Temperature, salinity, and density for 148 sea ice cores drilled dring the N-ICE2015 cruise. Ice cores were retrieved from a variety of ice types at different locations on each of four floes.

Sensible & latent heat fluxes, momentum flux, roughness length and boundary layer height from eddy-covariance system deployed on the ice floe, nearby surface meteorology and radiation observations.

Zooplankton taxonomy & abundance data sampled by Multinet and WP2 (200 micron & 64 micron mesh) and swim net (200 micron). The data is presented as abundance (ind/m3) for all identified mesozooplankton taxa.

We performed measurements of carbon dioxide fugacity (fCO2) in the surface water under 8 Arctic sea ice from January to June 2015 during the Norwegian young sea ICE (N-ICE2015) expedition. Over 9 this period, the ship drifted with four different ice floes and covered the deep Nansen Basin, the slopes 10 north of Svalbard, and the Yermak Plateau. This unique winter-to-spring data set includes the first 11 winter-time under-ice water fCO2 observations in this region. The observed under-ice fCO2 ranged between 12 315 matm in winter and 153 matm in spring, hence was undersaturated relative to the atmospheric fCO2. 13 Although the sea ice partly prevented direct CO2 exchange between ocean and atmosphere, frequently 14 occurring leads and breakup of the ice sheet promoted sea-air CO2 fluxes. The CO2 sink varied between 0.3 15 and 86 mmol C m22 d21, depending strongly on the open-water fractions (OW) and storm events. The 16 maximum sea-air CO2 fluxes occurred during storm events in February and June. In winter, the main drivers 17 of the change in under-ice water fCO2 were dissolution of CaCO3 (ikaite) and vertical mixing. In June, in 18 addition to these processes, primary production and sea-air CO2 fluxes were important. The cumulative loss 19 due to CaCO3 dissolution of 0.7 mol C m22 in the upper 10 m played a major role in sustaining the 20 undersaturation of fCO2 during the entire study. The relative effects of the total fCO2 change due to CaCO3 21 dissolution was 38%, primary production 26%, vertical mixing 16%, sea-air CO2 fluxes 16%, and temperature 22 and salinity insignificant.

Sodar; Horizontal wind speed & direction, vertical wind speed, and standard deviation of vertical velocity

The phytoplankton and ice-algae dataset contains counts of algae performed in an Imaging Flow Cytobot (IFCB) and in an inverted microscope. The counts were performed after the cruise using fixed samples collected from the sea ice and the water column.

On the four N-ICE2015 floes we installed in total seven hot-wire fields and seven snow-stake fields following the routine outlined in Perovich [2003]. A rectangular hot-wire field with a side length of approximately 10 m was designed in a way that in each corner a wire was installed close to an ablation stake, and in the middle of the hot-wire field nine snow-stakes with even spacing were set up. Snow depth and ice thickness changes were recorded on a regular basis, and the readings were averaged in space to cover small scale spatial variability. For the hot wire readings see https://data.npolar.no/dataset/263a317f-5a65-4776-8f53-ef2c2857fc33

On the four N-ICE2015 floes we installed in total seven hot-wire fields and seven snow-stake fields following the routine outlined in Perovich [2003]. A rectangular hot-wire field with a side length of approximately 10 m was designed in a way that in each corner a wire was installed close to an ablation stake, and in the middle of the hot-wire field nine snow-stakes with even spacing were set up. Snow depth and ice thickness changes were recorded on a regular basis, and the readings were averaged in space to cover small scale spatial variability. For the snow stake readings see https://data.npolar.no/dataset/3099ea95-c3cd-4a8b-af5d-73750e46d791.

Sea ice, snow depth, freeboard, and total thickness (ice + snow) measurements from 2" drillholes or ice core holes during the N-ICE2015 expedition. EM31 conductivity value for EM31 calibration are included where available.

Particle absorption coefficients (m-1) in seawater and sea ice. Samples were filtered onto 0.7 µm glassfiber filters and stored in -80 °C until analysis. Filters were measured between 240 and 800 nm with a Shimadzu UV-2450 dual-beam spectrophotometer with a Shimadzu ISR-2200 integrating sphere. Measurement was made before and after bleaching of pigments with 400 µl of sodium hypochlorite (NaClO) solution with 1 % active chlorine for 10 minutes. Additional 20 minutes was used if the first bleaching was not complete. Filters were rinsed with artificial sea water (60 g Na2SO4 in 1 L of ultrapure water). As wet reference an average of 20 filters (filtered with ultrapure water throughout the campaign) was used. A baseline correction was done relative to the mean absorption values at 750–800 nm. Method used follows the method by Tassan and Ferrari 2002 (https://doi.org/10.1093/plankt/24.8.757). More sampling details in Kauko et al. 2017 (https://doi.org/10.1002/2016JG003626), Pavlov et al. 2017 (https://doi.org/10.1002/2016JC012471) and Kowalczuk et al. 2017 (https://doi.org/10.1002/2016JC012589). Separate files for water column samples from ship CTD and on-ice CTD (mini-rosette), and for sea ice samples from different environments (thick second-year/first-year ice and young ice in a refrozen lead).