with increased light availability is of specific significance for biological production within the Arctic system (Smith et al. 2023). Over a period of 3.5 months, 46 samples have been collected and analysed. After extraction in 90 % acetone, samples were analysed using high-performance liquid chromatography (HPLC) on a Waters system. Algal pigments contain a multiple set of information. Firstly, pigment concentrations can show the presence of algal biomass in the various water masses sampled. Secondly, marker pigments can reveal seasonal and temporal dynamics in algal community structure, by discerning specific algal classes like diatoms, cryptophytes, haptophytes and chlorophytes that have specific roles in biogeochemical cycles. Thirdly, certain pigments are indicative of the (photo)-physiological state of micro-algae and fourth, degradation products of the main chlorophyll a pigment further give an indication about senescence and grazing in the various habitats.

Description: Near-daily concentrations of Chlorophyll a and phaeopigments from RV Polarsterns underway water supply system (inlet at 11m water depth) collected during legs 1,3,4, and 5 of the MOSAiC (PS122) drift expedition in the central Arctic Ocean. 2-4L of water were filtered onto pre-combusted GF/F filters (nominal pore size 0.7µm) and frozen at -80°C. Samples were subsequently extracted in 90°C acetone, homogenized using a cell mill, and measured on the following day using a Turner flourometer, followed by an acidification step to determine phaeopigments (see Knap et al. 1996 for details and calculations). Samples were collected from end of October 2019 to beginning of October 2020, with a gap between mid-December and the end of February.

Description: Concentrations of Chlorophyll a and phaeopigments from CTD-rosette casts during the MOSAiC (PS122) drift expedition in the central Arctic Ocean. 2-4L of water were filtered onto precombusted GF/F filters (nominal pore size 0.7µm) and frozen at -80°C. Samples were subsequently extracted in 90°C acetone,homogenized using a cell mill, and meaured on the following day using a Turner flourometer, followed by an acidification step to determine pheopigments (see Knap et al. 1994 for details and calculations). Samples were collected roughly once per week from end of October 2019 to beginning of October 2020.

Description: First-year sea-ice thickness, draft, salinity, temperature, and density were measured during near-weekly surveys at the main first-year ice coring site (MCS-FYI) during the MOSAiC expedition (legs 1 to 4). The ice cores were extracted either with a 9-cm (Mark II) or 7.25-cm (Mark III) internal diameter ice corers (Kovacs Enterprise, US). This data set includes data from 23 coring site visits and were performed from 28 October 2019 to 29 July 2020 at coring locations within 130 m to each other in the MOSAiC Central Observatory. During each coring event, ice temperature was measured in situ from a separate temperature core, using Testo 720 thermometers in drill holes with a length of half-core-diameter at 5-cm vertical resolution. Ice bulk practical salinity was measured from melted core sections at 5-cm resolution using a YSI 30 conductivity meter. Ice density was measured using the hydrostatic weighing method (Pustogvar and Kulyakhtin, 2016) from a density core in the freezer laboratory onboard Polarstern at the temperature of –15°C. Relative volumes of brine and gas were estimated from ice salinity, temperature and density using Cox and Weeks (1983) for cold ice and Leppäranta and Manninen (1988) for ice warmer than –2°C. The data contains the event label (1), time (2), and global coordinates (3,4) of each coring measurement and sample IDs (13, 15). Each salinity core has its manually measured ice thickness (5), ice draft (6), core length (7), and mean snow height (22). Each core section has the total length of its top (8) and bottom (9) measured in situ, as well estimated depth of section top (10), bottom (11), and middle (12). The depth estimates assume that the total length of all core sections is equal to the measured ice thickness. Each core section has the value of its practical salinity (14), isotopic values (16, 17, 18) (Meyer et al., 2000), as well as sea ice temperature (19) and ice density (20) interpolated to the depth of salinity measurements. The global coordinates of coring sites were measured directly. When it was not possible, coordinates of the nearby temperature buoy 2019T66 were used. Ice mass balance buoy 2019T66 installation is described in doi:10.1594/PANGAEA.938134. Brine volume (21) fraction estimates are presented only for fraction values from 0 to 30%. Each core section also has comments (23) describing if the sample is from a false bottom, from rafted ice or has any other special characteristics. Macronutrients from the salinity core, and more isotope data will be published in a subsequent version of this data set.

