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Land use changes and forest management practices (eg, prescribed burning, mechanical thinning, etc.) are known to alter ecosystem structure and water quality. However, little is known about their short-and long-term impacts on the biogeochemical cycling of mercury (Hg), a global pollutant that can be bioaccumulated and biomagnified in natural food webs, posing a threat to the health of top predators and humans through fish consumption. Forest ecosystems are significant “sinks” for atmospheric Hg, due to the stomatal uptake by leaves. However, silvicultural practices such as prescribed burning and mechanical thinning interferes with Hg storage in forests and potentially increase its export to downstream environment, where it can be a hotspot for transformation into methylmercury (MeHg) production by different anaerobic microbes. Therefore, a better understanding is needed for the impacts of the forest management on Hg bioaccumulation and its export in forested watersheds.

We conducted this study in a paired experimental watershed (WS77 and WS80) with similar size (~ 150-160 ha) at Santee Experimental Forest on the Atlantic coastal plain of South Carolina managed by US Forest Service Southern Research Station (as shown in Figure 1). WS77 has been managed for various silvicultural treatments, and recently burned in March 2018. WS80 serves as an unburned control. The burn severities were mostly considered as light to moderate burn, with 33% light burned area, 66% moderate burned area, and~ 1% not burned area. We collected surface water samples biweekly before and after burning from the outlet of both gauged