Grantee Research Project Results

Final Report: Methylmercury Sources to Lakes in Forested Watersheds: Has Enhanced Methylation Increased Mercury in Fish Relative to Atmospheric Deposition?

EPA Grant Number: R827630
Title: Methylmercury Sources to Lakes in Forested Watersheds: Has Enhanced Methylation Increased Mercury in Fish Relative to Atmospheric Deposition?
Investigators: Swain, Edward B. , Engstrom, Daniel , Nater, Edward , Jeremiason, Jeff , Cotner, Jim , Almendinger, James E.
Institution: Minnesota Pollution Control Agency
EPA Project Officer: Chung, Serena
Project Period: October 1, 1999 through September 30, 2002 (Extended to September 30, 2003)
Project Amount: $847,690
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Watersheds , Heavy Metal Contamination of Soil/Water , Water , Safer Chemicals

Objective:

Mercury (Hg) contamination of fish is controlled not just by atmospheric mercury deposition, but also by the efficiency that deposited inorganic mercury is converted to methylmercury (MeHg), the form that accumulates in the aquatic food chain. We hypothesized that mercury concentrations in fish in sensitive ecosystems have been exacerbated relative to increases in atmospheric mercury deposition because of enhanced mercury methylation occurring in lake sediments or associated wetlands. We hypothesized that a variety of anthropogenic ecosystem changes that became widespread in the post-World War II era could be responsible for enhanced methylation, including: increased sulfate deposition, increased nutrient loads (N, P), increased loading of mercury to wetlands because of soil disturbance, and other unknown processes. The specific objectives of this research project were to: (1) conduct microcosm (wetland and lake sediment) and whole wetland experiments to elucidate methylation enhancing processes; (2) establish the relative importance of atmospheric, in-lake, and wetland sources of MeHg to lakes in forested watersheds; (3) determine the net retention and source strength of different wetland types; and (4) delineate and classify wetlands in other watersheds and estimate Hg and MeHg watershed loads to the receiving lakes.

Summary/Accomplishments (Outputs/Outcomes):

Background and Experimental Wetland Design

Wetlands are thought to be major sites of Hg methylation and are often the dominant MeHg source to surface waters containing fish. Sulfate-reducing bacteria are thought to be the primary methylators of Hg. Previous laboratory and field mesocosm studies have demonstrated an increase in MeHg levels in sediment and peatland porewaters following additions of sulfate. In this ecosystem-scale study, sulfate was sprayed onto half of a 2-ha peatland at the Marcell Experimental Forest in northeastern Minnesota, increasing the annual sulfate load by approximately four times relative to the control half of the wetland. Sulfate was added on four separate occasions during 2002 and delivered via a sprinkler system constructed on the lower half (1.0 ha) of the wetland. MeHg levels were monitored in porewater and at outflow from the wetland. MeHg also was measured at outflows of several other wetlands near the experimental wetland.

Results of Sulfate Addition to Wetland

Prior to the first sulfate addition, porewater MeHg concentrations (filtered, 0.45 µm) were not statistically different between the control (0.47 ± 0.10 ng·L^-1, n=12; mean ± one standard error) and experimental 0.52 ± 0.05 ng·L^-1, n=18) halves. Two weeks after the first addition in May 2002, MeHg porewater concentrations in the control half had not changed significantly, whereas the treated half increased to 1.63 ± 0.27 ng·L^-1, a three-fold increase. Subsequent additions of sulfate in 2002 did not raise porewater MeHg, but MeHg in the treated half remained elevated relative to the control. MeHg concentrations in flow from the wetland increased following each addition. The timing and intensity of these increases was dependent on flow characteristics and sulfate reduction rates at the time of each addition. Our results clearly demonstrate that sulfate additions enhance methylation and increased MeHg concentrations within the wetland and in outflow from the wetland. This experimental wetland is largely ombrotrophic and therefore sulfate-poor. Wetlands richer in sulfate from groundwater may not respond to sulfate additions in the same way. Our results suggest that in sulfate-poor systems MeHg production, and probably fish contamination, is as much a function of atmospheric deposition of sulfate as of mercury.

Experimental Additions to Lake Sediment

Mercury cycling in Spring Lake, an 8.9 ha lake in the Marcell Experimental Forest, was studied at the same time as the sulfate additions to the wetland. We hypothesized that methylation would respond to sulfate addition in a similar fashion to the wetland sulfate addition. Studies performed on Spring Lake sediment indicated that nutrients such as sulfate, glucose, and ammonium all had negative effects on net Hg methylation rates, probably by reducing the bioavailability of Hg through the binding of sulfides, reduction in pH, increased formation of Hg-Cl species, or a combination of these. Of all the different additions, pyruvate treatments alone increased net Hg methylation rates. This exception was probably because the sulfate reducing bacteria were fermenting pyruvate and not producing sulfides, which reduce mercury bioavailability. A sediment core transect across Spring Lake indicated that MeHg was negatively correlated with total reducible inorganic sulfides and, generally, with lake depth, indicating that the bioavailability of Hg may be controlling the spatial distribution of MeHg in Spring Lake sediment.

