Climate change and permafrost thaw are unlocking the vast storage of organic carbon held in northern frozen soils. Here, we evaluated the effects of thawing ice‐rich permafrost on dissolved organic matter (DOM) in freshwaters by optical analysis of 253 ponds across the circumpolar North. For a subset of waters in subarctic Quebec, we also quantified the contribution of terrestrial sources to the DOM pool by stable isotopes. The optical measurements showed a higher proportion of terrestrial carbon and a lower algal contribution to DOM in waters affected by thawing permafrost. DOM composition was largely dominated (mean of 93%) by terrestrial substances at sites influenced by thawing permafrost, while the terrestrial influence was much less in waterbodies located on bedrock (36%) or with tundra soils unaffected by thermokarst processes (42%) in the catchment. Our results demonstrate a strong terrestrial imprint on freshwater ecosystems in degrading ice‐rich permafrost catchments, and the likely shift toward increasing dominance of land‐derived organic carbon in waters with ongoing permafrost thaw.

Warming climate is thawing the permafrost in arctic and subarctic regions, leading to formation of thermokarst ponds. During the formation and geomorphological succession of these ponds, carbon that has been trapped in frozen soils for thousands of years is hydrologically mobilized and returned to the active carbon cycle. We sampled 12 thermokarst ponds representing three different stages of pond succession to study the potential of microbial communities to metabolize the organic carbon in the water. We investigated the quality of the dissolved organic carbon (DOC) in the water column based on the spectrophotometric and fluorometric properties of the chromophoric dissolved organic matter combined with parallel factor analysis and the potential of the microbial community for degrading these carbon compounds based on genetic markers related to carbon degradation. Our analysis showed a clear difference in the DOC quality across the different developmental stages. In the younger ponds, organic matter quality suggested that it was originating from the degrading permafrost and in the metagenomes collected from these ponds, the normalized abundance of genes related to degradation of carbon compounds was higher. There was also a shift in the degradation potential in the water column of the ponds, with higher potential for organic matter degradation in deeper, anoxic layers. In conclusion, our results show that the DOC quality and the genetic potential of the microbial community for carbon cycling change across the pond ontogeny, suggesting a capacity of the microbial communities to adapt to changing environmental conditions. Permafrost soils contain one of the major carbon storages on Earth, with an estimated 950-1672 Gt of carbon (Zimov et al.

Frozen tundra soils hold one of the Earth's largest pools of organic carbon. Climate warming and the associated permafrost thaw release a large fraction of this carbon into circumpolar lakes, inducing extreme browning that fuels the heterotrophic microbial food web. How this permafrost carbon affects organisms higher in the food chain remains unknown. Using dissolved organic matter properties, total phosphorus, chlorophyll a, fatty acids, and stable isotopes, we investigated the influence of thawing permafrost on primary producers and primary consumers of the planktonic food web. We sampled four subarctic thaw ponds that were affected by permafrost carbon and another four ponds that were not. Our results highlight the stimulating influence of eroding and degrading ice‐rich permafrost on nutrients and planktonic algae. Relative to the non‐thaw ponds, the permafrost thaw‐influenced freshwaters had higher total phosphorus concentrations (14.8 vs. 70.4 µg/L, respectively). This in turn led to a higher chlorophyll a (2.7 vs. 45.2 µg/L) and seston omega‐3 fatty acid concentrations (7.3 vs. 53.5 µg/L) despite significantly reduced light for primary production. Differences between the thaw and non‐thaw ponds were less marked at the primary consumer level. Daphnia pulex, which dominated the crustacean zooplankton community, did not respond to the higher omega‐3 availability in the thaw ponds but rather assimilated the high‐quality fatty acids equally in all ponds, possibly because their metabolic needs were already saturated. However, some lower quality terrestrial carbon compounds from permafrost ended up in the D. pulex body mass, resulting in a median allochthony of 18% based on fatty acid mixing model; non‐thaw ponds had median allochthony mixing model estimates of 8%. The high availability of algal resources seemed to prevent extensive zooplankton allochthony in subarctic thaw ponds.

