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Climate warming and related drivers of soil thermal change in the Arctic are expected to modify the distribution and dynamics of carbon contained in perennially frozen grounds. Thawing of permafrost in the Mackenzie River watershed of northwestern Canada, coupled with increases in river discharge and coastal erosion, triggers the release of terrestrial organic matter (OMt) from the largest Arctic drainage basin in North America into the Arctic Ocean. While this process is ongoing and its rate is accelerating, the fate of the newly mobilized organic matter as it transits from the watershed through the delta and into the marine system remains poorly understood. In the framework of the European Horizon 2020 Nunataryuk programme, and as part of the Work Package 4 (WP4) Coastal Waters theme, four field expeditions were conducted in the Mackenzie Delta region and southern Beaufort Sea from April to September 2019. The temporal sampling design allowed the survey of ambient conditions in the coastal waters under full ice cover prior to the spring freshet, during ice breakup in summer, and anterior to the freeze-up period in fall. To capture the fluvial–marine transition zone, and with distinct challenges related to shallow waters and changing seasonal and meteorological conditions, the field sampling was conducted in close partnership with members of the communities of Aklavik, Inuvik and Tuktoyaktuk, using several platforms, namely helicopters, snowmobiles, and small boats. Water column profiles of physical and optical variables were measured in situ, while surface water, groundwater, and sediment samples were collected and preserved for the determination of the composition and sources of OMt, including particulate and dissolved organic carbon (POC and DOC), and colored dissolved organic matter (CDOM), as well as a suite of physical, chemical, and biological variables. Here we present an overview of the standardized datasets, including hydrographic profiles, remote sensing reflectance, temperature and salinity, particle absorption, nutrients, dissolved organic carbon, particulate organic carbon, particulate organic nitrogen, CDOM absorption, fluorescent dissolved organic matter intensity, suspended particulate matter, total particulate carbon, total particulate nitrogen, stable water isotopes, radon in water, bacterial abundance, and a string of phytoplankton pigments including total chlorophyll. Datasets and related metadata can be found in (10.1594/PANGAEA.937587). [ABSTRACT FROM AUTHOR]

Climate warming and related drivers of soil thermal change in the Arctic are expected to modify the distribution and dynamics of carbon contained in perennially frozen grounds. Thawing of permafrost in the Mackenzie Delta region of northwestern Canada, coupled with increases in river discharge and coastal erosion, trigger the release of terrestrial organic matter (OMt) from the largest Arctic drainage basin in North America into the Arctic Ocean. While this process is ongoing, well-established, and its rate is accelerating, the fate of the newly-mobilized organic matter, as it transits from the watershed through the delta and into the marine system, remains poorly understood. In the framework of the European Horizon 2020 Nunataryuk programme, and as part of theWork Package 4 (WP4) CoastalWaters theme, four field expeditions were conducted in the Mackenzie Delta region and southern Beaufort Sea from April to September 2019. The temporal sampling design allowed the survey of ambient conditions in the coastal waters under full ice cover prior to the spring freshet, during ice break-up in summer, as well as anterior to the freeze-up period in fall. To capture the fluvial-marine transition zone, and with distinct challenges related to shallow waters and changing seasonal and meteorological conditions, the field sampling was conducted in close partnership with members of the communities of Aklavik, Inuvik and Tuktoyaktuk, using several platforms: helicopters, snowmobiles and small boats. Water column profiles of physical and optical variables were measured in situ, while surface water, groundwater and sediment samples were collected and preserved for the determination of the composition and sources of OMt, including particulate and dissolved organic carbon (POC, DOC), and chromophoric dissolved organic matter (CDOM), as well as a suite of physical, chemical and biological variables. Here we present an overview of the standardized datasets, including hydrographic profiles, remote sensing reflectance, temperature and salinity, particle absorption, nutrients, dissolved organic carbon, particulate organic carbon, particulate organic nitrogen, colored dissolved organic matter absorption, fluorescent dissolved organic matter intensity, suspended particulate matter, total particulate carbon, total particulate nitrogen, stable water isotopes, radon in water, bacterial abundance, and a string of phytoplankton pigments including total chlorophyll. Datasets and related metadata can be found in Juhls et al. 2021. https://doi.pangaea.de/10.1594/PANGAEA.937587. [ABSTRACT FROM AUTHOR]

