From Drought to Deluge: Implications of variable hydrologic connectivity on lake ecosystem functions in the Boreal Plains of Western Canada

• Author / Creator

  Pugh, Emily Audrey

• Presence and functions of lakes are dependent on hydrological connectivity to the terrestrial landscape. Inter-annual wet-dry periods, and their amplification through climate change, can influence hydrological connectivity and affect the delivery of water and solutes from various terrestrial sources. Altered hydrological connectivity and delivery of water to lakes influences lake water residence times and the role of within-lake processes, but can also influence the risk of a lake to contract and even completely disappear. This thesis examined the influence of inter-annual wet-dry periods and subsequent hydrologic connectivity on different ecosystem functions of 34 lakes on the Boreal Plains in Western Canada. In Chapters 2 and 3, I focused on the influence of inter-annual wet-dry periods on hydrological connectivity of terrestrial sources and subsequently the concentration and chemical composition of lake dissolved organic carbon (DOC), a key water quality parameter that influences lake ecosystem functioning. In Chapter 4, I focused on how lake extent and water level responded to inter-annual wet-dry periods and show how lakes can have different combinations of resistance and resilience to dry periods, whilst identifying watershed and lake attributes which can increase lake susceptibility to terrestrialisation.
  In Chapter 2, I found large spatial variability of lake DOC concentrations and aromaticity (as indicated by specific UV absorbance at 254 nm – SUVA254) that was closely linked to surficial geology and thus wetland connectivity in the watershed. The aromaticity of DOC was further influenced by lake water residence time and the extent of within-lake processing of aromatic DOC via photodegradation. Large inter-annual variability in lake DOC characteristics (concentration and composition) was attributed to interactions between surficial geology and sub-humid climate. Inter-annual variability in DOC concentration had low synchronicity among lakes, with patterns of variability linked to surficial geology and short-term precipitation shifting connectivity of terrestrial DOC sources. However, inter-annual variability in DOC composition had high synchronicity among lakes, where lake water residence time related to longer-term precipitation, and the extent of within-lake DOC degradation. This study shows that lake DOC characteristics vary spatially, but many lakes on the Boreal Plains are also highly sensitive to inter-annual wet-dry periods.
  In Chapter 3, end-member mixing model analysis showed that lake DOC characteristics did not conform to conservative mixing of terrestrial sources, with lower observed DOC concentrations and A254 than modelled, suggesting net DOC and A254 losses in terrestrial end-members along the soil-stream-lake continuum. Losses were associated with lakes in coarse and fine hummocky watersheds. However, we found that in fine hummocky lakes with large proportions of connected wetland and fine plains watersheds, observed DOC concentrations exceeded modelled, suggesting autochthonous production of DOC. On an inter-annual basis, losses in A254 were greater during dry years, when lake water residence times are longer and within-lake photodegradation processes enhanced. This study highlights that the DOC pool of Boreal Plains lakes reflects mixing of water from terrestrial sources, but also processes of within-lake degradation of DOC which may be influenced by inter-annual wet-dry periods, and in lakes with high nutrients and productivity, autochthonous production of DOC.
  In Chapter 4, I found large spatial variability in lake extent responses to dry periods, with many lakes exhibiting transient or long-term reduction in lake area. Lakes located in higher landscape positions and those with a lower proportion of connected wetland area in the watershed showed quicker responses to dry periods (i.e. shorter lag times), thus lake extent contracted relatively quickly. Resistance and resilience of lake extent to dry periods, i.e. the ability to withstand change and to return to pre-dry period extents, were highest for lakes with greater wetland connectivity and for lakes with straight-sided basin morphology. This study suggests that isolated lakes, with lower proportions of connected wetland in the watershed and lake beds that slope towards the lake centre with shallower depths of loose sediment are most susceptible to long-term terrestrialisation processes.
  Overall, in this thesis, I analyzed both spatial and inter-annual variability of lake ecosystem functions at a landscape scale across the Boreal Plains, my results identified watershed and lake attributes (HRA, wetland connectivity, lake water residence time, lake basin morphology) which can be used to predict lakes on the Boreal Plains where the most rapid changes in lake function can be expected.

• Subjects / Keywords

  • Ecohydrology
  • Wetlands
  • Surficial Geology
  • Terrestrialisation
  • Dissolved Organic Carbon

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-mz5w-wq09

• 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.