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In formerly glaciated permafrost regions, extensive areas are still underlain by a considerable amount of glacier ice buried by glacigenic sediments. It is expected that large parts of glacier ice buried in the permafrost will melt in the near future, although the intensity and timing will depend on local terrain conditions and the magnitude and rate of future climate trends in different Arctic regions. The impact of these ice bodies on landscape evolution remains uncertain since the extent and volume of undisturbed relict glacier ice are unknown. These remnants of glacier ice buried and preserved in the permafrost contribute to the high spatial variability in ground ice condition of these landscapes, leading to the formation of lakes with diverse origins and morphometric and limnological properties. This study focuses on thermokarst lake initiation and development in response to varying ground ice conditions in a glacial valley on Bylot Island (Nunavut). We studied a lake-rich area using lake sediment cores, detailed bathymetric data, remotely sensed data and observations of buried glacier ice exposures. Our results suggest that initiation of thermokarst lakes in the valley was triggered from the melting of either buried glacier ice or intrasedimental ice and ice wedges. Over time, all lakes enlarged through thermal and mechanical shoreline erosion, as well as vertically through thaw consolidation and subsidence. Some of them coalesced with neighbouring water bodies to develop larger lakes. These glacial thermokarst lakes formed in buried glacier ice now evolve as “classic” thermokarst lakes that expand in area and volume as a result of the melting of intrasedimental ground ice in the surrounding material and the underlying glaciofluvial and till material. It is expected that the deepening of thaw bulbs (taliks) and the enlargement of Arctic lakes in response to global warming will reach undisturbed buried glacier ice where it is still present, which in turn will substantially alter lake bathymetry, geochemistry and greenhouse gas emissions from Arctic lowlands.

While the sentinel nature of freshwater systems is now well-recognized, widespread integration of freshwater processes and patterns into our understanding of broader climate-driven Arctic terrestrial ecosystem change has been slow. We review the current understanding across Arctic freshwater systems of key sentinel responses to climate, which are attributes of these systems with demonstrated and sensitive responses to climate forcing. These include ice regimes, temperature and thermal structure, river baseflow, lake area and water level, permafrost-derived dissolved ions and nutrients, carbon mobilization (dissolved organic carbon, greenhouse gases, and radiocarbon), dissolved oxygen concentrations, lake trophic state, various aquatic organisms and their traits, and invasive species. For each sentinel, our objectives are to clarify linkages to climate, describe key insights already gained, and provide suggestions for future research based on current knowledge gaps. We suggest that tracking key responses in Arctic freshwater systems will expand understanding of the breadth and depth of climate-driven Arctic ecosystem changes, provide early indicators of looming, broader changes across the landscape, and improve protection of freshwater biodiversity and resources., Arctic Science, 9 (2), ISSN:2368-7460

Permafrost thaw has been widely observed to alter the biogeochemistry of recipient aquatic ecosystems. However, research from various regions has shown considerable variation in effect. In this paper, we propose a state factor approach to predict the release and transport of materials from permafrost through aquatic networks. Inspired by Hans Jenny's seminal description of soil-forming factors, and based on the growing body of research on the subject, we propose that a series of state factors—including relief, ice content, permafrost extent, and parent material—will constrain and direct the biogeochemical effect of thaw over time. We explore state-factor-driven variation in thaw response using a series of case studies from diverse regions of the permafrost-affected north, and also describe unique scaling considerations related to the mobile and integrative nature of aquatic networks. While our cross-system review found coherent responses to thaw for some biogeochemical constituents, such as nutrients, others, such as dissolved organics and particles, were much more variable in their response. We suggest that targeted, hypothesis-driven investigation of the effects of state factor variation will bolster our ability to predict the biogeochemical effects of thaw across diverse and rapidly changing northern landscapes.

Surface waters are widely used as sources of drinking water in Canada. The identification of the main sources of pollutants to surface waters, often associated with increasing urbanization, is need...

In formerly glaciated permafrost regions, extensive areas are still underlain by a considerable amount of glacier ice buried by glacigenic sediments. Although the extent and volume of undisturbed relict glacier ice are unknown, these ice bodies are predicted to melt with climate warming but their impact on landscape evolution remains poorly studied. The spatial distribution of buried glacier ice can play a significant role in reshaping periglacial landscapes, in particular thermokarst aquatic systems. This study focuses on lake initiation and development in response to the melting of buried glacier ice on Bylot Island, Nunavut. We studied a lake-rich area using lake-sediment cores, detailed bathymetric data, remotely sensed data and observations of buried glacier ice exposures. Our results suggest that initiation of deeper thermokarst lakes was triggered by the melting of buried glacier ice. They have subsequently enlarged through thermal and mechanical shoreline erosion, as well as vertically through thaw consolidation and subsidence, and they later coalesced with neighbouring water bodies to form larger lakes. Thus, these lakes now evolve as “classic” thermokarst lakes that expand in area and volume as a result of the melting of intrasedimental ground ice in the surrounding material and the underlying glaciofluvial and till material. It is expected that the deepening of thaw bulbs (taliks) and the enlargement of Arctic lakes in response to global warming will reach undisturbed buried glacier ice, if any, which in turn will substantially alter lake bathymetry, geochemistry and greenhouse gas emissions from Arctic lowlands.