Description: The CO2 fluxes were measured over the surfaces of snow, ice, water with LI-COR 8100-104 chambers connected to a LI-8100A soil CO2 flux system (LI-COR Inc., USA) during expedition PS122 (MOSAiC Legs 1−5) to the central Arctic in October 2019−September 2020. A chamber was connected via a closed loop to an infrared gas analyzer (LI-8100A, LI-COR Inc., USA) to measure CO2 concentrations with an air pump at a rate of 3 L min−1 during 20-minute intervals. Power was supplied by a battery (8012−254, Optima Batteries Inc., USA). We also used a Teflon-coated metal chamber (0.50 m in diameter and 0.30 m high with a serrated bottom edge) (Nomura et al., 2010, 2012). Every 5 or 10 minutes during an experiment, about 500 mL of air was collected from the chamber using a 50-mL glass syringe with a three-way valve and then transferred to a 3000-mL Tedlar bag (AAK 3L, GL Sciences Inc., Japan). After collection, air samples were quickly transported in a dark container to a laboratory onboard the R/V Polarstern, which was moored near our sampling site. The CO2 concentrations were measured with a CO2 analyzer (Picarro 2132-i, Los Gatos Research Inc., USA) used for continuous measurements of atmospheric CO2/CH4 concentrations on board. The CO2 fluxes (in mmol C m−2 day−1) (a negative value indicates CO2 being absorbed from the atmosphere) were calculated with LI-COR software (model: LI8100PC Client v.3.0.1.) based on the changes of the CO2 concentrations within the headspace of the LI-COR 8100−104 chambers. For the metal chamber, we took into account the changes of CO2 concentrations and the volume of the chamber (Nomura et al., 2010, 2012) to calculate the CO2 fluxes. The detection limit of this system was about +0.1 mmol C m−2 day−1 (Nomura et al., 2010; 2018). An inter-comparison experiment between the metal chamber and the LI-COR 8100−104 chamber in the home laboratory indicated good agreement (Nomura et al., 2022). Data obtained with both methods were therefore comparable. When the surface of the melt ponds/leads was frozen, flux measurements were made over the frozen surface. Then, a 1 m x 1 m hole was cut with a hand saw, and chambers were installed over the water surface with buoyant material (Nomura et al., 2020; 2022). In addition, chambers were installed over snow/slush/frost flower surface, and ice surface after removing snow by shovel. We conducted water mixing experiments at St. 4 (melt pond) on September 2, 2020 and at the ROV lead site on September 5, 2020 to understand how the carbonate chemistry and CO2 fluxes responded to changes in the marine environment caused by agitation in the melt pond and lead by wind and movement of sea ice. We measured the fluxes between the atmosphere and the surfaces of the melt ponds and leads using a floating metal chamber. After the measurements, the water in the melt ponds or leads was mixed for 30 minutes by two persons using a shovel and an oar. After mixing, the pre-mixing measurements were repeated.

