Final Report: Response of Methylmercury Production and Accumulation to Changes in Hg Loading: A Whole-ecosystem Mercury Loading Study

EPA Grant Number: R827631
Title: Response of Methylmercury Production and Accumulation to Changes in Hg Loading: A Whole-ecosystem Mercury Loading Study
Investigators: Gilmour, Cynthia C. , Heyes, Andrew , Mason, Robert P. , Rudd, John M.
Institution: Academy of Natural Sciences , Chesapeake Biological Laboratory , Freshwater Institute , University of Maryland Research Centers
EPA Project Officer: Hiscock, Michael
Project Period: October 1, 1999 through September 30, 2002 (Extended to September 30, 2003)
Project Amount: $848,029
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text |  Recipients Lists
Research Category: Water and Watersheds , Mercury , Water , Safer Chemicals


The response of methylmercury (MeHg) production and accumulation leading to a change in Hg loading was examined as part of two large, multi-disciplinary, multi-investigator ecosystem-level Hg loading studies. These are the Mercury Experiment To Assess Atmospheric Loading in Canada and the United States (METAALICUS) study in a boreal watershed in northwest Ontario and the Aquatic Cycling of Mercury in the Everglades (ACME) study.

During 2001-2003, the Lake 658 (L658) basin at the Experimental Lakes Area, in northwestern Ontario, has been the site of a whole-ecosystem experiment designed to understand the impact of elevated rates of atmospheric deposition of mercury on mercury concentrations in fish. The METAALICUS experiment is a Hg-loading experiment in which the deposition of inorganic Hg to the uplands, wetland, and lake surface is being experimentally elevated by the additions of isotopically enriched Hg to simulate an approximate four-fold increase in Hg in wet deposition.

The primary objective of the METAALICUS study is to provide the first direct ecosystem-level data to support or refute the hypothesis that a change in atmospheric Hg deposition changes MeHg concentrations in biota. Within that objective are detailed studies of many aspects of the Hg cycle, at numerous spatial scales, including robust mass balances for both ambient Hg and the Hg spikes. METAALICUS is being accomplished by a multidisciplinary team of approximately 50 researchers from the United States and Canada. Our overall objective was to resolve a basic and unanswered question in our understanding of the Hg cycle, “How much does MeHg in ecosystems change in response to a change in Hg loading?” METAALICUS incorporates two powerful techniques that are novel to the Hg research community to accomplish that objective: the use of stable Hg isotopes and the manipulation of a whole watershed. The Hg load to the study ecosystem is being increased by a factor of approximately four over current local atmospheric ambient deposition using highly enriched stable Hg isotopes. Three separate isotopes have been used to dose the upland, wetland, and lake. This unique approach allowed us to track the fate of newly deposited Hg separately from the larger existing pools through time and across various habitats. The U.S. Environmental Protection Agency (EPA) Science To Achieve Results (STAR) program supported the microbial methylation component of the METAALICUS study.

To provide comparison in a very different ecosystem type, and a different region of North America, we carried out Hg-loading projects in a subtropical wetland, the Florida Everglades. In the Everglades, Hg addition experiments were conducted primarily within multiple 1m-diameter enclosures. Work in the Everglades is an extension of a study of biogeochemical cycling Hg across that ecosystem, the ACME study, conducted in collaboration with the U.S. Geological Survey (USGS). The EPA Science To Achieve Results (STAR) program has supported the methylation/demethylation work within ACME during 1999-2003, with additional funding from the USGS, South Florida Water Management District (SFWMD), and Florida Department of Environmental Protection.

The primary objective of this research project was to determine the response of MeHg production and accumulation to a change in ecosystem Hg loading.