Previous studies of thermokarst lakes have drawn attention to the potential for accumulation of CH₄ under the ice and its subsequent release in spring; however, such observations have not been available for thermokarst waters in carbon‐rich peatlands. Here we undertook a winter profiling of five black‐water lakes located on eroding permafrost peatlands in subarctic Quebec for comparison with summer profiles and used a 2‐year data set of automated water temperature, conductivity, and oxygen measurements to evaluate how the annual mixing dynamics may affect the venting of greenhouse gases to the atmosphere. All of the sampled lakes contained large amounts of dissolved CH₄ under their winter ice cover. These sub‐ice concentrations were up to 5 orders of magnitude above air equilibrium (i.e., the expected concentration in lake water equilibrated with the atmosphere), resulting in calculated emission rates at ice breakup that would be 1–2 orders of magnitude higher than midsummer averages. The amount of CO₂ dissolved in the water column was reduced in winter, and the estimated ratio of potential diffusive CO₂ to CH₄ emission in spring was half the measured summer ratio, suggesting a seasonal shift in methanogenesis and bacterial activity. All surface lake ice contained bubbles of CH₄ and CO₂, but this amounted to

In this paper, an ensemble-based system (EBS) for estimating chlorophyll-a concentrations (Chl-a) in inland water bodies using downscaled MODIS images was developed. Seeking additional opinions before making a decision is part of human nature, particularly for decisions involving health issues. The general concept behind EBS algorithms is based on this principle. Forty-six Chl-a measurements, collected over four water bodies between 2000 and 2008, were used to calibrate the EBS. Measurements ranged from 2.7 (oligotrophic waters) to 91 000 mg Chl-a $\text{m}^{{-3}}$ (hypertrophic waters). The EBS performance was evaluated by cross validation, and also using an independent database (Chl-a ranging from 1 to 14 mg $\text{m}^{{-3}}$ ). Cross-validation results were satisfactory both under high-blooming conditions ( $\text{R}^{2} = 0.98$ , relative RMSE (RMSEr) = 15%, relative Bias (BIASr) = −2%, and relative Nash (NASHr) = 0.95) and at the initialization of blooms ( $\text{R}^{2} = 0.77$ , RMSEr = 37%, BIASr = −8%, and NASHr = 0.70). The EBS also performed well on the independent database ( $\text{R}^{2} =0.93$ , RMSEr = 50%, BIASr = −27%, and NASHr = 0.70). A visual comparison mapping Chl-a on the Missisquoi Bay of Lake Champlain additionally illustrates the potential of the EBS to detect early phases of algal growth contrary to other models (adaptive model, Kharu, and APProach by Elimination). This approach is a proof of concept for a more efficient way to quantify algal biomass from remote sensing and could be exported to any types of satellite imagery in a context of water quality monitoring.

Lakes and ponds can be hotspots for CO₂ and CH₄ emissions, but Arctic studies remain scarce. Here we present diffusive and ebullition fluxes collected over several years from 30 ponds and 4 lakes formed on an organic‐rich polygonal tundra landscape. Water body morphology strongly affects the mixing regime—and thus the seasonal patterns in gas emissions—with ice‐out and autumnal turnover periods identified as hot moments in most cases. The studied thermokarst lake maintained relatively high ebullition rates of millennia‐old CH4 (up to 3405 ¹⁴C YBP). Larger and deeper kettle lakes maintained low fluxes of both gases (century to millennium‐old), slowly turning into a CO₂ sink over the summer. During winter, lakes accumulated CO₂, which was emitted during the ice‐out period. Coalescent polygonal ponds, influenced by photosynthesizing benthic mats, were continuous CO₂ sinks, yet important CH₄ emitters (modern carbon). The highest fluxes were recorded from ice‐wedge trough ponds (up to 96 mmol CO₂ equivalent m⁻² d⁻¹). However, despite clear signs of permafrost carbon inputs via active shore erosion, these sheltered ponds emitted modern to century‐old greenhouse gases. As the ice‐free period lengthens, scenarios of warmer and wetter conditions could favor both the production of CO₂ and CH₄ from thawing permafrost carbon, and CH₄ production from recently fixed carbon through an atmospheric CO₂₋to₋CH₄ shunt at sites in which primary production is stimulated. This must be carefully considered at the landscape scale, recognizing that older carbon stocks can be mineralized efficiently in specific locations, such as in thermokarst lakes.

