Landscape influences on downstream concentrations of mercury, methylmercury, and dissolved organic carbon in permafrost peatland catchments
Creators: Shewan, Renae L
Publisher: name: University of Alberta Library
Year: 2024

<p>Thawing permafrost in northern regions threatens to increase the downstream delivery of mercury (Hg) and its organic form, methylmercury (MeHg). Permafrost thaw may mobilize large Hg and dissolved organic carbon (DOC) stores from permafrost soils. Once mobilized, inorganic Hg (Hg(II)) can be transformed (methylated) by microbes into MeHg, a neurotoxin that bioaccumulates up aquatic food webs. Permafrost thaw may increase methylation in peatland complexes as elevated and dry permafrost peat plateaus thaw into peatland ecosystems, which can be favorable environments for microbes responsible for methylation. Transport of Hg and MeHg is often facilitated by binding to DOC and as peatlands become increasingly hydrologically connected with continued thaw, increased export of allochthonous DOC, Hg, and MeHg to downstream inland waters may occur. It remains uncertain how these changes will influence downstream concentrations of Hg, MeHg, and DOC as well as the composition of dissolved organic matter (DOM) at a catchment scale within the Taiga Plains of western Canada. <br>
In this study, I completed three sampling campaigns of 93 streams throughout the Northwest Territories and northern Alberta located within the Taiga Plains of western Canada (summer 2021 and 2022). The study area was characterized by extensive peatlands and represented a permafrost gradient with more southern catchments characterized by no permafrost or sporadic permafrost to the more northern catchments which were underlain by extensive discontinuous permafrost. Streams were sampled for Hg, MeHg, and DOC concentrations and DOM composition indices to: 1) Investigate the role of catchment characteristics, including landcover, size, climate, and seasonality (June vs July vs August) as it relates to flow conditions on downstream concentrations; 2) use a developed statistical model to estimate concentrations of unsampled streams in the study region under current and future climate conditions; and 3) understand how the composition of DOM in streams relates to downstream concentrations of Hg and MeHg. <br>
Random forest models were used to investigate the role of catchment characteristics in driving downstream concentrations. Mean annual temperature (MAT) and pH were established as the two most important variables for predicting MeHg and DOC concentrations, demonstrating the influence of peatlands and permafrost extent on these parameters. Turbidity was the most important variable for predicting Hg concentrations, indicating the importance of colloidal or particulate bound Hg in regulating concentrations. Using these models to predict unsampled streams within the region resulted in an overall trend of higher concentrations of Hg, MeHg, and DOC in more southern permafrost regions compared to more northern streams underlain by more extensive permafrost. <br>
More aromatic DOM was associated with higher flow and low electrical conductivity likely highlighting the influence of organic surface and shallow subsurface flow paths in contributing aromatic DOM in these catchments.  Bulk DOC concentrations were highly correlated with MeHg concentrations while Hg preferentially associated with DOM of high aromatic composition. <br>
As northern regions continue to warm, our findings suggest that concentrations of Hg, MeHg, and DOC will increase within the more northern catchments of the study region. Understanding factors that influence Hg export in northern rivers is important to anticipate and monitor the influence of climate change on northern aquatic ecosystems for the local communities which rely on them. </p>