Methyl and Total Mercury Mass Balances for Spring Lake

In Spring Lake, we quantified redox transformations, photodegradation of MeHg, internal and external loading for both HgT and MeHg, and modeled Hg cycling using the software Stella. Measurements on sediment cores allowed for separate treatment of lake and sediment environments. Atmospheric deposition is the main input of HgT, but MeHg is supplied by a combination of atmospheric, near-shore wetland, and in-lake methylation. Photodegradation annually removes about twice the lake burden of MeHg (20 mg), and burial removes slightly more than one lake burden per year. The residence time of MeHg in the lake was 84 days during the ice-free season, compared with only 4.5 days for the residence time of MeHg on settling particles (seston) in the spring. For elemental mercury (Hg0), the rate of photoreduction (1.1 h^-1) was twice that of photooxidation (0.58 h^-1). Losses of HgT from evasion of Hg0 during the daytime in 2001 and 2002 averaged 1.1 ng m -2h^-1. The evasional loss of Hg0, 365 mg (March. – November.), was much smaller than the HgT loss from burial (1,435 mg).

In this lake, which has little wetland coverage in the watershed, the majority of the input of MeHg is from internal methylation. The combination of demethylation and photodegradation consumes most of the MeHg available in the lake. Lakes with greater areas of wetland in the watershed would receive correspondingly greater MeHg inputs, and the biota would have greater opportunity for bioaccumulation of MeHg.

Conclusions:

Our experiments support the hypothesis that sulfate addition can stimulate the production and export of MeHg from a wetland. Surprisingly, our laboratory experimental additions of sulfate to lake sediment showed the opposite effect: added sulfate decreased the production of MeHg.

These results suggest that fish contamination may be controlled by atmospheric deposition of both sulfate and mercury in sulfate-poor aquatic systems with significant wetland components.

The relative importance of in-lake mercury methylation compared to loading of MeHg from wetlands in the watershed is subject to a variety of controlling factors, including relative wetland area, hydraulic transport from the wetlands, and losses of MeHg due to photodegradation and microbial demethylation. Even though some of these controlling factors are poorly characterized, it is our goal to utilize the results of our STAR project to build a model for Minnesota lakes, and to test the model against ten lakes with a spectrum of wetland areas within the watershed.

Sulfate additions will continue through 2004 and 2005 to determine the effect of longer-term sulfate deposition on a sulfate-limited wetland system.

Journal Articles on this Report : 1 Displayed | Download in RIS Format

Publications Views
Other project views:	All 16 publications	1 publications in selected types	All 1 journal articles

Publications
Type	Citation	Project	Document Sources
Journal Article	Hines NA, Brezonik PL. Mercury Dynamics in a small Northern Minnesota lake: water to air exchange and photoreactions of mercury. Marine Chemistry, 2004 90(1-4): 137-149	R827630 (Final)	not available

Supplemental Keywords:

mercury, Hg, monomethylmercury, methyl mercury, MMHg, MeHg, wetlands, peatlands, bog, biogeochemical cycling, fate and transport, microbiology, bacteria, bioavailability, watersheds, water, acid deposition, acid rain, sulfates, environmental chemistry, Minnesota, MN, EPA Region 5, lakes, sediment, mercury cycling, aquatic, mass balance studies, methylation, elemental mercury, evasion, photodegradation, sulfate reducing bacteria, SRB, sulfides,, Scientific Discipline, Water, Waste, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Nutrients, Ecology, Hydrology, Contaminated Sediments, Environmental Chemistry, State, Fate & Transport, Air Deposition, Geology, Mercury, EPA Region, aquatic ecosystem, nutrient supply, fate and transport, Minnesota, MN, forested watersheds, food chain, contaminated sediment, benthic food web, atmospheric deposits, mercury cycling, fish consumption, biological integrity, geochemistry, methylmercury, watershed influences, methylation, terrestrial and aquatic fate, ecosystem stress, Region 5, atmospheric deposition, benthic nutrients

Relevant Websites:

http://www.pca.state.mn.us/air/mercury.html Exit
http://www.tc.umn.edu/~cotne002/ Exit
Synthesis Report of Research from EPA’s Science to Achieve Results (STAR) Grant Program: Mercury Transport and Fate Through a Watershed (PDF) (42 pp, 760 K)

Progress and Final Reports:

Original Abstract

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.