The specific objectives were to: (1) determine the bioavailability of Hg delivered to different parts of the watershed for methylation; (2) determine the contribution of newly deposited Hg to MeHg production relative to existing Hg pools in sediments and soils and how the bioavailability of new Hg changes over time; (3) develop stable isotope techniques for tracing Hg cycling in watersheds, making simultaneous Hg methylation and demethylation rate measurements, and examining the bioavailable pool of Hg(II); and (4) examine regional and habitat type differences in the response of MeHg to Hg loading by conducting a second Hg loading project in a different ecosystem type and using process and model-based approaches to transfer results to other ecosystems.

We hypothesized that Hg stored in sediments and soils is less available for methylation than newly deposited Hg and that Hg deposited directly to the lake surface, and/or rapidly delivered to the lake in runoff, is most important to ecosystem methylation.

Overview of Work: METAALICUS

During 1999, preliminary studies were conducted at the Experimental Lakes Area, including assessment of candidate watersheds and development of stable isotope methylation/demethylation techniques. Work in 2000 included pilot-scale Hg isotope additions to lake enclosures, wetland plots, and upland plots. L658 was selected as the project site, and background data were collected for 1 year. Full-scale stable isotope Hg additions to L658 and its watershed began in June 2001 and continued through 2004. The biogeochemistry of net Hg methylation was followed intensively through time and space in lake sediments, the wetland, and upland each year. Experimental studies in lake enclosures, wetland plots, and upland plots outside of L658 continued throughout the project. To supplement the whole watershed Hg response project, a series of dose-response studies in lake enclosures was carried out at multiple doses during 2002 and 2003. These studies mimic dose-response studies in mesocosms in the Everglades.

Overview of Work: ACME

In spring 2000, stable isotope Hg-dosing studies were conducted in enclosures at four sites in the Everglades. Enclosures were sampled in spring and summer, dosed again in the fall, and sampled into 2002. Also during 2000, Hg and MeHg levels were examined in phosphate-amended mesocosms. This work piggy-backed on a phosphate mesocosm study being done by Newman, et al., at the SFWMD.

A second ACME enclosure experiment began in November 2001, in which the effects of Hg, sulfate, and dissolved organic carbon (DOC) on MeHg production were assessed separately and together at two different sites. Sulfate, Hg, and DOC additions were made to ACME mesocosms, with a focus on responses to sulfate additions at low-sulfate sites. Mesocosms were dosed biweekly with sulfate, monthly with Hg isotope (using separate isotopes for each spike), and once with DOC. DOC was isolated from Everglades surface waters. This experiment continued intensively through February 2002. In 2003, an additional study of sulfate, Hg, and DOC was conducted at one site in the central Everglades where MeHg production is generally maximal within the ecosystem. In this study, increased replication, a large number of sulfate doses, and a longer time frame of sulfate dosing provided a more accurate description of the sulfate:MeHg relationship in the long term.

Summary/Accomplishments (Outputs/Outcomes):

The METAALICUS and ACME studies have been able to answer a number of our questions about Hg bioavailability and methylation in the environment and to address the original objectives of this research project. Hg-stable isotopes now are in routine use in our laboratory for assessment of instantaneous methylation and demethylation rates in sediments and soils. The use of stable isotope spikes in the L658 watershed has allowed us to trace newly deposited Hg throughout the system. In both ecosystems, stable isotopes have allowed us to examine the timing and magnitude of the response to a change in Hg loading.

The aquatic ecosystems studied respond rapidly and linearly to a change in Hg load. Hg applied to the surface of L658 or to Everglades surface waters was delivered rapidly (days to months) to sites of methylation, methylated, and bioaccumulated. The response time, in terms of a change in MeHg production from a change in Hg load, for the shallow, warm Everglades was days to weeks depending on temperature. In Experimental Lakes Area projects, the response time varied depending on the size of the study system. For shallow lake enclosures, a new level of MeHg in the enclosures was achieved within weeks of a single Hg spike to the water surface. In the whole lake study in L658, the continuous Hg-dosing design (during open water) was used to mimic a long-term, approximately three-fold step change in Hg loading. Sediment MeHg data suggest that maximal MeHg production from the isotope spike added to the lake surface occurred by the end of the second summer. The concentration of MeHg in the water column and bioaccumulation of MeHg in fish will be used together with the sediment data to determine the time frame of the response to the change in load.

