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During the RV Polarstern expedition PS94, we gathered sediment samples in the Barents Sea and the central Arctic Ocean by deploying both a multiple corer (MUC) and a giant boxcorer (GKG). After retrieval of the MUC or GKG, replicate sediment cores with a visibly intact sediment surface were chosen for further laboratory analysis. The selected cores were brought to the laboratory on board RV Polarstern for further analysis. While data on chlorophyll pigments, total organic carbon, microbial cell numbers and diffusive oxygen uptake rates were taken from the same cores, total oxygen uptake rates were measured in three additional cores.

Low-salinity stress can severely affect the fitness of marine organisms. As desalination has been predicted for many coastal areas with ongoing climate change, it is crucial to gain more insight in mechanisms that constrain salinity acclimation ability. Low-salinity induced depletion of the organic osmolyte pool has been suggested to set a critical boundary in osmoconforming marine invertebrates. Whether inorganic ions also play a persistent role during low-salinity acclimation processes is currently inconclusive. We investigated the salinity tolerance of six marine invertebrate species following a four-week acclimation period around their low-salinity tolerance threshold. The species investigated were Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris. To obtain complete osmolyte budgets of seawater, body fluids and tissues we quantified total osmolality (via osmometer), organic osmolytes (methylamine and free amino acids) via 1H-NMR spectroscopy and inorganic osmolytes (anions and cations) via flame photometry and a novel protocol using ion-chromatography. We further determined the fitness proxies survival, growth and tissue water content. Our data show the importance of the organic and inorganic osmolyte pool during low-salinity acclimation. It also shows the importance of specific compounds in some species. This data can be used in future osmolyte and salinity tolerance research. This type of data is essential to establish reliable physiological limits of species in order to estimate consequences of future salinity changes with ongoing climate change. It can be used to assess the salinity tolerance capacity and to obtain a better understanding of the basic mechanisms that are utilized in a wide range of species. The established cellular inorganic and organic osmolyte profiles can build a foundation for applied cellular physiological research, for example for designing suitable buffers for in vitro assays as these buffers need to incorporate complex organic and inorganic osmolyte changes. Knowledge about cellular and whole-organism biochemistry and physiology is absolutely crucial for characterizing the functions of genes that are under selection by climate change stressors. A quantitative knowledge of cellular osmolyte systems is key to understand the evolution of euryhalinity and to characterize targets of selection during rapid adaptation to ongoing desalination.

Solar radiation over and under sea ice was measured by radiation station 2020R21, an autonomous platform, installed on a drifting melt pond in the Arctic Ocean during MOSAiC (Leg 5) 2019/20. The resulting time series describes radiation measurements as a function of place and time between 27 August 2020 and 14 November 2020 in sample intervals of 3 hours. The radiation measurements have been performed with spectral radiometers. All data are given in full spectral resolution interpolated to 1.0 nm, and integrated over the entire wavelength range (broadband, total: 320 to 950 nm). Two sensors, solar irradiance and upward reflected solar irradiance, were mounted on a on a platform about 1 m above the sea ice surface. The third sensor was mounted 0.5 m underneath the sea ice measuring the downward transmitted irradiance. Along with the radiation measurements, this autonomous platform consisted of a lightchain, which measured counts of red, green and blue light at 49 positions at hourly intervals. In addition, the evolution of snow height is measured at hourly intervals. All times are given in UTC.

As the atmospheric CO2 concentration rises, more CO2 will dissolve in the oceans, leading to a reduction in pH. Effects of ocean acidification on bacterial communities have mainly been studied in biologically complex systems, in which indirect effects, mediated through food web interactions, come into play. These approaches come close to nature but suffer from low replication and neglect seasonality. To comprehensively investigate direct pH effects, we conducted highly-replicated laboratory acidification experiments with the natural bacterial community from Helgoland Roads (North Sea). Seasonal variability was accounted for by repeating the experiment four times (spring, summer, autumn, winter). Three dilution approaches were used to select for different ecological strategies, i.e. fast-growing or low-nutrient adapted bacteria. The pH levels investigated were in situ seawater pH (8.15-8.22), pH 7.82 and pH 7.67, representing the present-day situation and two acidification scenarios projected for the North Sea for the year 2100. In all seasons, both automated ribosomal intergenic spacer analysis and 16S ribosomal amplicon pyrosequencing revealed pH-dependent community shifts for two of the dilution approaches. Bacteria susceptible to changes in pH were different members of Gammaproteobacteria, Flavobacteriaceae, Rhodobacteraceae, Campylobacteraceae and further less abundant groups. Their specific response to reduced pH was often context-dependent. Bacterial abundance was not influenced by pH. Our findings suggest that already moderate changes in pH have the potential to cause compositional shifts, depending on the community assembly and environmental factors. By identifying pH-susceptible groups, this study provides insights for more directed, in-depth community analyses in large-scale and long-term experiments.

This dataset contains energy content measurements performed on zooplankton collected in the Arctic Ocean during the MOSAiC expedition (PS122) from November 2019 untill September 2020. Energy content measurements were done on Apherusa glacialis, Themisto abyssorum, Chaetognatha, Thysanoessa longicaudata and Calanus hyperboreus. These species are all known prey of polar cod (Boreogadus saida), and their energy content was measured to be included in a bioenergetic model of the growth rate of this predator and to gain insight in the differences between prey species. The meaurements were performed on freeze-dried specimens using a 6725 semi-micro oxygen calorimeter (Parr, USA) connected to a 6772 calorimetric thermometer (Parr, USA).

Here we quantify the abundance and distribution of three primary permafrost region disturbances (PRD; lakes and their dynamics, wildfires, retrogressive thaw slumps) using trend analysis of 30-m resolution Landsat imagery from 1999-2014 and auxiliary datasets. The dataset spans four continental-scale transects in North America (Alaska, Eastern Canada) and Eurasia (Western Siberia, Eastern Siberia), covering 2.3M km² or ~10% of the permafrost region. This data publication contains geospatial vector files (polygons) of the perimeters of PRD. The data are subdivided by PRD type (lakes, wildfire, retrogressive thaw slumps) and further subdivided by study region (T1_WS, T2_ES, T3_AK, T4_EC). T1_WS: Western SIberia T2_ES: Eastern Siberia T3_AK: Alaska T4_EC: Eastern Canada The datasets are documented in detail in the linked document (Nitze_etal_2018: Data Documentation v1.0).