The net exchange of carbon greenhouse gases with high Arctic terrestrial and aquatic ecosystems

• Author / Creator

  Emmerton, Craig A.

• Accelerated climate warming of Canada’s sparsely vegetated high Arctic has resulted in rapid environmental changes including loss of glacial ice, permafrost thaw, decreased snow cover and changing plant communities. These responses are causing mostly unknown changes to the natural cycling of the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4) between northern landscapes and the atmosphere, therefore potentially perturbing global carbon feedbacks. From 2005-2012 at Lake Hazen, Quttinirpaaq National Park, Nunavut, Canada (82ºN), we investigated growing season (June-September) exchange of atmospheric CO2 and CH4 with high Arctic landscapes and aquatic systems, and scaled these ground-level measurements to larger regions to more broadly apply our findings. We used multi-year eddy covariance and static chamber measurements on contrasting high Arctic dry semidesert and meadow wetland landscapes to quantify their net exchange of CO2 and CH4 with the atmosphere. We used these rare high latitude data with ground and satellite productivity measurements (Normalized Difference Vegetation Index; NDVI) to evaluate the effectiveness of upscaling local to regional exchange of CO2. During the growing season, the semidesert landscape was a weak CO2 source to the atmosphere (+0.05 g C m-2 d-1) which was primarily driven by increasing surface soil respiration and moisture. However, rising soil temperatures and environmental conditions suitable for gas diffusion resulted in considerable consumption of atmospheric CH4 (-0.001±0.000 g-CH4 m-2 d-1) in semidesert soils. Greater access to water and resulting plant growth at the nearby wetland resulted in considerable uptake of CO2 (-0.63 g C m-2 d-1) relative to the semidesert during the growing season, rivaling rates observed at Arctic wetlands much further to the south. Emission of CH4 from the wet soils, however, was weak (+0.001±0.000 g-CH4 m-2 d-1) compared to other high Arctic sites likely because of shallow permafrost depths and limited microbial substrate. Our upscaling assessment found that semidesert ground NDVI was low and similar to satellite measurements, however, faint seasonal changes and poor relationships between CO2 exchange and measured NDVI suggested that high Arctic landscapes were too sparsely vegetated currently to accurately upscale ground measurements of productivity to broader regions. We also quantified dissolved CO2 and CH4 concentrations and fluxes of common aquatic systems in the Lake Hazen watershed by collecting water samples and deploying automated systems. Gas concentrations in oligotrophic Lake Hazen were near atmospheric equilibrium and associated closely with carbonate concentrations in the water and turbulence, resulting in near-zero exchange of each GHG with the atmosphere. Lakes higher in the watershed emitted CO2 in relation to heterotrophic signatures, while CH4 emission was low and declined with increasing incidence of dissolved sulfate in water columns. Shoreline ponds bordering Lake Hazen transitioned from weak CO2 sinks during drier conditions, to strong sources of CH4 when flooded by Lake Hazen. Finally, we weighted mean seasonal GHG exchange rates from measured landscapes and aquatic systems by total land cover in the Lake Hazen watershed. We found that despite existence of environments capable of exchanging considerable amounts of GHGs with the atmosphere (e.g., shoreline ponds, meadow wetlands), Lake Hazen watershed cycling of GHGs was dominated by exchange at the expansive, but relatively unproductive, semidesert soils and Lake Hazen. Therefore, we estimated that the watershed effectively transferred net-zero amounts of carbon GHGs (CO2: 20±267, CH4: -0.76±0.80 mg C m-2 d-1) with the atmosphere during the growing season. Continued climate warming in the watershed is expected to support greater vegetation growth and productivity in Lake Hazen. However, poor soil moisture retention and limited nutrient availability in soils and in Lake Hazen may hinder short-term changes in productivity and GHG exchange, at least until plant reproductive success improves, vegetation cover expands and accumulation of organic matter and moisture in soils occurs. These studies report the most northerly eddy covariance data in the literature and also concurrently compare GHG cycling between two contrasting high Arctic ecosystems, which has only been achieved at a small handful of high Arctic site globally. This important baseline data set may be important for the global carbon modeling community which has only rare high Arctic CO2 and CH4 exchange data to validate simulations.

• Subjects / Keywords

  • Arctic
  • Methane
  • Eddy covariance
  • Carbon dioxide

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3BR8MP2Z

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.