Description: Melt ponds water sampling for biogeochemical parameters such as dissolved inorganic carbon (DIC), total alkalinity (TA), oxygen isotopes were examined from August to September 2020. To obtain discrete water samples from the melt ponds and leads, we checked the vertical structure and depth of the meltwater layer from the same hole used for the RINKO Profiler by attaching a conductivity sensor (Cond 315i, WTW GmbH, Germany) to a 2-m-long ruler and inserting the ruler into the lead water until the salinity measured with the Cond 315i increased at the meltwater–seawater interface (Nomura et al., 2024) . Water was pumped up with a peristaltic pump through a 2-m-long PTFE tube (L/S Pump Tubing, Masterflex, USA) at depths corresponding to meltwater (surface), the interface between meltwater and seawater (interface), and seawater (bottom). Salinity was measured at each depth by attaching a Cond 315i conductivity sensor to the bottom of the ruler. The tube intake was likewise attached to the bottom of the ruler. Seawater was subsampled into a 250-mL glass vial (Duran Co., Ltd., Germany) for measurement of dissolved inorganic carbon (DIC) and total alkalinity (TA) and a 50-mL glass, screw-cap, narrow-neck vial (VWR international LLC, Germany) for measurement of the oxygen isotopic ratio (δ18O) of the water. Immediately after subsampling for measurement of DIC and TA, a 6.0% (wt.) mercuric chloride (HgCl2) solution (100 µL) was added to stop biological activity. Samples for DIC and TA were stored at +4°C on the R/V Polarstern. Samples for δ18O were stored at room temperature (20°C). During the discrete water sampling, the CO2 concentration in the water column was measured directly on site by passing the water through an equilibrator Liqui-Cel® (G542, S/N: 132462, 3M Company, USA) connected to an infrared gas analyzer (LI-8100A, LI-COR Inc., USA). The analyzer was calibrated with standard gases containing 0.0, 299.3, and 501.3 ppm CO2 before MOSAiC Leg 5. RMS (root means square) noise at 370 ppm with 1 sec signal averaging is 〈1 ppm (https://www.licor.com/env/products/soil-flux/LI-8100a). The equilibrator was connected in the loop for water sampling (vide supra), and a 2-m-long ruler was inserted into the water and kept at that depth until the CO2 was equilibrated with air (about 1 minute) by monitoring the CO2 values. The CO2 concentration was measured at each depth (i.e., surface, interface, and bottom). At the ROV lead sites, vertical CO2 measurements were made every 0.05 m for detailed profiles. The DIC of water was determined by coulometry (Johnson et al., 1985; Johnson, 1992) using a home-made CO2 extraction system (Ono et al., 1998) and a coulometer (CM5012, UIC, Inc., Binghamton, NY, USA). The TA of water was determined by titration (Dickson et al., 2007) using a TA analyzer (ATT-05, Kimoto Electric Co., Ltd., Japan). Both DIC and TA measurements were calibrated with reference seawater materials (Batch AR, AU, and AV; KANSO Technos Co., Ltd., Osaka, Japan) traceable to the Certified Reference Material distributed by Prof. A. G. Dickson (Scripps Institution of Oceanography, La Jolla, CA, USA). Oxygen isotope analyses were carried out at the ISOLAB Facility at AWI Potsdam (hdl:10013/sensor.ddc92f54-4c63-492d-81c7-696260694001) with mass spectrometers (DELTA-S Finnigan MAT, USA): hdl:10013/sensor.af148dea-fe65-4c87-9744-50dc4c81f7c9 and hdl:10013/sensor.62e86761-9fae-4f12-9c10-9b245028ea4c employing the equilibration method (details in Meyer et al., 2000). δ18O values were given in per mil (‰) vs. Vienna standard mean ocean water (V-SMOW) as the standard.

Description: Second-year sea-ice thickness, draft, salinity, temperature, and density were measured during near-weekly surveys at the main second-year ice coring site (MCS-SYI) during the MOSAiC expedition (legs 1 to 3) and new second-year ice coring site leg 4, since the earlier site was not accessible any longer. The ice cores were extracted either with a 9-cm (Mark II) or 7.25-cm (Mark III) internal diameter ice corers (Kovacs Enterprise, US). This data set includes data from 18 coring site visits and were performed from 28 October 2019 to 20 July 2020 at coring locations within 50 m to each other in the MOSAiC Central Observatory. During each coring event, ice temperature was measured in situ from a separate temperature core, using Testo 720 thermometers in drill holes with a length of half-core-diameter at 5-cm vertical resolution. Ice bulk practical salinity was measured from melted core sections at 5-cm resolution using a YSI 30 conductivity meter. Ice density was measured using the hydrostatic weighing method (Pustogvar and Kulyakhtin, 2016) from a density core in the freezer laboratory onboard Polarstern at the temperature of –15°C. Relative volumes of brine and gas were estimated from ice salinity, temperature and density using Cox and Weeks (1983) for cold ice and Leppäranta and Manninen (1988) for ice warmer than –2°C. The data contains the event label (1), time (2), and global coordinates (3,4) of each coring measurement and sample IDs (13, 15). Each salinity core has its manually measured ice thickness (5), ice draft (6), core length (7), and mean snow height (22). Each core section has the total length of its top (8) and bottom (9) measured in situ, as well estimated depth of section top (10), bottom (11), and middle (12). The depth estimates assume that the total length of all core sections is equal to the measured ice thickness. Each core section has the value of its practical salinity (14), isotopic values (16, 17, 18) (Meyer et al., 2000), as well as sea ice temperature (19) and ice density (20) interpolated to the depth of salinity measurements. The global coordinates of coring sites were measured directly. When it was not possible, coordinates of the nearby temperature buoy 2019T62 (legs 1-3) or 2019T61 (leg 4) were used. Ice mass balance buoy 2019T62 installation is described in doi:10.1594/PANGAEA.940231, ice mass balance buoy 2020T61 installation is described in doi: 10.1594/PANGAEA.926580. Brine volume (21) fraction estimates are presented only for fraction values from 0 to 30%. Each core section also has comments (23) describing if the sample is from a new coring site or has any other special characteristics. Macronutrients from the salinity core will be published in a subsequent version of this data set.

