To assess the affects of permafrost degradation on key components of the aquatic food web, an in situ manipulative mesocosm experiment was performed in an upland, unslumped Arctic lake located near Inuvik, Northwest Territories. In total, twelve replicate mesocosms were established, 3 control and 3 replicates of 3 treatment levels each dosed with differing amounts of sediments sourced from a nearby thermokarst slumped lake. Findings from the experiment showed that pelagic autotrophic processes had the greatest potential to contribute to higher trophic levels regardless of treatment. Even in the high sediment treatment level, which showed the least pelagic autotrophic production, pelagic autotrophic production was two orders of magnitude greater than pelagic heterotrophic production and 5 times greater than benthic autotrophic or heterotrophic production. Sediment treatment had no significant effect on benthic primary productivity; however, a 500% increase in benthic heterotrophic production was observed. This raises the possibility that increased activity in benthic heterotrophic production is the first step in thermokarst-affected lake ecosystem succession leading to the proliferation of benthic primary production observed in many slumped lakes in the western Canadian Arctic. Water column phosphorus concentrations increased with increasing sediment treatment while pelagic primary production decreased and zooplankton biomass increased. These results suggest that the initial effect of thermokarst disturbance is an enrichment of the system and that top-down predation from zooplankton regulate the abundance of phytoplankton in these systems. If incidences of thermokarst disturbance continue to increase as predicted by current climate models/scenarios, results from this study suggest that the structure and function of Arctic aquatic ecosystems will be significantly impacted. This study highlights the need for further research to obtain a better mechanistic and predictive understanding of the potential effects of thermokarst disturbance on the geochemistry and ecology of Arctic lakes at relevant spatial and temporal scales. / Graduate

Climate warming has the potential to alter the structure and function of Arctic ecosystems in ways that are not fully understood. Polygonal terrain is a widespread permafrost feature of Arctic landscapes that is likely to be impacted by warming ground temperatures. This is of particular relevance in the uplands in the Mackenzie Delta region, where high-centre ice wedge polygon fields comprise 10% of the terrestrial landscape, and mean annual ground temperatures have increased between 1 and 2°C over the last 40 years (Burn and Kokelj 2009). I used broad-scale airphoto analysis and fine-scale field studies to investigate the impacts and possible trajectories of ice wedge degradation in the upland tundra north of Inuvik, NWT. Field investigations were undertaken to characterize biotic and abiotic conditions and feedbacks in stable and degrading high-centre polygons. Field surveys were conducted along transects which crossed three polygon micropositions (centres, edges and troughs) and targeted a degradation sequence from stable troughs to ice wedge melt ponds. I measured surface microtopography, active layer depth, water depth, plant community composition, soil gravimetric moisture, late winter snow depth, and shallow annual ground temperatures. Field data showed that ice wedge degradation drove increases in soil moisture, standing water depth, ground surface collapse, ground temperature, and active layer thaw and snow pack compared to stable troughs. These changing abiotic conditions drove the shift from mesic upland tundra plant communities to unvegetated melt ponds. Interactions between abiotic and biotic factors in degrading troughs increase ground temperature and contribute to positive feedbacks for ice wedge degradation. Analysis of broad-scale factors affecting ice wedge degradation involved the mapping of high-centre polygon distribution across the study area and the distribution of ice wedge melt ponds using high-resolution aerial photographs from 2004. Recent changes in melt pond area were also mapped using imagery dating from 1972. Thermokarst activity in polygonal terrain adjacent to anthropogenic disturbances was also assessed. Polygon fields were more abundant and larger in the northern part of the study area, where ground temperature conditions were most favourable for ice wedge formation. Spatial variation in polygonal terrain density was also related to topography, drainage, and the distribution of lacustrine sediments. Melt pond mapping and assessment of thermokarst at anthropogenic disturbances showed that ice wedges at higher latitudes are more susceptible to degradation primarily because these areas are underlain by larger and more abundant ice wedges. Melt pond mapping confirmed that the polygonal fields north of 69.4°N have shown both large increases and decreases in area, and that polygons in the south have been relatively stable in recent decades. The increased thaw sensitivity of polygonal terrain at higher latitudes has implications for soil carbon dynamics, terrestrial ecosystems, and the planning and maintenance of infrastructure as air and ground temperatures continue to increase. / Graduate / 0329 / 0372 / 0388

<p>This thesis outlines the development and decay of palsas in northern Sweden in relation to climatic and environmental factors. Palsas were investigated on morphology, vegetation, peat porosity, ground temperature and local climate in four bogs situated on a north-south gradient in northern Sweden. A new classification for palsa stages (embryo, young, mature, degrading and remnant stage), based on morphological and vegetational characteristics, is proposed and described in the thesis. The start of palsa growth seems to be favoured by a low vegetation height and a high cover of <i>Sphagnum</i> mosses. Very high porosities were measured in this peat type, resulting in a high insulation capacity preserving the frozen ground in summer. A decrease of porosities in the surface peat layer from the embryo palsa stage to the mature and degrading palsa stage was found. This contributes to the degrading of palsas, due to higher thermal diffusivities in palsas with lower porosities. The aggradation of the ice core can also cease due to the observed change in vegetation from low vegetation on embryo and young palsas to vegetation with tall <i>Betula nana</i> shrubs on degrading palsas. This was found to cause thicker snow cover that prevents frost penetration into the palsa core. The present climate conditions (with mean annual temperature of –1.5°C and annual precipitation of 737 mm) at the boundary of palsa distribution in Sweden were found to be unsuitable for palsa development. Palsa growth started here during a cold period in the last part of the Little Ice Age, with estimated mean annual and mean winter temperatures of –2.3°C and –10.0°C, respectively. </p><p>The major conclusions are that besides air temperature, depth of snow cover, summer precipitation and hydrology, also vegetation cover and peat characteristics are important for palsa growth and decay.</p>

