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Differences in catchment properties may be major drivers in mercury (Hg) cycling. However, the complex interplay of these environmental drivers with Hg speciation, transport, and bioavailability is still not fully understood. To relate Hg speciation to different catchment types (tundra, birch, boreal) and their inherent differences in stream and lake chemistry, we studied Hg speciation and concentrations along a climate and vegetation gradient in sub-arctic northern Sweden (including 18 streams and 8 lakes). We find differences in Hg concentrations aligning with differences in water chemistry between the studied catchment types. All observed differences between catchments align with the gradient in aquatic and terrestrial biological productivity (tundra < birch < boreal). Moreover, we find higher methylmercury (MeHg) concentrations in lakes compared to streams. Overall, our data suggests that dissolved organic matter (DOM) components play a crucial role in (a) the concentrations of total Hg and MeHg in the studied waters (especially allochthonous DOM) and (b) Hg methylation (especially autochthonous DOM).