Description: This data set contains the hydrographic profile data collected with a CTD rosette in a shelter on the ice (Ocean City) during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC). The CTD is an SBE911plus with 12 bottles, 5 liters each, operated with a small winch and crane in the shelter on the ice. The data set contains calibrated and quality-controlled parameters (temperature, conductivity, oxygen and their derived variables) as well as only pre-cruise calibrated parameters where no post-cruise calibration or quality control was applied (all other). CDOM fluorescence data are the exception. Quality control was performed but data have to be handled with care, as the sensor seems to have broken down during leg 3 such that no post-cruise calibration could be applied. The data are provided as text file (all cruise legs in one file) as well as in netCDF format (one file per cruise leg). The accuracy for salinity and conductivity is 0.004 while the accuracy for temperature is 0.002. Additional information on the sensor used for the final data set, the water depth as well as the availability of profile or bottle data is given in a separate info-text-file. Contact: Sandra.Tippenhauer@awi.de Quality flags are given based on paragraph 6. "Quality flags" from https://www.seadatanet.org/content/download/596/file/SeaDataNet_QC_procedures_V2_%28May_2010%29.pdf. QC flag meanings: 0 = unknown, 1 = good_data, 2 = probably good_data, 3 = probably bad data, 4 = bad data set to nan. This work was carried out and data was produced as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020. We thank all persons involved in the expedition of the Research Vessel Polarstern during MOSAiC in 2019-2020 (AWI_PS122_00) as listed in Nixdorf et al. (2021).

Description: This data set contains the hydrographic profile data collected with the ship based CTD rosette during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC). The CTD is an SBE911plus with 24 bottles, 12 liters each, operated with a winch and crane on the side of Polarstern. The data set contains calibrated and quality-controlled parameters (temperature, conductivity, oxygen and their derived variables) as well as only pre-cruise calibrated parameters where no post-cruise calibration or quality control was applied (all other). CDOM fluorescence data are the exception. Quality control was performed but data have to be handled with care, as the sensor seems to have broken down during leg 3 such that no post-cruise calibration could be applied. The data are provided as text file (all cruise legs in one file) as well as in netCDF format (one file per cruise leg). The accuracy for salinity and conductivity is 0.004 while the accuracy for temperature is 0.002. Additional information on the sensor used for the final data set, the water depth as well as the availability of profile or bottle data is given in a separate info-text-file. Contact: Sandra.Tippenhauer@awi.de. Quality flags are given based on paragraph 6. "Quality flags" from https://www.seadatanet.org/content/download/596/file/SeaDataNet_QC_procedures_V2_%28May_2010%29.pdf. QC flag meanings: 0 = unknown, 1 = good_data, 2 = probably good_data, 3 = probably bad data, 4 = bad data set to nan. This work was carried out and data was produced as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020. We thank all persons involved in the expedition of the Research Vessel Polarstern during MOSAiC in 2019-2020 (AWI_PS122_00) as listed in Nixdorf et al. (2021).

