|dc.description.abstract||Mercury (Hg) is a global pollutant that in its organic and more toxic form, methyl Hg (MeHg), can cause detrimental health effects on fish, wildlife, and humans. In lake ecosystems, concentrations of Hg ([Hg]) in fish reflect a complex outcome of interactions among variables that act at different ecological and spatial scales. From catchment and lake characteristics to fish and food web ecology, numerous processes interact to influence Hg uptake, methylation, bioaccumulation, and biomagnification, and ultimately, [Hg] in fish. As a result, fish [Hg] often vary among lakes and species, and among individuals of the same population. While factors that affect fish [Hg] in lakes are well studied and generally well understood, the relative importance and influence of these factors can vary among populations, species, ecosystems, and geographical regions, making it difficult to generalize and predict variability in fish [Hg].
Generally, little is known about controls of fish [Hg] in northern regions, where ecosystems and biota can be profoundly influenced by environmental perturbations, and where fish provide a critically important subsistence food source. In the subarctic Dehcho Region, Northwest Territories, Canada, fish [Hg] vary widely among lakes due to largely unknown mechanisms, and have, on occasion, triggered both general and site-specific consumption notices (sometimes referred to as consumption advisories or contaminant advisories). These advisories, and the lack of understanding about sources of among-lake variation in fish [Hg], are of concern for First Nation communities that rely on wild-caught fish as a subsistence food source. This thesis is aimed to address knowledge gaps regarding causes of among-lake variation in fish [Hg] in the Dehcho Region, and to improve the current understanding of what variables and processes control fish [Hg] in remote and understudied subarctic lakes.
In Chapter 1, I provide a detailed overview of freshwater fish and fisheries, Hg cycling in the environment, and controls of [Hg] in fish, with a focus on lakes, especially in the Canadian subarctic. I then describe my study area, the Dehcho Region, and elaborate on the importance of wild-caught freshwater fish for local communities and Hg-related fish safety concerns, which frames my specific objectives for each data chapter (i.e., Chapters 2, 3, and 4). My data chapters, and associated analyses and findings, use comprehensive geospatial, environmental, and biological data that were collected from eleven lakes in the Dehcho Region over the period of seven years, from 2013 to 2019.
In Chapter 2, I conducted an ecosystem-scale study to investigate causes of among-lake variability in fish [Hg] in the study area. I investigated how interacting processes at scales ranging from whole catchments to individual organisms influence [Hg] in Northern Pike (Esox lucius), a predatory fish of widespread subsistence and commercial importance. Findings indicated that more than four-fifths of the among-lake variability in [Hg] in Northern Pike was explained by fish growth rates (negative) and concentrations of methyl Hg ([MeHg]) in benthic invertebrates (positive). Fish growth rates and [MeHg] in benthic invertebrates were, in turn, influenced by concentrations of dissolved organic carbon and MeHg in water, and total Hg in sediment. These in lake variables were ultimately driven by catchment characteristics. Lower-elevation lakes in relatively larger catchments with proportionally more temperate/subpolar needleleaf and mixed forests had Northern Pike with higher [Hg].
In Chapter 3, my goal was to understand causes of among-lake variability in fish growth rates in the study area. I investigated how fish trophic ecology (i.e., trophic level and foraging area) and lake-catchment interactions influence growth rates in Northern Pike. Growth was slower in lakes that were subject to greater inferred catchment influence, which were lakes located at lower elevation and in relatively larger catchments with proportionally greater forest cover. Northern Pike in lakes subject to greater inferred catchment influence also had more depleted carbon stable isotope ratios. As carbon stable isotope ratios in benthic invertebrates were also more depleted in lakes that were subject to greater inferred catchment influence, slower Northern Pike growth in lakes with greater catchment influence may reflect the fact that terrestrially derived organic matter has lower nutritional value and less bioaccessibility relative to internally derived organic matter.
In Chapter 4, I investigated biomagnification of Hg through lake food webs in the study area. I examined how estimates of Hg biomagnification rates can be affected by the species of mercury (total Hg vs methyl Hg) that is measured in fish and used in biomagnification models. I also investigated relationships between rates of Hg biomagnification and [Hg] in resident fish, and how rates of Hg biomagnification are related to ecosystem characteristics, including in-lake and in-catchment variables. Results showed that uncertainty around estimates of Hg biomagnification rate was lower when estimated using measured concentrations of MeHg, the biomagnifying form of Hg. Rates of Hg biomagnification were positively but not significantly related to [Hg] in resident fish used for subsistence. Rates of Hg biomagnification were on average higher than previously reported for lakes at lower latitudes, and were higher in lakes surrounded by catchments with proportionally greater forest cover.
In Chapter 5, I synthesized findings that were discussed in Chapters 2 through 4. Together, results from this thesis show that mercury levels in resident fish are influenced by biological processes (growth rates in fish, Hg concentrations in primary consumers, and biomagnification of Hg through food webs), that are, in turn, influenced by lake-catchment interactions (higher concentrations of DOC and both total and methyl Hg in lakes located in lower-elevation catchments with relatively larger size and proportionally more forest cover). Given that catchment physical attributes (elevation, relative size, and proportional forest cover) appeared to result in changes in abiotic and biotic ecosystem compartments in downstream lakes and ultimately influenced fish [Hg], findings provide invaluable direction for prioritizing lakes for monitoring and management, especially because physical characteristics of lakes and catchments can be sensed remotely. Ongoing monitoring and assessment of fish [Hg] are necessary in northern lakes due to profound effects of climate warming on northern latitudes and the heavy reliance of northern communities on wild-caught freshwater fish. This thesis reveals insights that address knowledge gaps regarding causes of among-lake variability in fish [Hg] in the Dehcho Region and improve the current and general understanding of fish [Hg] controls in subarctic lakes.||en