Mercury bioaccumulation in relation to changing physicochemical and ecological factors across a large and undisturbed boreal watershed

Within a drainage basin, the position of a lake or river can greatly affect its limnological and ecological characteristics. These properties influence the cycling of mercury (Hg), a neurotoxic and bioaccumulative metal prevalent in remote northern watersheds. In this study, we examined how 43 physical, chemical, and ecological endpoints change across 58 lake and river sites within an undisturbed boreal watershed in Ontario, Canada, and assessed the influence of these endpoints on aqueous and biotic Hg concentrations ([Hg]). We found that several physicochemical parameters, but few ecological factors, varied in systematic patterns across the watershed. Overall, [Hg] in water and some fish increased in systems with decreasing landscape positions. Aqueous and biotic [Hg] were strongly related to dissolved organic carbon and nutrient concentrations. Biotic [Hg] was lower in higher-nutrient systems, potentially due to biodilution, but higher in systems with more nitrates + nitrites, suggesting an indirect relationship between Hg and nitrogen cycling. This study is the first to assess patterns of [Hg] across an entire intact watershed and provides valuable results for a region anticipating substantial industrial development.