In the ice-rich permafrost area of Central Yakutia (Eastern Siberia, Russia), climate warming and other natural and anthropogenic disturbances have caused permafrost degradation and soil subsidence, resulting in the formation of numerous thermokarst (thaw) lakes. These lakes are hotspots of greenhouse gas emissions, but with substantial spatial and temporal heterogeneity across the Arctic. We measured dissolved CO₂ and CH₄ concentrations in thermokarst lakes of Central Yakutia and their seasonal patterns over a yearly cycle. Lakes formed over the Holocene (alas lakes) are compared to lakes that developed over the last decades. The results show striking differences in dissolved greenhouse gases (up to two orders of magnitude) between lake types and seasons. Shallow lakes located in hydrologically closed alas depressions acted as CO₂ sinks and strong sources of diffusive CH₄ during some seasons (ebullition was not assessed). Recent thermokarst lakes were moderate to extremely high sources of diffusive CO₂ and CH₄, with considerable accumulation of greenhouse gas under the ice cover (winter) or in the deepest water layers (summer), highlighting the need to include spring and autumn as critical periods for integrated assessments. The water column was stratified in winter (all lake types) and especially in summer (recent thermokarst lakes), generating anoxia in bottom waters and favoring CH₄ production and storage, particularly in the most organic-rich lakes. The diffusive fluxes measured from thermokarst lakes of this typical taiga alas landscape of Central Yakutia are among the highest presented across Arctic and subarctic regions.

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.

Permafrost thaw leads to the formation of shallow water bodies in which large quantities of terrestrial organic carbon are mobilized as dissolved organic matter (DOM), partly turned into greenhouse gases (GHG). DOM comes from ancient carbon pools trapped in frozen soils for hundreds to thousands of years but also from present-day primary producers. Determining the fate of these pools is fundamental to evaluate the potential of these water bodies to amplify climate warming through their GHG emissions. In addition to the microbial degradation pathways producing CO2 and CH4, DOM can be directly mineralized into CO2 by sunlight. The CO2 production rates from photodegradation vary extensively across Arctic regions. The controlling factors and interactions with the microbial communities are not well understood, while photodegradation is likely to rise as the open-water season extends. Determining the photo- and bio-lability of the carbon pools available on thawing permafrost landscapes is needed to predict to what extent these systems can affect the global carbon cycle.Various DOM and environmental characteristics are considered in my PhD project, including mixing regime, seasonal exposure and light attenuation, as well as the microbial community response to photo-induced chemistry changes in DOM. Study sites include subarctic and arctic peatland areas of Eastern Canada, rich in thaw ponds and where organic matter started to accumulate between 3700 and 5600 years BP. These are non-Yedoma systems that have been poorly studied despite the large amount of organic carbon they store. This presentation will show the results of a lab experiment using a solar simulator where DOM of various origins and ages were tested: thaw pond water and leachates from plants, permafrost active layer, and previously unthawed permafrost. Short term incubations were carried out under five treatments: exposure to light without bacteria (0.2 µm filtration), exposure to light followed by a dark incubation with a bacterial inoculum, dark incubation with a bacterial inoculum, dark incubation with the whole bacterial community (2.7 µm filtration), and dark control without bacteria. A set of optical, biological and chemical characteristics were measured at the beginning and end of incubation. DOM losses (DOC, CDOM, and FDOM) and CO2 production vary extensively among treatments and DOM pools. They were the highest in dark bacterial incubations of plants leachates. DOM of the subarctic area was quite refractory to degradation in general, except for the biodegradation of the unthawed permafrost leachate (- 50%). Photodegradation was observed in all water types, with DOM losses faster than biodegradation ones for the Arctic soils leachates and all the ponds waters. The highest CO2 photoproduction was measured in Arctic unthawed permafrost leachates. Finally, the enhancement of DOM lability to microbes caused by photodegradation was generally observed for unthawed permafrost leachates. Incoming biological and 14C data, along with multivariate analyses, will improve the characterisation of the trends.

Chronic wasting disease (CWD), a degenerative and fatal prion disease affecting cervids, was detected for the first time in the province of Qu&eacute, bec, Canada, in a red deer (Cervus elaphus) farm in the Laurentides region on 10 September 2018. To assess CWD prevalence and control the disease in the free-ranging white-tailed deer (Odocoileus virginianus) population, a response plan including enhanced surveillance, population control, regulatory measures, and public outreach was deployed by the Ministry of Forests, Wildlife, and Parks (MFFP). In the 401 km2 white-tailed deer control area, a total of 750 free-ranging white-tailed deer were culled over 70 days, from 22 September to 15 December 2018. Of the culled deer, 534 were tested for CWD. We also tested for CWD a total of 447 white-tailed deer hunted from the enhanced surveillance zone and 2584 free-ranging white-tailed deer harvested outside this zone. Regulations were applied to prevent the spread of the disease through movements of infected animals harvested by hunters. Although no case of CWD was detected in free-ranging cervids in Qu&eacute, bec in 2018, this does not confirm the absence of the disease in these populations. However, the results suggest that if CWD is present, few free-ranging cervids are infected, making it possible to prevent its establishment in the province of Qu&eacute, bec.

