Mercury cycling in a remote boreal drainage basin

The consumption of freshwater fish and seafood is the main source of mercury (Hg), a widespread neurotoxic pollutant, in humans, a fact which has sparked decades of research on Hg cycling in aquatic systems. More specifically, the formation and bioaccumulation of methylmercury (MeHg) is of particular importance because it biomagnifies through aquatic food webs, resulting in relatively high levels in predatory fish despite typically low concentrations in the surrounding water. The main goal of this thesis was to assess how various watershed-level processes affect Hg bioaccumulation and biomagnification through freshwater food webs across the relatively pristine Attawapiskat Drainage Basin (ADB) in the remote Far North of Ontario. This watershed overlaps with the mineral-rich region known as the “Ring of Fire” which is expected to be heavily developed in the coming decades, likely altering the physico-chemical environments of surrounding lakes and rivers. In total, 58 lakes and river sites across the ADB were sampled for surface water quality, aquatic macroinvertebrates, and fish from 2014 to 2016. Water samples were analyzed for 39 chemical parameters including total Hg (THg; the sum of all Hg species) and MeHg concentrations ([MeHg]). Biotic samples were analyzed for [THg] and/or [MeHg], as well as carbon and nitrogen stable isotope ratios, which are indicative of an animal’s food web position. In Chapter 1 of this thesis, I provide an overview of our current knowledge on Hg cycling in aquatic systems of the boreal region. In Chapter 2, I present an extensive assessment of the chemical, physical, and ecological gradients across the ADB, and an analysis of the relationships between Hg and these environmental gradients. I determined that less productive systems with higher concentrations of dissolved organic matter (DOM) had higher aqueous and biotic [Hg]. In Chapter 3, I examined how changes in the quality of DOM across the ADB relate to [Hg] in water and biota. Findings from this study suggest that more labile DOM complexes enhance MeHg bioaccumulation into food webs, while systems with more humic and aromatic DOM had higher aqueous total [Hg]. The fourth chapter critically examined the speciation of Hg in fish from across the ADB and showed substantially lower percentages of MeHg (relative to total Hg) in muscle of smaller-sized fish, particularly those which feed on littoral-based food webs and had higher lipid content in their tissue. These novel findings challenge the general assumption, used in many biomagnification studies and consumption guidelines, that all fish muscle tissue has > 95% MeHg. Finally, in Chapter 5, I discuss the implications of my research for subsistence fishers, specifically those from remote communities, where freshwater fish are important for both culture and sustenance. Here I developed some preliminary approaches to better communicating the risks and benefits of consuming fish when presenting fish tissue contaminant results in remote northern communities, including those in and around the ADB. Understanding the major influences on MeHg cycling is vital to properly monitoring the effects of industrial development (e.g., the Ring of Fire development) and climate change, which can greatly alter the physico-chemical environment of surrounding lakes and rivers. The results from my thesis indicate significant physical and chemical differences between waters in the two ecozones (i.e., the Boreal Shield and Hudson Bay Lowlands) across the ADB, presumably due to changes in bedrock geology and riparian characteristics. No other study, to my knowledge, has explored the effects of landscape position and the associated changes in physico-chemical characteristics on Hg bioaccumulation, speciation, and biomagnification on such a large scale. My findings demonstrate that monitoring programs will need to effectively track changing nutrient concentrations, DOM characteristics, and Hg bioaccumulation patterns that vary across large spatial gradients.