Net ecosystem carbon dioxide exchange (NEE) was measured using the eddy covariance (EC) technique at a wetland tundra-sedge fen near Churchill, Manitoba, Canada during two consecutive growing seasons (2007 and 2008). Mean daily NEE at the fen (DOY 157-254) was -3.5 (± 0.26 S.E.) g CO2 m-2 d-1 in 2007 and -4.6 (± 0.36) g CO2 m-2 d-1 in 2008. The fen was a net carbon dioxide (CO2) sink during both the 2007 and 2008 growing seasons of -343 (± 79) and -450 (± 87) g CO2 m-2, respectively. Mean air temperature during the summer (June 1-August 31) was about 1°C greater than the historical average (1971-2000) in 2007 and about 2°C greater in 2008. Growing season precipitation was 107.5 mm below normal in 2007 and 359.5 mm above normal in 2008. These data suggest that if future climate change brings warmer temperatures and near-to-above average precipitation maintaining the water table near the surface, similar subarctic ecosystems will experience increased gross ecosystem productivity enhancing CO2 sequestration during the growing season.

I used a specific inhibitor approach to systematically measure NH4+ oxidation rates through the euphotic zone of three distinct oceanographic regimes. Study sites included Saanich Inlet, a highly productive British Columbia fjord, the Line P oceanographic transect in the NE subarctic pacific, and the Bermuda Atlantic Time-series Study (BATS) station in the oligotrophic, sub-tropical Sargasso Sea. Nitrate uptake rates were also measured at select stations on a number of research cruises. NH4+ oxidation rates were found to proceed throughout the euphotic zone in each of my study regions, and, overall, euphotic zone NH4+ oxidation rates ranged from undetectable to 203 nmol L-1 d-1. A general characterization of the rates observed in each of my study regions shows that euphotic zone NH4+ oxidation rates were typically highest in Saanich Inlet, intermediate along Line P, and lowest at BATS. The observation that NH4+ oxidation occurred throughout the euphotic zone in each of my study regions was in contrast to the traditional assumption of no euphotic zone nitrification, and it should now be considered a ubiquitous process in the euphotic regions of the ocean. Results found that euphotic zone nitrification could have potentially supported, on average, 15, 53 and 79% of the phytoplankton NO3- requirements in Saanich Inlet, and along Line P and at BATS, respectively, and this underscores the need for a major re-evaluation of the new production paradigm. Light, substrate concentrations, and potentially substrate supply rates were all found to play a role in regulating NH4+ oxidation, albeit to varying degrees, and I discuss the influence that each of these variables may have had on controlling NH4+ oxidation rates at regionally specific scales in Chapters 2 (Saanich Inlet), 3 (Line P) and 4 (BATS). Finally, a cross study-region comparison of results showed that the relative degree by which new production estimates were reduced, when euphotic zone nitrification was taken into consideration, decreased exponentially as total NO3- uptake rates increased; the relationship I describe between these two variables may potentially provide a simple and rapid means of estimating the extent to which new production may have been overestimated at regionally specific and global scales. My Line P sampling program also provided me with an opportunity to conduct the first investigation of intermediate depth N2O distributions along the Line P oceanographic transect. My results demonstrated that nitrification is the predominant production pathway for N2O in the NE subarctic Pacific. N2O distributions along Line P were variable, however, and I also consider the role of different transiting water masses and potential far-field denitrification in contributing to this variability. Finally, I estimated sea-to-air fluxes of N2O and based on these results I have demonstrated that the NE subarctic Pacific is a “hotspot” for N2O emissions to the atmosphere. / Graduate

There is growing concern for increased frequency of extreme events due to several severe floods and droughts occurring globally in recent years. Improving knowledge on the complexity of hydrological systems and interactions with climate is essential to be able to determine drivers and predict changes in the future. This is especially true in cold regions such as the Swedish Subarctic. This thesis explored changes in high peak flows and linked trends to climate. Trend analyses were applied on 18 catchments in the Swedish Subarctic over their entire periods of record and a common period (1990-2013) among the data to explore changes in flood magnitude, flood occurrence, mean summer flow, snowmelt onset and center of mass. Further, a flood frequency analysis was applied using the extreme value type I (Gumbel) distribution and selected flood percentiles were tested for stationarity. The results show the complexity of the hydrological system and interactions with climate. No clear overall pattern could be determined suggesting that changes are happening at catchment scale. Indications for a shift in flow regime from snowmelt-dominated to rainfall-dominated are evident with all significant trends pointing towards lower flood magnitudes in the spring flood, earlier flood occurrence and snowmelt onset, and decreasing mean summer flows. The shift in flow regime suggests that air temperature is more clearly reflected in streamflow than precipitation in the Swedish Subarctic. Decreasing trends in flood magnitude and mean summer flows are suggestive of permafrost thawing, which agrees with the increasing trends in the annual minimum flow. Long streamflow records can further link variability in streamflow to multidecadal atmospheric circulations over the North Atlantic. Most evident are changes towards lower mean summer flows (ten catchments significant at a 95% confidence interval) and earlier snowmelt onset (eight catchments significant). Trends in the selected flood percentiles show indications towards an increase in extreme events over the entire period (significant for four catchments), with all significant trends being positive. Over the common period, no pattern is notable and the sensitivity of trend analyses is evident.

