Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Abstract

Changes in sulfate (SO 4 2−) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO 4 2− deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO 4 2− export from a peatland-dominated catchment as a function of changing SO 4 2− deposition in a long-term (1998− 2011), whole-ecosystem, control-impact experiment. Annual SO 4 2− deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO 4 2− additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO 4 2− addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO 4 2− inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO 4 2− deposition declines.

Methylmercury Export From a Headwater Peatland Catchment Decreased With Cleaner Emissions Despite Opposing Effect of Climate Warming

“…Gerson and Driscoll (2016) also observed no correlation between summertime MeHg concentration and $${{\text{SO}}_{4}}^{2-}$$ in the inflow and outflow of a temperate lake watershed over a 10‐year period of decreasing $${{\text{SO}}_{4}}^{2-}$$ and Hg deposition. As such, current $${{\text{SO}}_{4}}^{2-}$$ deposition rates are not controlling streamwater MeHg as would have been during past higher $${{\text{SO}}_{4}}^{2-}$$ deposition rates that stimulated elevated streamwater MeHg prior to clean air legislation (McCarter, Sebestyen, et al., 2022).…”

“…The solid lines are the loess smoothed lines illustrating the results of the Mann‐Kendall tests to illustrate the average temporal trends. Note the gap in the MeHg record due to a resolved analytical contamination issue (McCarter, Sebestyen, et al., 2022).…”

“…Peatlands are often disproportionately large sources of MeHg to surface waters relative to upland mineral soils (Branfireun et al., 1998; Lam et al., 2022; Skyllberg et al., 2003; Woerndle et al., 2018). The net export of MeHg from peatlands to surface waters is a balance between Hg methylation and demethylation (Kronberg et al., 2018; Tjerngren, Meili, et al., 2012), MeHg solubility (Skyllberg, 2008), and the hydrological transport of Hg from a peatland to a stream (McCarter, Sebestyen, et al., 2022). In peatlands, wet atmospheric Hg deposition is usually lower than Hg associated with litterfall (Grigal et al., 2000; Woerndle et al., 2018) but is often more readily available to resident microbial communities (Hsu‐Kim et al., 2013), likely resulting in quicker incorporation into the Hg cycle.…”

“…The bioaccumulating neurotoxin methylmercury (MeHg) is a contaminant of global concern linked to adverse human health conditions (O'Connor et al., 2019). Mercury mobilization, either as MeHg or total‐Hg (THg), from uplands and wetlands to downstream water bodies is controlled by interacting hydrological and biogeochemical processes that affect both concentrations and fluxes (Bishop et al., 2020; Branfireun et al., 2020; McCarter, Sebestyen, et al., 2022; O'Connor et al., 2019; Woerndle et al., 2018). Peatlands are often disproportionately large sources of MeHg to surface waters relative to upland mineral soils (Branfireun et al., 1998; Lam et al., 2022; Skyllberg et al., 2003; Woerndle et al., 2018).…”

“…(2023) observed a strong correlation between atmospheric Hg and climate signals with Hg concentrations in Arctic char. Moreover, elevated atmospheric $${{\text{SO}}_{4}}^{2-}$$ deposition to peatlands has been linked to increased MeHg concentrations in receiving streamwater (Jeremiason et al., 2006; McCarter, Sebestyen, et al., 2022) but the effect is rapidly reversible once $${{\text{SO}}_{4}}^{2-}$$ additions decline (McCarter et al., 2017; McCarter, Sebestyen, et al., 2022). While both $${{\text{SO}}_{4}}^{2-}$$ and Hg deposition have decreased in response to clean air legislation, climate change likely affects the ecohydrological and biogeochemical process that control Hg transformation and movement through catchments (Bishop et al., 2020; Gerson & Driscoll, 2016; Yang et al., 2016).…”