Accurate estimations of gaseous emissions and carbon sequestration in wastewater processing are essential for the design, operation and planning of treatment infrastructure, particularly considering greenhouse gas reduction targets. In this study, we look at the interplay between biological productivity, hydrodynamics and evasion of carbon-based greenhouse gases (GHG) through diffusion and ebullition in order to provide direction for more accurate assessments of their emissions from waste stabilization ponds (WSPs). The ponds stratified in the day and mixed at night. Buoyancy flux contributed between 40 and 75% to turbulence in the water column during nocturnal cooling events, and the associated mixing lead to increasing carbon dioxide (CO2) and methane (CH4) concentrations by up to an order of magnitude in the surface. The onset of stratification and phytoplankton surface blooms, associated with high pH as well as low and variable CO2 partial pressure resulted in an overall reduction of CO2 efflux. Ebullition represented between 40 and 99% of the total CH4 efflux, and up to 95% of the integrated GHG release during wastewater treatment (in CO2 equivalents). Hydrodynamic conditions, diurnal variability and ebullition need to be accounted for reliable assessments of GHG emissions from WSPs. Our study is an important step towards gaining a deeper understanding in the functioning of these hot spots of carbon processing. The contribution of WSPs to atmospheric GHG budget is likely to increase with population growth unless their performance is improved in this regard.

Thermokarst lakes are widespread and diverse across permafrost regions and they are considered significant contributors to global greenhouse gas emissions. Paleoenvironmental reconstructions documenting the inception and development of these ecologically important water bodies are generally limited to Pleistocene-age permafrost deposits (Yedoma) of Siberia, Alaska, and the western Canadian Arctic. Here we present the gradual transition from syngenetic ice-wedge polygon terrains to a thermokarst lake in the Eastern Canadian Arctic. We combine geomorphological surveys with paleolimnological reconstructions from sediment cores in an effort to characterize local landscape evolution from terrestrial to freshwater environment. Located on an ice-rich and organic-rich polygonal terrace, the studied lake is now evolving through active thermokarst, as revealed by subsiding and eroding shores, and was likely created by water pooling within a pre-existing topographic depression. Organic sedimentation in the valley started during the mid-Holocene, as documented by the oldest organic debris found at the base of one sediment core and dated at 4.8 kyr BP. Local sedimentation dynamics were initially controlled by fluctuations in wind activity, local moisture and vegetation growth/accumulation, as shown by alternating loess (silt) and peat layers. Fossil diatom assemblages were likewise influenced by local hydro-climatic conditions and reflect a broad range of substrates available in the past (both terrestrial and aquatic). Such conditions likely prevailed until ~ 2000 BP, when peat accumulation stopped as water ponded the surface of degrading ice-wedge polygons, and the basin progressively developed into a thermokarst lake. Interestingly, this happened in the middle of the Neoglacial cooling period, likely under wetter-than-average conditions. Thereafter, the lake continued to develop as evidenced by the dominance of aquatic (both benthic and planktonic) diatom taxa in organic-rich lacustrine muds. Based on these interpretations, we present a four-stage conceptual model of thermokarst lake development during the late Holocene, including some potential future trajectories. Such a model could be applied to other formerly glaciated syngenetic permafrost landscapes.

Mixing regime and CO2 availability may control cyanobacterial blooms in polymictic lakes, but the underlying mechanisms still remain unclear. We integrated detailed results from a natural experiment comprising an average-wet year (2011) and one with heat waves (2012), a long-term meteorological dataset (1960–2010), historical phosphorus concentrations and sedimentary pigment records, to determine the mechanistic controls of cyanobacterial blooms in a eutrophic polymictic lake. Intense warming in 2012 was associated with: 1) increased stability of the water column with buoyancy frequencies exceeding 40 cph at the surface, 2) high phytoplankton biomass in spring (up to 125 mg WW L−1), 3) reduced downward transport of heat and 4) depleted epilimnetic CO2 concentrations. CO2 depletion was maintained by intense uptake by phytoplankton (influx up to 30 mmol m−2 d−1) in combination with reduced, internal and external, carbon inputs during dry, stratified periods. These synergistic effects triggered bloom of buoyant cyanobacteria (up to 300 mg WW L−1) in the hot year. Complementary evidence from polynomial regression modelling using historical data and pigment record revealed that warming explains 78% of the observed trends in cyanobacterial biomass, whereas historical phosphorus concentration only 10% thereof. Together the results from the natural experiment and the long-term record indicate that effects of hotter and drier climate are likely to increase water column stratification and decrease CO2 availability in eutrophic polymictic lakes. This combination will catalyze blooms of buoyant cyanobacteria.