Significant warming of Arctic and northern regions is ongoing and may greatly alter the carbon cycle of these regions. During the International Polar Year, an extensive study was carried out in the Eastern Canadian subarctic and Arctic in order to characterize CO2 and CH4 exchanges from these potentially sensitive ecosystems. The main objectives of this study were to identify the land cover and environmental factors leading to greatest CO2 and CH4 emissions in a highly heterogeneous subarctic landscape, to quantify interannual variability in the net ecosystem exchange of CO2 (NEE) in subarctic forest tundra and investigate the weather conditions that increase net uptake of CO2, and finally, to evaluate the general trends of mid-summer NEE along a latitudinal gradient spanning from 55° to the 72° north. At the landscape level, CO2 and CH4 exchanges showed large variability. Although CH4 emissions were greatest in wetlands, their areal coverage is small in the Kuujuarapik area and limited the influence of these CH4 sources. At the ecosystem level, large-scale atmospheric processes controlled growing season length and cumulative growing degree days which greatly influenced annual and seasonal NEE trends. The subarctic forest tundra near Kuujuarapik was a net source of CO2 in all 3 study years but the source strength was least with the greatest growing degree days while the length of the snow-free period appeared to be less important. Across a latitudinal gradient covering subarctic forest tundra to Arctic tundra, variations in summer NEE could be linked to surface organic carbon content with higher net CO2 uptake at sites with greater soil organic carbon. Warmer days tended to correlate with smaller daily net CO2 uptake (or greater net CO2 losses) but overall, warmer growing seasons reduced the net losses of CO2 on an annual basis. Carbon fluxes in Eastern Canadian subarctic and Arctic regions are highly variable in space and time but these observations help establish a baseline for future examinations of how these carbon exchanges may change with further warming.

High latitude and altitude ecosystems are currently undergoing rapid and unprecedented warming in response to anthropogenically induced climate change. Subarctic, alpine regions are particularly vulnerable to increases in air temperature and changing precipitation regimes, which have caused cascading hydrological and ecological impacts. In addition to changing flow regimes, thawing permafrost, and declining glaciers, widespread changes in vegetation composition, density and distribution have been observed across northern regions. Specifically, treeline is advancing with increasing latitude and altitude and shrubs are increasing in height, extent, and density. Despite widespread documentation of this northern greening, few field-based studies have evaluated the hydrological implications of these changes. Quantification of total evapotranspiration (ET) across a range of vegetation gradients is essential for predicting water yield, yet challenging in cold alpine catchments due to heterogeneous land cover. Direct field-level measurements of transpiration (T) and evaporative partitioning across subarctic, alpine ecosystems and species are rare, yet essential to assess sensitivities and hydrological response to changing climate drivers. This thesis presents six years of surface energy balance components and ET dynamics and two years of sap flux measurements and critical zone stable water isotope sampling at three sites along an elevational gradient in a subarctic, alpine catchment near Whitehorse, Yukon Territory, Canada. These sites span a gradient of thermal and vegetation regimes, providing a space-for-time comparison for future ecosystem shifts: 1) a low-elevation boreal white spruce forest (~12-20 m), 2) a mid-elevation subalpine taiga comprised of tall, dense willow (Salix) and birch (Betula) shrubs (~1-3 m) and 3) a high-elevation subalpine taiga with short, sparse shrub cover (< 0.75 m) and moss, lichen, and bare rock. We utilize both mass flux measurements and stable water isotopes to evaluate the timing, magnitude, sensitivities, and sources of plant water uptake across these vegetation covers. Total ET decreased and interannual variability increased with elevation, with mean May to September ET totals of 349 (±3) mm at the forest, 249 (±10) mm at the tall, dense shrub site, and 240 (± 26) mm at the short, sparse shrub site. The shrub sites exhibited similar ET losses over 6 years despite differences in shrub height and abundance, although daily rates were higher at the tall shrub site in the peak growing season. From May to September, ET:R ratios were the highest and most variable at the forest (2.19 ± 0.37) and similar at the tall, dense shrub (1.22 ± 0.09) and short, sparse shrub (1.14 ± 0.05) sites. In the mid-growing season, mean T rates were greater at the dense shrub site (2.0 ± 0.75 mm d-1) than the forest (1.47 ± 0.52 mm d-1). During this time, T:ET was lower at the forest (0.48) than at the tall, dense shrub site (0.80). During the growing season between the two years, 2020 was considerably wetter and cooler than 2019. At the tall shrub site, during the mid-growing season (July 1-Aug 15), T dropped considerably in 2020 (-26%), as T was suppressed during the short, wet growing season. In contrast, T at the forest was only moderately suppressed (-3%) between years in this same period. Evapotranspiration was more strongly controlled by air temperature during the early and late season at the forest, while ET at the shrub site was more sensitive to warmer temperatures in the mid-growing season. At the shrub sites, ET was energy limited with no observed soil moisture limitation on T. While 2H and 18O of volume weighted precipitation became more depleted with elevation, the opposite was true in xylem water, where 2H and 18O became more enriched with elevation. Plant water uptake was more reflective of snow water at the forest site than both shrub sites, particularly early in the year and during dry periods. Near-surface bulk soil water had more negative lc-excess at the forest throughout the season and with depth, highlighting increased contributions from soil evaporation. This study combined direct measurements of sap flux, ET, and critical zone isotopes to provide new details on multi-year plant-soil-water dynamics, critical zone water cycling, and species-specific plant water uptake patterns in seasonally frozen soils, which have not previously been reported in cold regions. Our results suggest that advances in treeline will increase overall ET and lower interannual variability; however, the large growing season water deficit and stable water isotope signature at the forest indicates strong reliance on soil moisture from late fall and snowmelt recharge and the potential for plant water stress. Differences between the shrub species were apparent in the sap flux and stable isotope measurements, highlighting the need to further evaluate species specific responses and feedbacks when predicting hydrological fluxes across subarctic ecosystems. Overall, our results suggest that predicted changes in vegetation type and structure in northern regions will have a considerable impact on water partitioning and will vary in a complex way in response to changing precipitation timing, phase and magnitude. / Thesis / Candidate in Philosophy

