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 , Harris, Reed , Almendinger, James E.
Current 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: Water and Watersheds , Mercury , Water , Safer Chemicals


Efforts are being made to reduce mercury emissions in Minnesota and throughout the nation (MPCA 1994; EPA 1997), under the implicit assumption that in most lakes mercury accumulation in fish is more or less proportional to atmospheric deposition. Given the strong evidence that mercury deposition is now about 3 to 4 times greater than natural rates (Swain et al. 1992), a first estimation would be that fish are 3 to 4 times more contaminated than natural levels. Surprisingly, a comparison of modern fish to fish caught and preserved in a Minnesota Museum in the 1930s suggests that mercury concentrations in fish have increased by a factor of 10 in low-alkalinity systems in contrast to the 3 to 4x increase based on mercury deposition (Swain and Helwig 1989). It is possible that enhanced bioaccumulation could be a result of changes in the food chain, such as additional trophic levels (Rasmussen et al. 1990). However, recent Hg sediment core profiles from 16 lakes in Minnesota demonstrate that, beginning about 1940, the mercury present as methylmercury (MeHg) began to comprise a larger and larger percentage of Hg accumulation in the lakes (Engstrom et al. 1999). This finding suggests a fundamental change in methylation efficiency, weakening support for the competing hypothesis that the food chains have simply lengthened. The sediment data supports the idea of enhanced MeHg loads over the past few decades to many lakes resulting in elevated mercury concentrations in fish above what would be expected based on total mercury alone. As a reasonable explanation for these observations, we hypothesize that Hg concentrations in fish in sensitive ecosystems have been exacerbated relative to atmospheric deposition due to enhanced methylation occurring in lake sediments or associated wetlands. A variety of anthropogenic ecosystem changes that became widespread in the post-World War II era are possibly responsible for enhanced methylation, including: increased sulfate deposition (Gilmour et al. 1992), increased nutrient loads (Marvin-Dipasquale and Oremland 1998), increased loading of mercury to wetlands because of soil disturbance, and other unknown processes. The specific objectives of this study are to: (1) establish the relative importance of atmospheric, in-lake, and wetland sources of MeHg to a lake in a forested watershed containing three types of wetlands; (2) determine the net retention and source strength of different wetland types; (3) conduct mesocosm (wetland and lake) and whole wetland experiments to elucidate methylation enhancing processes (4) add a hydrologically-based wetland GIS module to the Mercury Cycling Model (MCM) to apply our findings to a larger set of lakes where extensive GIS and mercury data have been collected (Engstrom et al. 1999).


We are proposing a three-tiered approach involving microscale experiments, lake/wetland studies, and evaluation of the new understanding through modeling a set of lakes for which we already possess data. Microscale studies will focus on testing for enhanced methylation or inhibited demethylation in wetlands and lake sediments using benthic chambers and wetland mesocosms. Field studies will allocate external and internal MeHg loads to the lake (and, ultimately, to fish), focusing on transport from different types of wetlands within a single watershed. A full-scale addition of sulfate to a wetland will also be conducted. A hydrologically-based wetland GIS module will be linked to the MCM model to better quantitatively understand watershed inputs of Hg and MeHg to lakes. This model will then be applied to a larger set of lakes in Minnesota where extensive food web and sediment core data are available.

Expected Results:

An improved understanding of the sources of MeHg to fish and the process-driven model derived from this research will aid in explaining the large observed variability in fish mercury levels in Minnesota and elsewhere. The GIS module addition to the MCM model, using standard Cowardin wetland classifications will be applicable elsewhere in the United States. Minnesota and other states also have a responsibility under the Total Maximum Daily Load (TMDL) process to investigate measures that can reduce mercury in surface waters and in fish. This proposed project not only would satisfy the need for scientific information on the processes that control fish contamination, but could lead to mitigative measures that are practical to implement in specific watersheds.

Publications and Presentations:

Publications have been submitted on this project: View all 16 publications for this project

Journal Articles:

Journal Articles have been submitted on this project: View all 1 journal articles for this project

Supplemental Keywords:

EPA Region 5, heavy metals, terrestrial, biology, hydrology, geology, environmental chemistry, Scientific Discipline, Geographic Area, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Nutrients, Ecology, Contaminated Sediments, Environmental Chemistry, State, Fate & Transport, Air Deposition, Geology, EPA Region, Mercury, 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:
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:

  • 2000 Progress Report
  • 2001 Progress Report
  • 2002
  • Final Report