Production and Loss of Methylmercury and Loss of Total Mercury from Boreal Forest Catchments Containing Different Types of Wetlands

Abstract

Four terrestrial boreal forest catchments containing different types of wetlands were studied to determine their strength as sources or sinks of methylmercury (MeHg) and total mercury (THg) to downstream ecosystems and to determine if patterns seen in one year were consistent over several years. All catchments were sinks for THg. The wetland type, percentage wetland area (0−25%), or annual water yield did not appear to have a consistent effect on the magnitude of this retention. Wetland areas of the catchments were always net sources of MeHg. Unlike for THg, there were large and consistent differences in the source strength among wetland types for MeHg. These differences appeared to be related to differences in the internal hydrology of the wetlands. All types of wetlands were greater sources of MeHg during years of high water yield, but even during years of low flow all wetland types were sources of MeHg. Thus, we conclude that wetlands are important sites of MeHg production in boreal ecosystems on the long term. Upland areas of catchments were consistently sinks for MeHg, and so whole catchment sink/source values were strongly affected by the percentage of wetland areas within a catchment. Mass balance estimates of MeHg input from wetland areas to a lake indicate that the annual input of MeHg from wetlands is larger than the annual uptake of Hg by fish and is similar to the amount of MeHg produced in the lake. Because of the predictable patterns between terrestrial catch ments in their strength as sources or sinks of MeHg, it is possible to model inputs of MeHg from lake catchments with knowledge of the percentage wetland area in a catchment, the type of wetland contained in a catchment, and the annual water yield of a catchment.

Temporal and spatial distribution and production of dissolved gaseous mercury in the Bay St. François wetland, in the St. Lawrence River, Quebec, Canada

“…However, total Hg levels in the BSF were lower than concentrations measured by Thompson-Roberts and Pick (2000) $300 km upstream the BSF, in water from inland and riverine wetlands of the St. Lawrence River (range from 3 to 34 ng L À1 , mean 12.9± 8.1 ng L À1 ). Total Hg in the BSF was also in the lower end of the range observed in Florida Everglades wetlands (0.3-15.5 ng L À1 , Strober et al, 1995) or in various boreal forest wetlands (range 3.4-13.5 ng L À1 , St. Louis et al, 1996).…”

Tidal exchange of total mercury and methylmercury between a salt marsh and a Chesapeake Bay sub-estuary

“…It is well known that freshwater wetlands are large sinks for THg inputs (StLouis et al 1996;Driscoll et al 1998;Grigal 2003;Galloway and Branfireun 2004), but much less is known about the magnitude of THg sequestration in tidal marsh systems. In general, the net annual fluxes of THg into Kirkpatrick Marsh compare similarly to other studies.…”

Mercury inputs and outputs at a small lake in northern Minnesota

“…The high fraction of sestonic MeHg early in the year may reflect inputs of MeHg on particles from near-shore wetlands but the seston data are limited. Several studies have found that wetlands provide high inputs of MeHg to surface waters (Branfireun et al 1996(Branfireun et al , 1998St Louis et al 1996). However, dissolved MeHg concentrations in near-shore wetlands generally were low (<0.3 ng L À1 ), suggesting that these wetlands may not supply the bulk of the MeHg to the lake.…”

“…Moreover, conclusions about the relative importance of sediments, atmospheric inputs, chemical transformations, and effects of landscape conditions on mercury fluxes and concentrations are not consistent among available mass balances for total mercury (Hg T ) and especially methylmercury (MeHg) in aquatic systems (Henry et al 1995;Iverfeldt et al 1996;Kotnik et al 2002;Meili et al 1991;Sellers et al 2001;Sullivan and Mason 1998;Watras 1994). MeHg inputs to surface waters are thought to depend on catchment conditions (e.g., wetlands versus uplands) (Allan et al 2001;St Louis et al 1996, 1994, microbial activity (Gilmour et al 1992;Compeau and Bartha 1985), hydrology (Branfireun et al 1996), and geology . Lakes are thought not to be net generators of MeHg, but the bioaccumulative nature of MeHg nonetheless results in greatly elevated MeHg concentrations in biota compared to concentrations in lake water.…”