The Rise and Fall of Mercury Methylation in an Experimental Reservoir

Abstract

For the past 9 years, we experimentally flooded a wetland complex (peatland surrounding an open water pond) at the Experimental Lakes Area (ELA), northwestern Ontario, Canada, to examine the biogeochemical cycling of methyl mercury (MeHg) in reservoirs. Using input-output budgets, we found that prior to flooding, the wetland complex was a net source of approximately 1.7 mg MeHg ha(-1) yr(-1) to downstream ecosystems. In the first year of flooding, net yields of MeHg from the reservoir increased 40-fold to approximately 70 mg MeHg ha(-1) yr(-1). Subsequently, annual net yields of MeHg from the reservoir declined (10-50 mg MeHg ha(-1) yr(-1)) but have remained well above natural levels. The magnitude and timing of Hg methylation in the flooded peat portion of the wetland reservoir were very different than in the open water region of the reservoir. In terms of magnitude, net Hg methylation rates in the peat in the first 2 years of flooding were 2700 mg ha(-1) yr(-1), constituting over 97% of the MeHg produced at the whole-ecosystem level. But in the following 3 years, there was a large decrease in the mass of MeHg in the flooded peat due to microbial demethylation. In contrast, concentrations of MeHg in the open water region and in zooplankton, and body burdens of Hg in cyprinid fish, remained high for the full 9 years of this study. Microbial activity in the open water region also remained high, as evidenced by continued high concentrations of dissolved CO2 and CH4. Thus, the large short-term accumulation of MeHg mass in the peat appeared to have only a small influence on concentrations of MeHg in the biota; rather MeHg accumulation in biota was sustained by the comparatively small ongoing net methylation of Hg in the flooded pond where microbial activity remained high. In large reservoirs, where the effects of wind and fetch are greater than in the small experimental reservoir we constructed, differences can occur in the timing and extent of peat and soil erosion, effecting either transport of MeHg to the food chain or the fueling of microbial activity in open water sediments, both of which could have important long-term implications for MeHg concentrations in predatory fish.

Freshwater discharges drive high levels of methylmercury in Arctic marine biota

“…Research in the Experimental Lakes Area of Canada showed a 40-fold increase in aqueous MeHg concentrations following large-scale flooding and increased biological concentrations that persisted for more than a decade (4,14,15). Here we discuss the magnitude of potential changes driven by hydroelectric development compared with future climate-driven effects.…”

“…Flooding associated with reservoirs causes a long-term increase in MeHg production resulting from decomposing organic matter and changes in the geochemical environment that stimulate methylating bacteria (4,14,15). With increasing demand for renewable energy,…”

Recent advances in the study of mercury methylation in aquatic systems

“…Methylation has been extensively studied in boreal lakes and wetlands (Beijer and Jernelov 1979;Regnell 1994;Matilainen and Verta 1995;Pak and Bartha 1998;Benoit et al 2002;Eckley and Hintelmann 2006), notably at the Experimental Lakes Area in northwestern Ontario, where much of the early work surrounding factors influencing methylation was done (Furutani and Rudd 1980;Ramlal et al 1986Ramlal et al , 1993Miskimmin et al 1992;Kelly et al 1997;St. Louis et al 2004;Harris et al 2007).…”

Are Arctic Ocean ecosystems exceptionally vulnerable to global emissions of mercury? A call for emphasised research on methylation and the consequences of climate change

“…The latter process leads to the release of Hg (e.g. see Klaminder et al [36] ) as well as the creation of MeHg (St. Louis et al [37] ). In particular, recently deposited Hg may be more susceptible to release and uptake by freshwater fish.…”