Synopsis: Prior studies have identified that additional deposition of sulfate can lead to greatly elevated levels of methy-lmercury in wetland ecosystems. Methyl-mercury is the form of mercury responsible for most mercury-related health concerns, including neurological impairments in developing children. Methyl-mercury is produced in many water bodies, especially wetlands, by certain bacteria that require sulfate. Such wetlands are believed to be the most important source of methyl-mercury to the waters of the Great Lakes. The dependence of methyl-mercury creation on sulfate deposition is therefore a very important finding. Prior work by this research team showed that when sulfate deposition is experimentally increased in a wetland, the methyl-mercury levels increase dramatically.

This study will examine this relationship between sulfate and methyl-mercury more closely. In particular, it will examine what happens to methyl-mercury levels within a wetland when sulfate deposition levels that were previously elevated are decreased to a normal level. This mimics the change that has actually taken place over much of the Great Lakes region over the past 15 years due to improved controls on sulfur emissions. It also examines the extent of the benefits of further sulfur emission reductions and how rapidly they might be seen. Additional experimental techniques will be used to examine the influence of water level fluctuations on mercury methylation in wetlands. Prior evidence suggests the extent and timing of these fluctuations is an important determinant of methyl-mercury production.

Chemicals Studied: Total and methyl mercury will be monitored in the experimental wetland. In addition, a suite of general water chemistry parameters will be monitored, including cations, anions, dissolved organics, nitrogen and sulfate. Methyl-mercury a substance of very high concern within the Great Lakes region due to its high toxicity and tendency to accumulate at high levels within fish.
Geographic Areas: The experiment is taking place at the Marcell Experimental Forest in Northern Minnesota. The results will be applicable to similar ecosystems, which are common across the Great Lakes basin.
Project Duration: Field work began in the summer of 2007 and will conclude in the fall of 2008. Final project results should be available by Spring of 2009.

Methods Used: The approximately two hectare wetland will be artificially amended with additional sulfate though a sprinkler system designed to simulate rain events. The sulfate added will be approximately four times the ambient contribution. Water chemistry and mercury levels will be monitored in the wetland following sulfate amendment events and during control periods. The data will be analyzed to determine the impact of the increased sulfate deposition on the methylmercury production within the wetland. To examine the impacts of decreases in sulfur deposition, a portion of the previously treated wetland will not receive additional sulfur. The impact of water fluctuations will be studied by constructing experimental areas within each control or treatment zone of the wetland where a confined area can receive water inputs. These areas will consist of impermeable walls beneath the surface of the wetland that will prevent water movement between these areas and the surrounding wetland areas.

Potential Results and Implications: Wetlands are widely believed to be the major location where methyl-mercury is produced that enters the Great Lakes. This project will answer several important questions regarding how this methyl-mercury production occurs and what factors control how much methyl-mercury is produced. Several of these factors, such as sulfur deposition and water level fluctuations, are items for which there may be opportunities to use this knowledge to decrease mercury methylation on a regional scale (such as by further decreasing sulfur emissions) or on smaller scales (such as by modifying the water fluctuations in wetlands where this can be altered).

Project Contact:
Daniel R. Engstrom, Ph.D.
St. Croix Watershed Research Station
16910 152nd St. North
Marine on St. Croix, MN 55047
Phone: (651) 433-5953
Email: dre@smm.org