Permafrost thawing mobilizes large quantities of organic carbon that was sequestered in Arctic regions over the last glacial cycle. Processes involved in the oxidation of this carbon need to be further assessed to estimate the fraction to be released into the atmosphere. Shallow tundra ponds are sites of active carbon turnover on the landscape and significant sources of greenhouse gases. Dissolved organic matter (DOM) leached from thawing peat into these ponds is exposed to sunlight, with the potential to accelerate its mineralization directly into CO2 or through the production of more labile molecules. We tested the catalytic effect of sunlight on DOM mineralization in tundra ponds formed on organic‐rich polygonal landscapes originating from syngenetic permafrost, including a pond exposed to active permafrost erosion. Microbial decay rates, measured as the loss of chromophoric DOM, were similar to photodecay rates (1%–3% d−1). Groups of fluorescing molecules were formed through microbial transformation or lost through photolysis at differing rates among studied ponds, with the erosive trough pond presenting a unique response suggesting the involvement of soil microbes. Despite the stimulation of microbial growth under sunlight and the dynamic response of DOM optical properties, the loss of dissolved organic carbon was not significant under any treatment. This suggests that microbial and photochemical mineralization of DOM was slow and potentially substrate‐limited during the dry period when ponds were sampled. The static nature of tundra ponds, with their long water retention time, may thus constrain hot moments when water moves and transports carbon on the landscape. [ABSTRACT FROM AUTHOR]

Sea ice plays a fundamental role in Arctic marine environments, by driving primary productivity and sustaining ice-associated ecosystems. Simultaneously, sea ice influences the contamination of Arctic marine organisms, by modifying contaminant cycles or their bioavailability. Changes in sea ice conditions could therefore profoundly impact the functioning of Arctic marine food webs and their contamination. Top predators such as seabirds, which are subject to bioaccumulation and biomagnification of contaminants, are particularly exposed. In this context, the present study aims to investigate the influence of sea ice and of the use of ice-derived resources on the contamination of seabirds by mercury (Hg). To this end, eggs of thick-billed murres (Brünnich's guillemots, Uria lomvia ; n = 60) were collected on Prince Leopold Island (Canadian High Arctic) during four years of varying ice conditions (2010–2013). Trophic tracers (i.e., Highly Branched Isoprenoids, HBIs – an indicator of the use of ice-derived resources; carbon and nitrogen stable isotopes – indicators of foraging habitats and trophic status), as well as total Hg concentrations were quantified. Results showed that feeding on ice-derived resources (as indicated by HBI concentrations) was positively correlated to sea ice cover, and both positively influenced Hg concentrations in murre eggs. However, when testing for the best predictor with model selection, sea ice concentration only drove Hg contamination in murres. This work provides new insights into the role of sea ice and ice-derived resources in the contamination by Hg of Arctic wildlife. Further research is now needed to better understand the relationship between sea ice and Hg contamination in Arctic biota and its underlying mechanisms, but also to identify Hg sources in rapidly changing environmental conditions in the Arctic. [Display omitted] • Trophic biomarkers and total Hg concentrations were measured in thick-billed murres. • Egg Hg concentrations were higher during icier years. • Hg concentrations were higher when murres relied heavily on ice-derived resources. • Ice concentrations were the best predictor for egg Hg contamination. [ABSTRACT FROM AUTHOR]

In the Arctic, sea-ice plays a central role in the functioning of marine food webs and its rapid shrinking has large effects on the biota. It is thus crucial to assess the importance of sea-ice and ice-derived resources to Arctic marine species. Here, we used a multi-biomarker approach combining Highly Branched Isoprenoids (HBIs) with δ 13 C and δ 15 N to evaluate how much Arctic seabirds rely on sea-ice derived resources during the pre-laying period, and if changes in sea-ice extent and duration affect their investment in reproduction. Eggs of thick-billed murres (Uria lomvia) and northern fulmars (Fulmarus glacialis) were collected in the Canadian Arctic during four years of highly contrasting ice conditions, and analysed for HBIs, isotopic (carbon and nitrogen) and energetic composition. Murres heavily relied on ice-associated prey, and sea-ice was beneficial for this species which produced larger and more energy-dense eggs during icier years. In contrast, fulmars did not exhibit any clear association with sympagic communities and were not impacted by changes in sea ice. Murres, like other species more constrained in their response to sea-ice variations, therefore appear more sensitive to changes and may become the losers of future climate shifts in the Arctic, unlike more resilient species such as fulmars.