Description: This dataset gives an overview of the abundance of microorganisms (smaller than 20 µm) enumerated using flow cytometry (FCM) during the Multidisciplinary drifting observatory for the study of Arctic Climate (MOSAiC) sampled from ship-based and on-ice CTD rosettes during leg 1, 2, 3, 4 and 5 (November 2019 – September 2020). Additional expedition and sampling details can be found in the ECO-overview paper (Fong et al., to be submitted to Elementa). We thank all persons involved in the expedition of the Research Vessel Polarstern during MOSAiC in 2019-2020 (AWI_PS122_00) as listed in Nixdorf et al. (2021). Flow cytometry (FCM) is a fast, high-throughput method to enumerate the abundance of microorganism (smaller than 20 µm). FCM uses the hydrodynamic focusing of a laminar flow to separate and line up microscopic particles. When particles pass a laser beam, the generated light scattering can be used to estimate their cell size, obtain information about cell granularity and surface characteristics and determine fluorescence from inherent pigments or applied stains, such as DNA binding ones. Photosynthetic microorganisms have auto-fluorescent pigments, such as chlorophylls which in combination with the light scattering properties (cell size) or surface properties, can be used to group them into clusters of similar or identical organism types. Heterotrophic microorganisms, including archaea, bacteria and heterotrophic nanoflagellates, and virus do not have fluorescent pigments and require staining, for example using SYBR Green to stain Nucleic Acids (DNA/RNA) in order to distinguish these cells from other organic and inorganic particles in the sample. Samples for flow cytometric analysis were taken in triplicates or quadruplicates of 1.8 mL of sample water and fixed with 36 μL 25 % glutaraldehyde (0.5 % final concentration) at 4 °C in the dark for approximately 2 hours, then flash frozen in liquid nitrogen and stored at -80 °C until analysis. The abundance of pico- and nano-sized phytoplankton and heterotrophic nanoflagellates (HNF) were determined using an Attune® NxT, Acoustic Focusing Cytometer (Invitrogen by Thermo Fisher Scientific) with a 20 mW 488 nm (blue) laser. Autotrophic pico-and nano-sized plankton were counted directly after thawing and the various groups discriminated based on their red fluorescence (BL3) vs. orange fluorescence (BL2), red fluorescence (BL3) vs. side scatter (SSC) and orange fluorescence (BL2) vs. side scatter (SSC). For HNF analysis, the samples were stained with SYBR Green I for 2 h in the dark and 1-2 mL were subsequently measured at a flow rate of 500 µl min-1 following the protocol of Zubkov et al. 2007. The abundance of virus and bacteria was determined using a FACS Calibur (Becton Dickinson) flow cytometer with a 15 mW 480 nm (blue) laser. Prior analysis of virus and bacteria, samples were first thawed, diluted x10 and x100 with 0.2 μm filtered TE buffer (Tris 10 mM, EDTA 1 mM, pH 8), stained with a green fluorescent nucleic acid dye (SYBR Green I ; Molecular Probes, Eugene, Oregon, USA) and then incubated for 10 min at 80°C in a water bath (Marie et al. 1999). Stained samples were counted at a flow rate of around 60 µL min-1 and different groups discriminated on a biparametric plot of green florescence (BL1) vs. side scatter (SSC). This allowed to distinguish virus particles of different sizes, and different bacterial groups including low nuclear acid (LNA) and high nuclear acid (HNA) bacteria. Names of size groups of photosynthetic and heterotrophic organisms are in accordance to "Standards and Best Practices For Reporting Flow Cytometry Observations: a technical manual (Version 1.1)" (Neeley et al., 2023). A short summary is listed here: RedPico = picophytoplankton (1-2 µm); RedNano = Nanophytoplankton (2-20µm), which includes subgroups RedNano_small (2-5 µm), RedNano_large (5-20 µm); OraPico = Nanophytoplankton with more orange fluorescence; OraNano = Cryptophytes; OraPicoProk = Synechococcus; HetNano = heterotrophic nanoflagellates; HetProk = bacteria (and when present archaea); HetLNA = low nucleic acid (LNA) containing bacteria; HetHNA = high nucleic acid (HNA) containing bacteria with the subgroups HetProk_medium = HNA-bacteria subgroup with less fluorescence signal, HetProk_large = HNA-bacteria subgroup with more fluorescence signal and HetProk_verylarge = HNA-bacteria subgroup with very strong fluorescence signal; Virus = virus-like particles, including size refined subgroups: LFV (low fluorescence virus or small virus); MFV (medium fluorescence virus or medium virus); HFV (high fluorescence virus or large virus) according to Larsen et al., 2008. Exemplary plots showing the gating strategies that were followed can be found in "Interoperable vocabulary for marine microbial flow cytometry" (Thyssen et al., 2022).