Fine roots constitute a large part of the primary production in northern (arctic and boreal) ecosystems, and are key players in ecosystem fluxes of water, nutrients and carbon. Data on root dynamics are generally rare, especially so in northern ecosystems. However, those ecosystems undergo the most rapid climatic changes on the planet and a profound understanding of form, function and dynamics of roots in such ecosystems is essential. This thesis aimed to advance our knowledge about fine root dynamics in northern ecosystems, with a focus on fine root phenology in natural plant communities and how climate change might alter it. Factors considered included thickness and duration of snow cover, thawing of permafrost, as well as natural gradients in temperature. Experiments and observational studies were located around Abisko (68°21' N, 18°45' E), and in a boreal forest close to Vindeln (64°14'N, 19°46'E), northern Sweden. Root responses included root growth, total root length, and root litter input, always involving seasonal changes therein, measured with minirhizotrons. Root biomass was also determined with destructive soil sampling. Additionally, aboveground response parameters, such as phenology and growth, and environmental parameters, such as air and soil temperatures, were assessed. This thesis reveals that aboveground patterns or responses cannot be directly translated belowground and urges a decoupling of above- and belowground phenology in terrestrial biosphere models. Specifically, root growth occurred outside of the photosynthetically active period of tundra plants. Moreover, patterns observed in arctic and boreal ecosystems diverged from those of temperate systems, and models including root parameters may thus need specific parameterization for northern ecosystems. In addition, this thesis showed that plant communities differ in root properties, and that changes in plant community compositions can thus induce changes in root dynamics and functioning. This underlines the importance of a thorough understanding of root dynamics in different plant community types in order to understand and predict how changes in plant communities in response to climate change will translate into root dynamics. Overall, this thesis describes root dynamics in response to a variety of factors, because a deeper knowledge about root dynamics will enable a better understanding of ecosystem processes, as well as improve model prediction of how northern ecosystems will respond to climate change.

