Importance of Wetlands as Sources of Methyl Mercury to Boreal Forest Ecosystems

Abstract

Wetlands were found to be important sources of methyl mercury to the boreal forest ecosystem. Yields of methyl mercury were about 26–79 times higher from wetland portions of catchments (1.84–5.55 mg∙ha−1∙yr−1) than from purely upland areas (0.07 mg∙ha−1∙yr−1). Mass-balance estimates using methyl mercury inputs in wet deposition and outputs in runoff water indicated that purely upland catchments and lakes were sites of methyl mercury retention or demethylation, while catchments with wetland areas were sites of net methyl mercury production. These observations may explain the high concentrations of mercury in fish taken from lakes that are high in colour because they receive water from wetlands. There was no relationship between the concentration of total mercury and the concentration of methyl mercury in runoff water. Total mercury yields were low from a wetland-dominated catchment, higher from a combination upland/riparian wetland catchment, and highest from a purely upland catchment. The opposite was true for methyl mercury yields from these same catchments. This indicates that environmental factors other than total mercury concentration are controlling the production and loss of methyl mercury from catchments.

Recent advances in the study of mercury methylation in aquatic systems

“…Louis et al 2004;Harris et al 2007). The importance of wetlands as sites of methylation was first determined in boreal wetlands (Zillioux et al 1993;St. Louis et al 1994Branfireun et al 1998Branfireun et al , 1999Tjerngren et al 2012b) and has been recently observed in wetlands in the northern Great Plains (Hoggarth et al 2015), southern Louisiana (Hall et al 2008), the high Arctic , in agricultural and non-agricultural wetlands (MarvinDiPasquale et al 2014), and temperate forests (Selvendrian et al 2008).…”

“…As a result, organisms in larger lakes with greater catchment areas may be less subject to the deleterious effects of methylated Hg, as photodemethylation processes may actively be breaking down this highly bioavailable Hg species. However, catchment wetlands are believed to be the primary location of methylation (St. Louis et al 1994;Tjerngren et al 2012a;Braaten et al 2014), suggesting that such effects may be mitigated if the catchment wetland area is sufficiently large.…”

Survey of mercury in boreal chorus frog (Pseudacris maculata) and wood frog (Rana sylvatica) tadpoles from wetland ponds in the Prairie Pothole Region of Canada

“…A "clean-hands-dirty-hands" technique was used to collect unfiltered water samples into precleaned glass bottles for Hg analysis (St. Louis et al 1994). Hg samples were preserved by acidification using trace metal grade HCl (at 0.2% and 0.4% by volume for THg and MeHg, respectively).…”

Widespread Effects of Climatic Warming on Freshwater Ecosystems in North America

“…Increased light penetration causes increased losses of methyl mercury by photolytic conversion to Hg 0 and subsequent loss to the atmosphere (Sellers et al, 1996). Drying of catchments may greatly decrease exports of mercury to freshwaters, since it is complexed with DOC (Miskimmin et al, 1992;St Louis et al, 1994). In-lake methylation would also be expected to increase as DOC declines (Miskimmin et al, 1992).…”