Over the last few decades, global warming has led to widespread shrinking of the cryosphere and has brought a sense of urgency toward the study of ice habitats. One of the most heavily impacted cryospheric habitats is freshwater ice. Climate change is leading to substantial reductions in the thickness and duration of lake and river ice cover worldwide (Sharma et al., 2019;Wrona et al., 2016). In the northern hemisphere, ice in lakes and rivers freezes an average of 5.8 days later and melts 6.5 days earlier than 100 years ago, based on the measurements taken between the years 1846and 1995(Magnuson, et al., 2000. Global air temperatures increased 1.2°C during this period. The majority of boreal and Arctic lakes are still ice-covered for six to ten months a year, and the potential importance of this cold and dark period on ecosystem structure and function is increasingly recognized (Grosbois & Rautio, 2018;Hampton et al., 2017;Schneider et al., 2017). Considering that ice-covered lakes include nearly 50% of the world's lakes, there is a pressing Abstract Around 50% of the world's lakes freeze seasonally, but the duration of ice-cover is shortening each year and this is likely to have broad limnological consequences. We sampled freshwater ice and the underlying water in 19 boreal and polar lakes to evaluate whether lake ice contains an inoculum of algae, nutrients, and carbon that may contribute to lake ecosystem productivity. Boreal and Arctic lakes differed in ice duration (6 vs. >10 months), thickness (70 vs. 190 cm), and quality (predominantly snow ice vs. black ice), but in all lakes, there were consistent differences in biological and biogeochemical composition between ice and water. Particulate fractions were often more retained while most dissolved compounds were excluded from the ice; for example, the ice had more terrestrial particulate carbon, measured as fatty acid biomarkers (averages of 1.1 vs. 0.3 µg L −1 ) but lower dissolved organic carbon (2.2 vs. 5.7 mg C L −1 ) and inorganic phosphorus concentrations (4.0 vs. 7.5 µg C L −1 ) than the underlying water. The boreal ice further had three times higher chlorophyll-a, than the water (0.9 vs. 0.3 µg L −1 ). Of the dissolved fractions, the contribution of protein-like compounds was higher in the ice, and this in all lakes. These labile compounds would become available to planktonic microbes when the ice melts. Our results show that freshwater ice has an underestimated role in storage and transformation in the biogeochemical carbon cycle of ice-covered lake ecosystems.Plain Language Summary Winter ice cover of 1-2 m thickness can comprise 20%-70% of the total lake depth in boreal and Arctic lakes. While sea ice is known to contain substantial quantities of carbon and organisms that at ice melt contribute to biological production in the underlying water column, little is known about the composition of lake ice and its role in storage of organic carbon. Our analyses of 19 boreal and Arctic lakes revealed large but different stores of organic material in lake ...

Thermokarst activity at permafrost sites releases considerable amounts of ancient carbon to the atmosphere. A large part of this carbon is released via thermokarst ponds, and fungi could be an important organismal group enabling its recycling. However, our knowledge about aquatic fungi in thermokarstic systems is extremely limited. In this study, we collected samples from five permafrost sites distributed across circumpolar Arctic and representing different stages of permafrost integrity. Surface water samples were taken from the ponds and, additionally, for most of the ponds also the detritus and sediment samples were taken. All the samples were extracted for total DNA, which was then amplified for the fungal ITS2 region of the ribosomal genes. These amplicons were sequenced using PacBio technology. Water samples were also collected to analyze the chemical conditions in the ponds, including nutrient status and the quality and quantity of dissolved organic carbon. This dataset gives a unique overview of the impact of the thawing permafrost on fungal communities and their potential role on carbon recycling.

Small lentic freshwater ecosystems play a disproportionate role in global biogeochemical cycles by processing large amounts of carbon (C), nitrogen (N), and phosphorus (P), but it is unlikely that they behave as one homogenous group for the purpose of extrapolation. Here, we synthesize biogeochemical data from >12,000 geographically distinct freshwater systems: lakes, peatland ponds, and thermokarst waterbodies. We show that peatland ponds are biogeochemically distinct from the more widely studied lake systems, while thermokarst waterbodies share characteristics with peatland ponds, lakes, or both. For any given size or depth, peatland ponds tend to have dissolved organic carbon concentrations several‐fold higher and are 100‐fold more acidic than lakes because of the organic matter‐rich settings in which they develop. The biogeochemical distinctiveness of freshwater ecosystems highlights the need to account for the fundamental differences in sources and processing of organic matter to understand and predict their role in global biogeochemical cycles.

Ice cover persists throughout summer over many lakes at extreme polar latitudes but is likely to become increasingly rare with ongoing climate change. Here we addressed the question of how summer ice-cover affects the underlying water column of Ward Hunt Lake, a freshwater lake in the Canadian High Arctic, with attention to its vertical gradients in limnological properties that would be disrupted by ice loss. Profiling in the deepest part of the lake under thick mid-summer ice revealed a high degree of vertical structure, with gradients in temperature, conductivity and dissolved gases. Dissolved oxygen, nitrous oxide, carbon dioxide and methane rose with depth to concentrations well above air-equilibrium, with oxygen values at > 150% saturation in a mid-water column layer of potential convective mixing. Fatty acid signatures of the seston also varied with depth. Benthic microbial mats were the dominant phototrophs, growing under a dim green light regime controlled by the ice cover, water itself and weakly colored dissolved organic matter that was mostly autochthonous in origin. In this and other polar lakes, future loss of mid-summer ice will completely change many water column properties and benthic light conditions, resulting in a markedly different ecosystem regime.

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