Fine roots constitute a large part of the primary production in northern (arctic and boreal) ecosystems, and are key players in ecosystem fluxes of water, nutrients and carbon. Data on root dynamics are generally rare, especially so in northern ecosystems. However, those ecosystems undergo the most rapid climatic changes on the planet and a profound understanding of form, function and dynamics of roots in such ecosystems is essential. This thesis aimed to advance our knowledge about fine root dynamics in northern ecosystems, with a focus on fine root phenology in natural plant communities and how climate change might alter it. Factors considered included thickness and duration of snow cover, thawing of permafrost, as well as natural gradients in temperature. Experiments and observational studies were located around Abisko (68°21' N, 18°45' E), and in a boreal forest close to Vindeln (64°14'N, 19°46'E), northern Sweden. Root responses included root growth, total root length, and root litter input, always involving seasonal changes therein, measured with minirhizotrons. Root biomass was also determined with destructive soil sampling. Additionally, aboveground response parameters, such as phenology and growth, and environmental parameters, such as air and soil temperatures, were assessed. This thesis reveals that aboveground patterns or responses cannot be directly translated belowground and urges a decoupling of above- and belowground phenology in terrestrial biosphere models. Specifically, root growth occurred outside of the photosynthetically active period of tundra plants. Moreover, patterns observed in arctic and boreal ecosystems diverged from those of temperate systems, and models including root parameters may thus need specific parameterization for northern ecosystems. In addition, this thesis showed that plant communities differ in root properties, and that changes in plant community compositions can thus induce changes in root dynamics and functioning. This underlines the importance of a thorough understanding of root dynamics in different plant community types in order to understand and predict how changes in plant communities in response to climate change will translate into root dynamics. Overall, this thesis describes root dynamics in response to a variety of factors, because a deeper knowledge about root dynamics will enable a better understanding of ecosystem processes, as well as improve model prediction of how northern ecosystems will respond to climate change.

Northern lakes and ponds are abundant and emit large amounts of the potent climate forcer methane to the atmosphere at rates prone to change with amplified Arctic warming. In spite of being important, fluxes from surface waters are not well understood. Long-term measurements are lacking and the dominant and irregular transport mode ebullition (bubbling) is rarely quantified, which complicate the inclusion of lakes and ponds in the global methane budget. This thesis focuses on variations in emissions on both local and regional scales. A synthesis of methane fluxes from almost all studied sites constrains uncertainties and demonstrates that northern lakes and ponds are a dominant source at high latitudes. Per unit area variations in flux magnitudes among different types of water bodies are mainly linked to water depth and type of sediment. When extrapolated, total area is key and thus post-glacial lakes dominate emissions over water bodies formed by peat degradation or thermokarst processes. Further, consistent multiyear measurements in three post-glacial lakes in Stordalen, northern Sweden, reveal that seasonal ebullition, primarily driven by fermentation of acetate, can be predicted by easily measured parameters such as temperature and heat energy input over the ice-free season. Assuming that most water bodies respond similarly to warming, this thesis also suggests that northern lakes and ponds will release substantially more methane before the end of the century, primarily as a result of longer ice-free seasons. Improved uncertainty reductions of both current and future estimates rely on increased knowledge of landscape-level processes related to changes in aquatic systems and organic loading with permafrost thaw, as well as more high-quality measurements, seldom seen in contemporary data. Sampling distributed over entire ice-free seasons and across different depth zones is crucial for accurately quantifying methane emissions from northern lakes and ponds. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.</p>

Cette thèse a pour but d'améliorer notre compréhension des processus d'altération des roches mafiques silicatées, de la spéciation des éléments et de leur migration dans le milieu boréal (bassin versant de la mer Blanche, Nord-Ouest de la Russie). Les objectifs principales du travail sont i) de caractériser et décrire les mécanismes responsables de l'altération chimique et de la formation des minéraux dans la zone subarctique, ii) évaluer le rôle de la lithologie (environnement granitique versus basaltique) dans la spéciation des éléments traces (ET) et la formation des colloïdes organo-minéraux dans les eaux de surface des bassins versants boréaux des hautes latitudes, et iii) révéler la dépendance de la spéciation des ET en fonction du pH de la solution pour prédire des changements éventuels dans la bioaccumulation des éléments dans les eaux naturelles à cause de leur acidification. L'originalité de cette thèse est de combiner, pour la première fois sur les mêmes objets naturels, les techniques physico-chimique, minéralogique et isotopique afin de mieux comprendre les facteurs qui contrôlent les cycles biogéochimiques des éléments dans les régions subarctiques.

A dissertation examining technological diversity in Indigenous copper metallurgy of the North American Arctic and Central Subarctic. Variation in technological diversity is assessed cross-culturally, chronologically, and geographically. This is accomplished using diversity statistics to characterize Richness and Evenness of spatiotemporal archaeological assemblages of copper artifacts, performing regression analysis to examine the relationship to the results of a GIS Path Distance analysis that models the cost of acquisition of raw or modified copper, and performing chi-square tests of independence to compare assemblages inter-regionally and temporally. Portable X-Ray Fluorescence was utilized to discriminate geologically pure copper from smelted trade copper and a comprehensive typology of copper artifacts was created using a compiled database of known copper artifacts from across the North American Arctic and central Subarctic. Inter-regional, chronological, and cross-cultural differences in technological diversity were identified and implications for Arctic and Subarctic archaeology and technological innovation are discussed.