Lakes are important sources of greenhouse gases (GHGs) to the atmosphere. Factors controlling CO2, CH4 and N2O fluxes include eutrophication and warming, but the integrated influence of climate-warming-driven stratification, oxygen loss and resultant changes in bloom characteristics on GHGs are not well understood. Here we assessed the influence of contrasting meteorological conditions on stratification and phytoplankton bloom composition in a eutrophic lake, and tested for associated changes in GHGs inventories in both the shallow and deep waters, over three seasons (2010-2012). Atmospheric heatwaves had one of the most dramatic effects on GHGs. Indeed, cyanobacterial blooms that developed in response to heatwave events in 2012 enhanced both sedimentary CH4 concentrations (reaching up to 1mM) and emissions to the atmosphere (up to 8 mmol m−2 d−1). That summer, CH4 contributed 52% of the integrated warming potential of GHGs produced in the lake (in CO2 equivalents) as compared to between 34 and 39% in years without cyanobacterial blooms. High CH4 accumulation and subsequent emission in 2012 were preceded by CO2 and N2O consumption and under-saturation at the lake surface (uptakes at -30 mmol m−2 d−1 and -1.6 µmol m−2 d−1, respectively). Fall overturn presented a large efflux of N2O and CH4, particularly from the littoral zone after the cyanobacterial bloom. We provide evidence that, despite cooling observed at depth during hot summers, CH4 emissions increased via stronger stratification and surface warming, resulting in enhanced cyanobacterial biomass deposition and intensified bottom water anoxia. Our results, supported by recent literature reports, suggests a novel interplay between climate change effects on lake hydrodynamics that impacts both bloom characteristics and GHGs production in shallow eutrophic lakes. Given global trends of warming and enrichment, these interactive effects should be considered to more accurately predict the future global role of lakes in GHG emissions.

The thawing and subsidence of frozen peat mounds (palsas) in permafrost landscapes results in the formation of organic-rich thermokarst lakes. We examined the effects of palsa degradation on CH4 and CO2 emissions by comparing thermokarst lakes at two peatland locations in subarctic Quebec, Canada: in the northern discontinuous permafrost region, and in southern sporadic permafrost where palsas are more rapidly degrading. The lakes were shallow (< 3 m) but stratified at both sites, and most had anoxic bottom waters. The surface waters at both sites were supersaturated in CH4 and CO2, and to a greater extent in the southern lakes, where the surface CH4 concentrations were up to 3 orders of magnitude above air equilibrium. Concentrations of CH4 and CO2 increased by orders of magnitude with depth in the southern lakes, however these gradients were less marked or absent in the North. Strong CH4 and CO2 emissions were associated with gas ebullition, but these were greatly exceeded by diffusive fluxes, in contrast to thermokarst lakes studied elsewhere. Also unusual relative to other studies to date, the surface concentrations of both gases increased as a linear function of water column depth, with highest values over the central, deepest portion of the lakes. Radiocarbon dating of ebullition gas samples showed that the CH4 had 14C-ages from 760 yr to 2005 yr before present, while the CO2 was consistently younger. Peatland thermokarst lakes may be an increasingly important source of greenhouse gases as the southern permafrost limit continues to shift northwards.

Permafrost thaw lakes (thermokarst lakes) are widely distributed across the northern landscape, and are known to be biogeochemically active sites that emit large amounts of carbon to the atmosphere as CH4 and CO2. However, the abundance and composition of the photosynthetic communities that fix CO2 have been little explored in this ecosystem type. In order to identify the major groups of phototrophic organisms and their controlling variables, we sampled 12 permafrost thaw lakes along a permafrost degradation gradient in northern Québec, Canada. Additional samples were taken from five rock-basin reference lakes in the region to determine if the thaw lakes differed in limnological properties and phototrophs. Phytoplankton community structure was determined by high-performance liquid chromatography analysis of their photoprotective and photosynthetic pigments, and autotrophic picoplankton concentrations were assessed by flow cytometry. One of the black-colored lakes located in a landscape of rapidly degrading palsas (permafrost mounds) was selected for high-throughput 18S rRNA sequencing to complement conclusions based on the pigment and cytometry analyses. The results showed that the limnological properties of the thaw lakes differed significantly from the reference lakes, and were more highly stratified. However, both waterbody types contained similarly diverse phytoplankton groups, with dominance of the pigment assemblages by fucoxanthin-containing taxa, as well as chlorophytes, cryptophytes and cyanobacteria. Chlorophyll a concentrations (Chl a) were correlated with total phosphorus (TP), and both were significantly higher in the thaw lakes (overall means of 3.3 µg Chl a L−1 and 34 µg TP L−1) relative to the reference lakes (2.0 µg Chl a L−1 and 8.2 µg TP L−1). Stepwise multiple regression of Chl a against the other algal pigments showed that it was largely a function of alloxanthin, fucoxanthin and Chl b (R2 = 0.85). The bottom waters of two of the thaw lakes also contained high concentrations of bacteriochlorophyll d, showing the presence of green photosynthetic sulphur bacteria. The molecular analyses indicated a relatively minor contribution of diatoms, while chrysophytes, dinoflagellates and chlorophytes were well represented; the heterotrophic eukaryote fraction was dominated by numerous ciliate taxa, and also included Heliozoa, Rhizaria, chytrids and flagellates. Autotrophic picoplankton occurred in biovolume concentrations up to 3.1 × 105 µm3 picocyanobacteria mL−1 and 1.9 × 106 µm3 picoeukaryotes mL−1, with large variations among lakes. Both groups of picophytoplankton were positively correlated with total phytoplankton abundance, as measured by Chl a; picocyanobacteria were inversely correlated with dissolved organic carbon, while picoeukaryotes were inversely correlated with conductivity. Despite their net heterotrophic character, subarctic thaw lakes are rich habitats for diverse phototrophic communities.