This thesis outlines the development and decay of palsas in northern Sweden in relation to climatic and environmental factors. Palsas were investigated on morphology, vegetation, peat porosity, ground temperature and local climate in four bogs situated on a north-south gradient in northern Sweden. A new classification for palsa stages (embryo, young, mature, degrading and remnant stage), based on morphological and vegetational characteristics, is proposed and described in the thesis. The start of palsa growth seems to be favoured by a low vegetation height and a high cover of Sphagnum mosses. Very high porosities were measured in this peat type, resulting in a high insulation capacity preserving the frozen ground in summer. A decrease of porosities in the surface peat layer from the embryo palsa stage to the mature and degrading palsa stage was found. This contributes to the degrading of palsas, due to higher thermal diffusivities in palsas with lower porosities. The aggradation of the ice core can also cease due to the observed change in vegetation from low vegetation on embryo and young palsas to vegetation with tall Betula nana shrubs on degrading palsas. This was found to cause thicker snow cover that prevents frost penetration into the palsa core. The present climate conditions (with mean annual temperature of –1.5°C and annual precipitation of 737 mm) at the boundary of palsa distribution in Sweden were found to be unsuitable for palsa development. Palsa growth started here during a cold period in the last part of the Little Ice Age, with estimated mean annual and mean winter temperatures of –2.3°C and –10.0°C, respectively. The major conclusions are that besides air temperature, depth of snow cover, summer precipitation and hydrology, also vegetation cover and peat characteristics are important for palsa growth and decay.

Thermokarst is a typical process that indicates widespread permafrost degradation in yedoma landscapes. The Lena-Aldan interfluvial area in Central Yakutia in eastern Siberia is now facing extensive landscape changes with surface subsidence due to thermokarst development during the past few decades. To clarify the spatial extent and rate of subsidence, multiple spatial datasets, including GIS and remote sensing observations, were used to analyze the Churapcha rural locality, which has a typical yedoma landscape in Central Yakutia. Land cover classification maps for 1945 and 2009 provide basic information on anthropogenic disturbance to the natural landscape of boreal forest and dry grassland. Interferometric synthetic aperture radar (InSAR) with ALOS-2/PALSAR-2 data revealed activated surface subsidence of 2 cm/yr in the disturbed area, comprising mainly abandoned agricultural fields. Remote sensing with an unmanned aerial system also provided high-resolution information on polygonal relief formed by thermokarst development at a disused airfield where InSAR analysis exhibited extensive subsidence. It is worth noting that some historically deforested areas have likely recovered to the original landscape without further thermokarst development. Spatial information on historical land-use change is helpful because most areas with thermokarst development correspond to locations where land was used by humans in the past. Going forward, the integrated analysis of geospatial information will be essential for assessing permafrost degradation.

Les changements climatiques mesurés dans le Nord-ouest canadien au cours du XXIe siècle entraînent une dégradation du pergélisol. Certaines des principales conséquences physiques sont la fonte de la glace interstitielle lors du dégel du pergélisol, l’affaissement du sol et la réorganisation des réseaux de drainage. L’effet est particulièrement marqué pour les routes bâties sur le pergélisol, où des dépressions et des fentes se créent de façon récurrente, rendant la conduite dangereuse. Des observations et mesures de terrain effectuées à Beaver Creek (Yukon) entre 2008 et 2011 ont démontré qu’un autre processus très peu étudié et quantifié dégradait le pergélisol de façon rapide, soit la chaleur transmise au pergélisol par l’écoulement souterrain. Suite aux mesures de terrain effectuées (relevé topographique, étude géotechnique du sol, détermination de la hauteur de la nappe phréatique et des chenaux d’écoulement préférentiels, température de l’eau et du sol, profondeur du pergélisol et de la couche active), des modèles de transfert de chaleur par conduction et par advection ont été produits. Les résultats démontrent que l’écoulement souterrain dans la couche active et les zones de talik contribue à la détérioration du pergélisol via différents processus de transfert de chaleur conducto-convectifs. L’écoulement souterrain devrait être pris en considération dans tous les modèles et scénarios de dégradation du pergélisol. Avec une bonne caractérisation de l’environnement, le modèle de transfert de chaleur élaboré au cours de la présente recherche est applicable dans d’autres zones de pergélisol discontinu. / Climate changes affecting the North West portion of Canada alter the thermal state of the permafrost and promote permafrost degradation. The results are permafrost thawing, ground ice melting, surface drainage changes and soil subsidence. Road infrastructures built on permafrost are particularly sensitive to permafrost stability and integrity. Depressions in the road pavement and development of cracks and potholes are recurrent problems for northern infrastructure. Field measurements done along a road transect in the discontinuous permafrost zone near Beaver Creek (Yukon) between 2008 and 2010 demonstrated that another process, advective heat transfer induced by groundwater flow, is promoting permafrost degradation. This process remains poorly known and has not been quantified sufficiently in permafrost environments. Field data on topography, soil geotechnical properties, water table and preferential flowpath characterization, ground and water temperature and active layer and permafrost depth were collected to build coupled models of seepage (mass transfer) and heat transfers. Results indicate that convective heat transfer processes associated with groundwater flow can have a substantial impact on permafrost degradation. Groundwater flow processes should therefore be taken into account in permafrost evolution models and climate warming scenarios. With a good characterization of the environment, the model that has been developed in this present research is relevant in other discontinuous permafrost environments.

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