Sea ice cores photos from Fort Ridge during MOSAiC 2019/20

Photos of ice cores are provided with file names including event label, core label, section number, and photo number. They include 35 photos of two cores from the PS122-2_19-116 event and 28 photos of a core from the PS122-2_21-124 event

Ice salinity measurements of Fort Ridge during MOSAiC 2019/20

Salinity data from sea ice samples taken during MOSAiC leg 2 (PS122/2). Sea ice samples were taken from Fort Ridge and surrounding undeformed first-year ice (FYI). Sea ice cores were extracted using a 9 cm inner diameter ice corer (Kovacs Enterprise Mark II coring system) powered by a cordless electric drill. Salinity ice cores were stored in sterile U-Lines bags and transported back to the ship within 2 hours of sampling. Ice cores were stored in a -20°C walk-in freezer until processing. The ice cores were cut into sections of various lengths using a stainless steel, electric butcher band saw. The samples were placed in plastic cups with lids and melted at room temperature. Bulk salinity was measured on the melted water sample using a calibrated YSI 30 conductivity meter (practical salinity scale, unitless). The table contains the event label (1), time (2), and global coordinates (3,4) of each coring measurement. Each separate core has its local coordinates X (5) in [m] across the corresponding ridge transect (6). For each drill hole, the depth relative to the waterline of the top (7) and bottom (8) interface of each separate layer is given together with its salinity (9), isotope composition ID (10), and nutrient ID (11). The ice type (11) includes first-year ice (FYI), ridge, and ridge (water-filled) void. Each core has also measurements of the total ice thickness (12), ice draft (13), snow depth (14), and additional information on sampling (16). Photos of ice cores are additionally provided: doi:10.1594/PANGAEA.962537. Ice mass balance buoy SIMBA 2020T60 was installed at the Fort Ridge: doi:10.1594/PANGAEA.924269. Ridge thickness drilling was performed at Fort Ridge: doi:10.1594/PANGAEA.960347

Protist (including algae) abundance and biodiversity data collected with short-term sediment traps deployed below level and ridged ice during MOSAiC legs 2 to 4 in 2019/2020

The data has been collected during the year-long drift expedition "Multidisciplinary drifting Observatory for the Study of Arctic Climate" (MOSAiC) from September 2019 to September 2020 on research vessel Polarstern. The dataset contains abundance of pelagic marine and sea ice protists, including algae (autotrophic) and protzoa (heterotrophic). Protists were identified and counted with light microscopy using the Utermöhl method and the result are given as cells per liter (cells/L) called Abundance. The samples were collected with short-term sediment traps deployed at 3-4 depths (1, 5, 15 and 50 m) below level ice and near sea-ice ridges. The samples were preserved with a few drops of Lugol and hexamethylenetetramine-buffered formalin at a final concentrations of 1%

Pelagic protist (including phytoplankton) abundance and biodiversity data collected from the water column and water filled voids inside ridges during MOSAiC legs 2 to 4 in 2019/2020

The data has been collected during the the year-long drift expedition "Multidisciplinary drifting Observatory for the Study of Arctic Climate" (MOSAiC) from September 2019 to September 2020 on research vessel Polarstern. The samples were collected with Niskin bottles attached to a CTD rosette, an Apstein net with 20 µm mesh size, a hand pump or a pump mounted on a ROV. The samples were preserved using a few drops of Lugol and hexamethylenetetramine-buffered formalin at a final concentration of 1%. The samples were collected with Niskin bottles attached to a CTD rosette at the following depths: 5, 10, 30, 60, 90 m and deep chlorophyll max (DCM). Protists were identified and counted with light microscopy using the Utermöhl method and the result are given as cells per liter (cells/L) called Abundance

Flow cytometry dataset from first year sea ice (FYI) core bottom 5 cm sections showing the abundance of microorganisms (< 20 µm) during leg 2, 3 (February, March and April 2020) of the Arctic MOSAiC expedition

This dataset is a subset 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 first year sea ice (FYI) core bottom 5 cm sections from leg 2 and 3 (February, March, April 2020). For sea ice derived FCM abundance data, subsamples of 15 mL were taken from pooled ice core sections that were melted in filtered sea water and correspondingly a correction factor applied (details provided in the data-file), to enumerate the abundance of microorganisms per mL of melted sea ice. 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) 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. Following the Zubkov protocol, HNF are enumerated using a fixed gate and in case of sea ice samples, there is an overlap between HNA-bacteria with very high fluorescence and HNF, which is not possible to disentangle with current methodology. 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)

Sea-ice protist (including ice algae) abundance and biodiversity data from ice coring at the main coring sites (MCS_FYI and MCS_SYI) and ridges during MOSAiC legs 2 to 4 in 2019/2020

The data has been collected during the expedition "Multidisciplinary drifting Observatory for the Study of Arctic Climate" (MOSAiC) from September 2019 to September 2020 on research vessel Polarstern. The dataset contains abundance of sea ice protists, including ice algae (autotrophic) and protozoa (heterotrophic). Protists were identified and counted with light microscopy using the Utermöhl method and the result are given as cells per liter (cells/L) called Abundance. Sea ice samples were collected with a 9 cm diameter ice corer (Kovacs Enterprise) from both level and ridge ice. The samples were collected from the bottom part of the ice core and generally sectioned from 0-3 cm, 3-10 cm and in 10 cm intervals thereafter. With some exceptions, ice core sections were melted in filtered sea water at 4°C. Melted samples were preserved using Lugol-formaldehyde mixture with a few drops of acidic Lugol solution and hexamethylenetetramine-buffered formalin at a final concentrations of 1%

Flow cytometry dataset from CTD casts showing the abundance of microorganisms (smaller than 20 µm) during the Arctic MOSAiC expedition

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

Second-year sea-ice salinity, temperature, density, oxygen and hydrogen isotope composition from the main coring site (MCS-SYI) during MOSAiC legs 1 to 4 in 2019/2020

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

First-year sea-ice salinity, temperature, density, oxygen and hydrogen isotope composition from the main coring site (MCS-FYI) during MOSAiC legs 1 to 4 in 2019/2020

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

Physical oceanography based on ship CTD during POLARSTERN cruise PS122

This dataset 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 dataset contains calibrated and quality-controlled parameters (temperature, conductivity, oxygen and their derived variables) as well as uncalibrated parameters where no quality control was applied (all other). Especially CDOM fluorescence has to be handled with care, as the sensor seems to have broken down during leg 3 (Contact/PI: for CDOM, PAR, SPAR: [email protected]). 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 The data is provided as text file (all cruise legs in one file) as well as in netCDF format (one file per cruise leg). 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)