The major locations of de novo methylation in both the Everglades and the boreal L658 watershed were saturated surface peats and sediments and in the case of L658, anoxic bottom waters. Hg isotope spikes that were transported to these zones were rapidly methylated, and subsequently bioaccumulated. Newly deposited isotopes spikes behaved differently than existing Hg pools after delivery to sediments, both in terms of sediment/water partitioning behavior, and in terms of net MeHg accumulation.

Once delivered to sites of methylation, Hg isotope spikes were more highly methylated than Hg previously accumulated in sediments and soils. The percent of total Hg as MeHg (%MeHg) in any given matrix provides an estimate of its bioavailability for methylation and of the balance between methylation, demethylation, and other MeHg-loss mechanisms. In L658 surface sediments, during the first 2 years of the addition study, the percent MeHg for the lake isotope spike was two to three times higher than that of previously existing Hg. In Everglades surficial soils, percent MeHg for new isotope spikes was often 5 to 10 times higher than native (or existing) Hg in the first weeks after the spike. We found that as isotope spikes aged in sediments and soils in both L658 and in the Everglades, the %MeHg dropped. New deposited Hg is relatively bioavailable for methylation, but bioavailability drops through time.

New MeHg production from Hg isotope spikes was a good predictor of total MeHg concentrations in surface sediments in both ecosystems. Taken together, these findings support the idea that newly deposited Hg contributes substantially to MeHg production and bioaccumulation within aquatic systems.

The change in MeHg production in response to a change in Hg loading depends on the timing and magnitude of new Hg transport to sites of methylation. In the METAALICUS study, Hg newly deposited to the lake surface was more readily methylated than existing Hg pools in sediments. For Hg isotope spikes deposited to uplands and wetlands, however, transport to sites of methylation in the lake and wetland has been slow. Only a small fraction of Hg isotope sprayed on the L658 upland migrated to the lake and wetland in the first 2 years after additions began. In the wetland, Hg isotope sprayed on the surface of the wetland has not penetrated into zones of methylation, which occur below the surface of the peat often near the surface of the water table. Because Hg spikes have not been exported substantially from the upland or wetland, the bioavailability of these Hg pools once delivered to sites of methylation remains unknown.

Formal budgets for each of the isotopes added to the upland, wetland, and lake surface of the L658 watershed are under construction. Draft budgets for Hg and MeHg suggest that most of the native and spike MeHg in the system is formed within the lake. Budgets for each isotope also will provide more quantitative information on the contribution of Hg deposited to the lake surface to the total MeHg production and bioaccumulation in the lake.

The bioaccumulation of isotope spikes in fish provides key information on the Hg sources that contribute to MeHg production. Two years after spiking began, approximately one quarter of the mercury in young perch was made up of the mercury isotope spike added to the lake surface. Given the approximately 5:1 ratio of upland to lake surface in the L658 basin, this finding supports the idea that Hg deposited to the lake surface contributes disproportionately to MeHg in fish relative to Hg deposited to the watershed.
In the Everglades, rapid and substantial transport of Hg through the shallow water column to soils surface flocs and peat meant that a relatively high fraction of Hg delivered to the water surface was methylated. The magnitude and timing of the response to a change in Hg load to the Everglades was substantially different from the boreal systems studied, in large part because of the speed and magnitude of delivery of new Hg to sites of methylation.

Our previous work has shown that the degree to which Hg is methylated depends heavily on the biogeochemical parameters that influence microbial activity and Hg complexation. For example, both sulfate and dissolved organic matter have major effects on methylation. In the Everglades enclosure studies, we found that sulfate and DOC have a much larger effect on the methylation of newly deposited Hg than on Hg already accumulated in soils.

