Assessing the Mining Effluent Impacts on Mercury Dynamics in Boreal Lakes

Abstract Mining wastewater and leakages into freshwater systems are a growing global concern, yet the impacts of bioleaching in open-pit mining on lake food webs and mercury dynamics remain understudied. This study examined three boreal lakes in eastern Finland: Jormasjärvi (primarily previously mining-impacted), Nuasjärvi (currently impacted), and Kiantajärvi (control). Food-web structure was analysed using stable carbon and nitrogen isotopes, while total mercury (THg) bioaccumulation in fish and biomagnification across the food web were examined using linear regressions. Jormasjärvi showed a simplified and less diverse pelagic food web. Top predators relied more on littoral resources, and key pelagic species, such as the acid-sensitive Mysis relicta , had disappeared from Jormasjärvi after mining impact. THg content in top predators were highest in Jormasjärvi where 20% of the analysed fillets exceeded mercury safety limits. On average, Jormasjärvi perch ( Perca fluviatilis ) over 27 cm, pikeperch ( Sander lucioperca ) over 40 cm and pike ( Esox lucius ) over 78 cm surpassed these thresholds (pike: 1.0 mg/kg; others: 0.5 mg/kg). Despite elevated THg levels, the trophic magnification slope and THg baseline in Jormasjärvi did not differ significantly from the control lake. In contrast, THg magnified fastest in Nuasjärvi where lowest THg baseline was observed, likely reflecting differences in water chemistry influencing Hg uptake. Overall, the findings suggest that bioleaching-related mining can increase Hg concentrations in top predators by altering food-web structure and energy pathways. However, Hg dynamics are highly context-dependent. The study highlights the importance of collecting baseline food-web data before mining begins to enable accurate before-and-after comparisons and better assess environmental impacts.