Investigating Changes in Mercury Concentrations in Small-Bodied Fishes Following Mine-Related Flooding in the Canadian Arctic

Abstract

Mercury (Hg) is a globally distributed contaminant that, in its methylated form (methyl mercury (MeHg)), is a potent neurotoxin that bioaccumulates and biomagnifies in aquatic food webs. At high enough exposures, Hg can pose risks to wildlife and human health. While Hg concentrations in aquatic ecosystems have been relatively well-studied in boreal systems, less research has been conducted in Arctic freshwater ecosystems, particularly in Barrenland tundra lakes that are increasingly affected by industrial activity and climate-driven change. This thesis examined the effects of a mine-related flooding event at the Amaruq project site in Nunavut, Canada, on aqueous and biological concentrations of Hg in a series of shallow Arctic lakes. Concentrations of total mercury (THg) and methyl mercury (MeHg) in water, indicators of organic matter (OM) quantity and quality in water, including concentrations of dissolved organic carbon (DOC), humification index (HIX), and specific ultraviolet absorbance (at 254 nm; SUVA) and concentrations of Hg in the tissues of Slimy Sculpin (Cottus cognatus) and Ninespine Stickleback (Pungitius pungitius), were compared: 1) before and after flooding in impacted lakes; and, 2) between flooded and reference lakes in the post-flood years. A series of potential covariates of fish Hg concentrations, including fish age, condition, C: N, δ13C, and δ15N, were also compared between pre- and post-flood years and between flooded and reference lakes.

Aqueous concentrations of THg and MeHg were significantly higher in flooded lakes compared to reference lakes (THg filtered: p = 0.01; MeHg filtered: p < 0.001) in post-flood years. In flooded lakes, concentrations of THg and MeHg were significantly higher in post-flood years compared to pre-flood years (p < 0.05) whereas there were no significant differences among years in reference lakes (p > 0.28). Concentrations of DOC were significantly higher in flooded lakes than in reference lakes in post-flood years (p < 0.001), suggesting increased terrestrial OM inputs. Higher MeHg concentrations and higher % MeHg in water in flooded lakes relative to reference lakes in post-flood years may indicate that terrestrial OM inputs increased net Hg methylation rates, but there was no difference in % MeHg between pre- and post-flood years in the flooded lakes (p < 0.05). Evidence for enhanced methylation post-flood is thus mixed, and more research is necessary.

Fish tissue Hg concentrations were significantly higher in flooded lakes than in reference lakes in the post-flood years for both Slimy Sculpin (p < 0.001 in 2020 and 2021) and Ninespine Stickleback (p = 0.002 in 2021). Within flooded lakes, tissue Hg concentrations in both species were significantly higher in the post-flood years relative to pre-flood years (Slimy Sculpin: F₃,₂₆₁.₇₁ = 14.53, p < 0.001; Ninespine Stickleback: F₃,₁₁.₀₆ = 33.96, p < 0.001). Flooding-induced increases in fish Hg concentrations appeared to be most closely related to concomitant decreases in δ¹³C; δ¹³C ratios were significantly more depleted in post-flood years (Slimy Sculpin: F₃,₃₁₅.₂₈ = 20.11, p < 0.0001; Ninespine Stickleback: F₃,₃₆₇.₃₃ = 5.38, p = 0.001) compared to pre-flood years, and were negatively correlated with fish Hg concentrations (Slimy Sculpin: r = -0.72; Ninespine Stickleback: r = -0.63). These results suggest a shift in basal carbon sources, likely reflecting increased reliance on terrestrially derived, allochthonous OM. The strong correlation between δ¹³C and Hg concentrations indicates that OM dynamics played a central role in driving Hg bioaccumulation in the food web. Other potential covariates of fish Hg concentrations, including relative condition, C:N ratio, and length-at-age, explained less variation in Hg concentrations and/or did not appear to be affected by flooding, although weak negative correlations between Hg concentrations and condition were observed.

While the general patterns observed in this study align with findings from boreal systems subjected to flooding, it is possible that Arctic lakes are uniquely vulnerable to flooding-induced Hg mobilization and increases in fish Hg concentrations due to their low productivity and catchments with permafrost. These characteristics may increase the release of OM and Hg during flooding and increase bioaccumulation of newly released Hg. Despite limitations in sampling design and data availability, results of this study provide the first empirical evidence linking industrial flooding to increases in Hg concentrations in Arctic lakes. Results underscore the need for long-term contaminant monitoring in remote tundra regions where climate warming, industrial development, and permafrost degradation are expected to intensify. Given the cultural importance of fish for northern communities, understanding how environmental disturbances affect Hg bioaccumulation is critical for protecting both ecosystem and human health in the Arctic.