Dose-response studies at multiple sites in the Everglades and in lake enclosures at the Experimental Lakes Area showed linear responses to Hg loading. In Florida, MeHg production in surface flocs and MeHg bioaccumulation in Gambusia were linear functions of Hg loading to surface waters over a range of loadings from one-half to two times annual wet deposition. The slope of the response, however, varied substantially across sites in the Everglades because of differences in biogeochemistry. In lake enclosures at the Experimental Lakes Area, responses in surface sediments and young perch were linear across a loading range of 5 to 120 mg/m2.

The results from both the METAALICUS and ACME studies show that aquatic ecosystems do respond to changes in Hg loading. Once delivered to sites of methylation, newly deposited Hg is more bioavailable for methylation than Hg already accumulated in sediments and soils. The change in MeHg production in response to a change in Hg loading, however, depends on the timing and magnitude of new Hg transport to sites of methylation. The response in MeHg production was a linear function of the loading for Hg depositing directly to water surfaces. Taken together, these findings support the idea that newly deposited Hg contributes substantially to MeHg production and bioaccumulation within aquatic systems. Especially for ecosystems that receive a substantial fraction of their Hg load from direct deposition to water and wetland surfaces, rapid reductions in Hg emissions should lead to rapid benefits to human and wildlife health.

Journal Articles on this Report : 4 Displayed | Download in RIS Format

Other project views: All 44 publications 5 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Babiarz CL, Hurley JP, Krabbenhoft DP, Gilmour CC, et al. Application of ultrafiltration and stable isotopic amendments to field studies of mercury partitioning to filterable carbon in lake water and overland runoff. Science of the Total Environment 2003;304(2):295-303. R827631 (Final)
not available
Journal Article Benoit JM, Mason RP, Gilmour CC, Aiken GR. Constants for mercury binding by dissolved organic matter isolates from the Florida Everglades. Geochimica et Cosmochimica Acta 2001;65(24):4445-4451. R827631 (2000)
R827631 (2001)
R827631 (Final)
R827653 (2000)
R827653 (2001)
R827653 (Final)
  • Full-text: ScienceDirect-Full Text PDF
  • Abstract: ScienceDirect-Abstract and Full Text HTML
  • Other: Smithsonian Library Online - Full Text PDF
  • Journal Article Benoit JM, Gilmour CC, Mason RP. The influence of sulfide on solid-phase mercury bioavailability for methylation by pure cultures of Desulfobulbus propionicus (1pr3). Environmental Science & Technology 2001;35(1):127-132. R827631 (2000)
    R827631 (2001)
    R827631 (Final)
    R827653 (2000)
    R827653 (2001)
    R827653 (Final)
  • Abstract from PubMed
  • Abstract: ACS - Abstract
  • Journal Article Jay JA, Murray KJ, Gilmour CC, Mason RP, Morel FMM, Roberts AL, Hemond HF. Mercury methylation by Desulfovibrio desulfuricans ND132 in the presence of polysulfides. Applied and Environmental Microbiology 2002;68(11):5741-5745 R827631 (Final)
    R824778 (Final)
    not available

    Supplemental Keywords:

    atmosphere, water, watersheds, land, soil, sediments, acid deposition, precipitation, chemical transport, risk assessment, human health, bioavailability, dose-response, metals, heavy metals, sulfates, bacteria, acid rain, ecosystem, restoration, regionalization, scaling, terrestrial, aquatic, habitat, public policy, decisionmaking, cost benefit, Everglades, Florida, FL,, Scientific Discipline, Geographic Area, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Contaminated Sediments, Environmental Chemistry, Chemistry, Fate & Transport, Ecological Risk Assessment, Ecology and Ecosystems, Biology, International, Environmental Engineering, Mercury, fate and transport, aquatic, contaminated sediment, mercury loading, fish consumption, mercury cycling, soils, biogeochemical cycling, methylation, methylmercury, terrestrial and aquatic fate, wetland

    Relevant Websites: Exit 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
  • 2000 Progress Report
  • 2001 Progress Report
  • 2002