Ponds and lakes are widespread across the rapidly changing permafrost environments. Aquatic systems play an important role in global biogeochemical cycles, especially in greenhouse gas (GHG) exchanges between terrestrial systems and the atmosphere. The source, speciation and emission rate of carbon released from permafrost landscapes are strongly influenced by local conditions, hindering pan-Arctic generalizations. This study reports on GHG ages and emission rates from aquatic systems located on Bylot Island, in the continuous permafrost zone of the Eastern Canadian Arctic. Dissolved and ebullition gas samples were collected during the summer season from different types of water bodies located in a highly dynamic periglacial valley: polygonal ponds, collapsed ice-wedge trough ponds, and larger lakes. The results showed strikingly different ages and fluxes depending on aquatic system types. Polygonal ponds were net sinks of dissolved CO2, but variable sources of dissolved CH4. They presented the highest ebullition fluxes, 1 or 2 orders of magnitude higher than from other ponds and lakes. Trough ponds appeared as substantial GHG sources, especially when their edges were actively eroding. Both types of ponds produced modern to hundreds of years old (< 550 yr BP) GHG, even if trough ponds could contain much older carbon (> 2000 yr BP) derived from freshly eroded peat. Lakes had small dissolved and ebullition fluxes, however they released much older GHG, including millennium-old CH4 (up to 3500 yr BP) from lake central areas. Acetoclastic methanogenesis dominated at all study sites and there was minimal, if any, methane oxidation in gas emitted through ebullition. These findings provide new insights on GHG emissions by permafrost aquatic systems and their potential positive feedback effect on climate.

Global warming has accelerated the formation of permafrost thaw ponds in several subarctic and arctic regions. These ponds are net heterotrophic as evidenced by their greenhouse gas (GHG) supersaturation levels (CO2 and CH4), and generally receive large terrestrial carbon inputs from the thawing and eroding permafrost. We measured seasonal and vertical variations in the concentration and type of dissolved organic matter (DOM) in five subarctic thaw (thermokarst) ponds in northern Quebec, and explored how environmental gradients influenced heterotrophic and phototrophic biomass and productivity. Late winter DOM had low aromaticity indicating reduced inputs of terrestrial carbon, while the high concentration of dissolved organic carbon (DOC) suggests that some production of non-chromophoric dissolved compounds by the microbial food web took place under the ice cover. Summer DOM had a strong terrestrial signature, but was also characterized with significant inputs of algal-derived carbon, especially at the pond surface. During late winter, bacterial production was low (maximum of 0.8 mg C m−3 d−1) and was largely based on free-living bacterioplankton (58 %). Bacterial production in summer was high (up to 58 mg C m−3 d−1), dominated by particle-attached bacteria (67 %), and strongly correlated to the amount of terrestrial carbon. Primary production was restricted to summer surface waters due to strong light limitation deeper in the water column or in winter. The phototrophic biomass was equal to the heterotrophic biomass, but as the algae were mostly composed of mixotrophic species, most probably they used bacteria rather than solar energy in such shaded ponds. According to the δ13C analyses, non-algal carbon supported 51 % of winter and 37 % of summer biomass of the phantom midge larvae, Chaoborus sp., that are at the top of the trophic chain. Our results point to a strong heterotrophic energy pathway in these thaw pond ecosystems, where bacterioplankton dominates the production of new carbon in both summer and winter.

