by Thomas Wu

The past half century of climate change has seen the Canadian boreal subarctic experience warming air temperatures, more thunderstorms, and changes to local vegetation, resulting in increased forest fires. This has accelerated permafrost thaw, converting boreal peatland forests into new wetlands and small lakes, completely altering water quality and how water flows throughout the landscape. Large areas of the Canadian subarctic with permafrost are now vulnerable to rapid thawing, especially the sporadic permafrost (i.e., permafrost that occurs in isolated fragmented patches beneath 10% - 50% of the landscape), of the Dehcho region (southwestern Northwest Territories, Canada).

The Dehcho region’s permafrost peatlands are home to globally significant carbon sinks in the form of peat (partially decomposed matter preserved by the cold over millennia). Climate warming and wildfire-stimulated permafrost thaw and altered hydrology could release much of this ancient organic material into local lakes as highly coloured dissolved organic carbon, reducing water clarity, and staining affected waters a deep yellow or brown colour in a process referred to as 'lake browning’. Since brownified lakes are associated with microbes that can recycle incoming carbon as greenhouse gas emissions, brownification has substantial implications for the current global climate crisis. However, much about lake brownification in the circumpolar north, including when it began and how prevalent it is, remains uncertain.

My project aimed to reconstruct a comprehensive paleoenvironmental history of light/clarity shifts in a small lake (unofficially referred to as SC11) in the Dehcho, using analyses of diatom (algae of the class Bacillariophyceae) subfossils extracted from lake sediments. Since lakes naturally accumulate material from the surrounding landscape and in-lake as sediment at the lake bottom, lake sediments act as a record of environmental conditions at time of burial, with recent events preserved in surficial sediments, while deeper sediments correspond to older deposits. Some materials buried at different sediment depths, called paleolimnological proxies, can then be used to construct a timeline of past environmental changes, based on the depth they were found in, and their known properties. One type of paleolimnological proxy are diatom subfossils, which are the remains of a type of microscopic algae. They make especially ideal ‘paleo-indicators’ of past environmental changes like brownification, due to their species-specific sensitivity to changes in water quality, such as clarity and pH, and their siliceous (i.e., glassy) cell walls that preserve well in sediments.

Preliminary comparisons of diatom assemblages from sediment intervals in SC11 corresponding to ~1850, ~1950, and 2018 (done previously by EUC PhD candidate Kristen Coleman) indicated that sometime after the 1950s, diatom assemblages in SC11 and other lakes became dominated by species associated with low clarity waters. My research seeks to identify a more precise date of when this ecological succession shift occurred, document its nature and character, and determine if the timing of these community shifts coincided with other environmental markers of climate-change stimulated extreme.

Conclusions:

            My results showed that the diatom assemblages of SC11 from at least the ~1850s, and possibly before, were originally dominated by only a few diatom taxa, largely those with large-sized cells, and or those that are dependent on or attached to vegetation on the lake bottom. These species form algal clumps and mats which require high clarity waters to survive. Over time, as the Canadian boreal subarctic warmed due to climate change, the assemblages at SC11 became more diverse, supporting a wider number of species, but otherwise remained firmly dominated by already present diatom species, albeit at a slightly lower abundance. However, circa 1930s, these diatom taxa associated with high water clarity began to diminish in abundance, and most abruptly disappeared by the 1950s, if not earlier. They were replaced by a few small-celled diatom species known for surviving even in harsh low clarity waters. These ‘low-clarity’ taxa continued to be the primary constituent species of SC11’s diatom assemblages, up to the surface sediments corresponding to ~2018, with their abundance only increasing over time. Notably, the beginning of this transition to a low-clarity diatom assemblage coincided with or occurred immediately after an anomalous geochemical signature that roughly matches the timing of a forest fire in the area, believed to have occurred around the ~1920s.

My research results clarified exactly when (~1930s) brownification occurred and revealed that it progressed relatively rapidly (<20 years). Brownification may be an abrupt irreversible state change that persists long (>50 years) after the initial perturbation. Extreme environmental disturbances, such as climate-change enhanced wildfires, and its effects on permafrost thaw and associated hydrological changes may be an important trigger of light/clarity shifts in discontinuous permafrost peatland lakes. As the Canadian boreal subarctic continues to warm, brownification in important water resources may become another important consideration when assessing the next steps in mitigating the impacts of climate change.

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Thomas Wu is a Geography MSc student in the Faculty of Environmental and Urban Change. His research deals with paleolimnological assessments of climate change and wildfire-stimulated permafrost thaw as drivers of long-term colour shifts in small shallow discontinuous permafrost peatland lakes in the Northwest Territories, Canada. He is one of the lead authors of the Frontiers article on Increasing fish biodiversity in high elevation Albertan lakes in response to global environmental change over the past 50 years. He is a project member of the Dehcho Collaborative on Permafrost and is part of Professor Jennifer Korosi's Limnology and Paleoenvironmental Research Group.