Phytoplankton bloom monitoring in freshwaters is a challenging task, particularly when biomass is dominated by buoyant cyanobacterial communities that present complex spatiotemporal patterns. Increases in bloom frequency or intensity and their earlier onset in spring were shown to be linked to multiple anthropogenic disturbances, including climate change. The aim of the present study was to describe the phenology of phytoplankton blooms and its potential link with morphological, physiographic, anthropogenic, and climatic characteristics of the lakes and their watershed. The spatiotemporal dynamics of near-surface blooms were studied on 580 lakes in southern Quebec (Eastern Canada) over a 17-year period by analyzing chlorophyll-a concentrations gathered from MODIS (Moderate Resolution Imaging Spectroradiometer) satellite images. Results show a significant increase by 23% in bloom frequency across all studied lakes between 2000 and 2016. The first blooms of the year appeared increasingly early over this period but only by 3 days (median date changing from 6 June to 3 June). Results also indicate that high biomass values are often reached, but the problem is seldom extended to the entire lake surface. The canonical correlation analysis between phenological variables and environmental variables shows that higher frequency and intensity of phytoplankton blooms and earlier onset date occurred for smaller watersheds and higher degree-days, lake surface area, and proportion of urban zones. This study provides a regional picture of lake trophic state over a wide variety of lacustrine environments in Quebec, a detailed phenology allowing to go beyond local biomass assessments, and the first steps on the development of an approach exploiting regional trends for local pattern assessments.

The distribution and quality of water resources vary dramatically across Canada, and human impacts such as land-use and climate changes are exacerbating uncertainties in water supply and security. At the national level, Canada has no enforceable standards for safe drinking water and no comprehensive water-monitoring program to provide detailed, timely reporting on the state of water resources. To provide Canada's first national assessment of lake health, the NSERC Canadian Lake Pulse Network was launched in 2016 as an academic-government research partnership. LakePulse uses traditional approaches for limnological monitoring as well as state-of-the-art methods in the fields of genomics, emerging contaminants, greenhouse gases, invasive pathogens, paleolimnology, spatial modelling, statistical analysis, and remote sensing. A coordinated sampling program of about 680 lakes together with historical archives and a geomatics analysis of over 80,000 lake watersheds are used to examine the extent to which lakes are being altered now and in the future, and how this impacts aquatic ecosystem services of societal importance. Herein we review the network context, objectives and methods.

In optically complex inland waters, the underwater attenuation of photosynthetically active radiation (PAR) is controlled by a variable combination of absorption and scattering components of the lake or river water. Here we applied a photon budget approach to identify the main optical components affecting PAR attenuation in Lake St. Charles, a drinking water reservoir for Quebec City, Canada. This analysis showed the dominant role of colored dissolved organic matter (CDOM) absorption (average of 44% of total absorption during the sampling period), but with large changes over depth in the absolute and relative contribution of the individual absorption components (water, nonalgal particulates, phytoplankton and CDOM) to PAR attenuation. This pronounced vertical variation occurred because of the large spectral changes in the light field with depth, and it strongly affected the average in situ diffuse absorption coefficients in the water column. For example, the diffuse absorption coefficient for pure-water in the ambient light field was 10-fold higher than the value previously measured in the blue open ocean and erroneously applied to lakes and coastal waters. Photon absorption budget calculations for a range of limnological conditions confirmed that phytoplankton had little direct influence on underwater light, even at chlorophyll a values above those observed during harmful algal blooms in the lake. These results imply that traditional measures of water quality such as Secchi depth and radiometric transparency do not provide a meaningful estimate of the biological state of the water column in CDOM-colored lakes and reservoirs.

Small lakes are likely to show considerable temporal variability in greenhouse gas emissions given their transient stratification and short residence time. To determine the extent that CO2 and CH4 emission and storage depends on surface meteorology, we studied a shallow lake during 2 years with contrasting rainfall and thermal stratification. Gas fluxes were estimated with wind-based and surface renewal models and compared to direct measurements obtained with floating chambers. The assessment of greenhouse gases storage revealed that the lake gained CO2 in association with rainfall in both the rainier (2011) and drier summer (2012). In 2011, stratification was less extensive and disrupted frequently. The lake was a source of CO2 and CH4, and ebullition exceeded diffusive fluxes of CH4. In 2012, stratification was more persistent, the lake was a sink for CO2 during dry periods, CO2 and CH4 accumulated in the hypolimnion later in the summer when rainfall increased, diffusive fluxes of CH4 were similar to those in 2011 mid-summer and over four times higher during overturn. Ebullition was lower in the drier summer. Fluxes measured with chambers were closer to estimations from the surface renewal model and about two times values estimated with wind-based models.

The ecological conditions and cellular mechanisms which affect the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in marine ecosystems are still enigmatic. This information is crucial for deriving accurate oceanic ecosystem models for the dynamics of these major players in the Earth’s sulfur cycle and climate. In the present study, we examined the effect of short-term increases in photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) on the production of DMSP and DMS and on DMSP lyase potential activity (DLPA) in an axenic culture of the coccolithophore Emiliania huxleyi (CCMP 1742). Algal cells were subjected to a rapid shift from a low intensity of PAR (50 μE m-2 s-1; low light, LL) to a high intensity of PAR (198 μE m-2 s-1) and elevated UVR conditions (high light, HL), simulating what may occur during upward mixing in the surface mixed layer or during changes in cloud cover. During the 4.5 h exposure to HL, the intracellular DMSP normalized to cell volume increased by ca. 30%, and dissolved DMSP doubled relative to control values. However, the DLPA per unit of cell volume decreased by ~45% compared to the control value. The up-regulation of cellular DMSP concentration is consistent with an antioxidant and/or energy dissipation role for DMSP. The decrease in DLPA may indicate that the DMSP lyase enzyme plays no role in antioxidant protection in this algal species, but rather serves some other cellular function, such as grazing protection.

Permafrost thawing in lowland Arctic tundra results in a polygonal patterned landscape and the formation of numerous small ponds. These ponds emit biologically mediated carbon dioxide (CO2) and methane (CH4). Their greenhouse gas (GHG) emissions are variable, for reasons that are not well understood. Emissions are related to a balance between GHG producers and consumers, as well as to physical properties of the water column controlling gas exchange rates with the atmosphere. Here, we investigated the bacterial diversity of polygonal and runnel ponds, two geomorphologically distinct pond types commonly found in continuous permafrost regions. Using a combination of 16S rRNA Sanger sequencing and high-throughput amplicon sequencing, we found that bacterial communities in overlying waters were clearly dominated by carbon degraders and were similar in both pond types, despite their variable physical and chemical properties. However, surface sediment communities in the two pond types were significantly different. Polygonal pond sediment was colonized by carbon degraders (46–29 %), cyanobacteria (20–27 %) that take up CO2 and produce oxygen, and methanotrophs (11–20 %) that consume CH4 and require oxygen. In contrast, cyanobacteria were effectively absent from the surface sediment of runnel ponds, which in addition to carbon degraders (65–81 %), were colonized by purple non-sulfur bacteria (5–21 %), and by fewer methanotrophs (1–5 %). The link between the methanotrophic community and the type of ponds could potentially be used to improve upscale estimates of GHG emissions based on landscape morphology in such remote regions.

Although widely distributed throughout Arctic and subarctic regions, thermokarst ponds and lakes remain relatively unexplored regarding geomorphological changes in their catchments and their internal properties in relation to climate change over the past decades. This study synthesizes recent landscape evolution and modern sedimentology of limnologically diverse thermokarst ponds near southeastern Hudson Bay, Canada. Spatio-temporal analysis of permafrost mounds, thermokarst ponds, and vegetation surface areas over the past five decades revealed that the recent climate-induced decrease of permafrost-affected areas was not primarily compensated by thermokarst pond development, but rather by a remarkable increase in vegetation cover. These changes appeared to be modulated by topographical and hydrological gradients at the study site, which are associated with east-ward increasing thickness of postglacial marine deposits. At a more contemporary timescale, physico-chemical measurements made on sedimenting materials (sediment traps) and freshly deposited lacustrine sediments of selected thermokarst ponds revealed striking differences both among ponds and between the oxic epilimnion and the oxygen-depleted hypolimnion. These findings underscore the major influence of local landscape properties and oxycline development on pond sedimentology and geochemistry, such as the transport of detritic particles and the concentration of redox-sensitive elements.

Colored permafrost thaw ponds (thermokarst ponds), are abundant freshwater eco- systems in the subarctic. The ponds can emit significant levels of greenhouse gases, which are drivers of climate change, but little is known of their microbial communities. The ponds are verti- cally stratified for most of the year with suboxic to anoxic hypolimnia, suggesting varied habitats despite their shallow depths (1 to 4 m). Pelagic bacterial communities in 4 contrasting ponds near the coast of Hudson Bay, Quebec, Canada, were investigated by cloning and sequencing the 16S rRNA gene from environmental DNA. Surface and bottom water libraries yielded a total of 109 operational taxonomical units (defined as 97% sequence similarity) from 261 clones. The majority of clones had their closest matches to other environmental clones originating from freshwater environments, especially lakes. Proteobacteria accounted for the majority of sequences (33%), fol- lowed by Bacteroidetes (19%), Actinobacteria (13%), Chlorobi (10%), Verrucomicrobia (8%), and Cyanobacteria (7%). There were large differences in the assemblages inhabiting the different ponds and water layers. Bacteroidetes were predominant at the surface, while Chlorobi were found in bottom waters. Sequences with best matches to known methanotrophic bacteria re - presented up to 20% of all sequences. Methanotrophs included Methylobacter psychrophilus, Crenothrix polyspora, and Methylocystis parvus as closest matches. Ordination analysis clearly separated surface and bottom waters, where relatively large quantities of organic matter, low light availability, cool temperatures, and suboxic conditions selected for the distinct communities. The separation of surface waters of different ponds showed that bacterial communities could differ between superficially similar environments. This study illustrates the high diversity in bacterial assemblages occurring in ponds separated by a few meters, and thus, the potentially complex response of thaw pond systems to climate change.