Wetland Management Technique Designed to Reduce Mercury in Water and Fish Tested During a Short-Term Field Study

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Results from a 3-year study indicate there was support for the use of open- and deep-water treatment pools at the downstream end of seasonal wetlands to reduce methylmercury concentrations in water exported from the wetlands, but the treatment had no measurable effect on wetland fish. Questions remain about the long-term potential for mercury removal using this wetland management strategy.


Ducks and geese float on the water at the Cosumnes River Preserve

The Cosumnes River Preserve, California.

(Credit: Bob Wick, BLM. Public domain.)

Wetlands are productive habitats for fish and wildlife and provide ecological and economic benefits; however, the biogeochemical conditions common in wetlands, particularly seasonal wetlands, have been shown to facilitate methylmercury production, making methylmercury available to wetland animals and to downstream habitats.

Land managers are seeking methods to reduce methylmercury export from seasonal wetlands because methylmercury is a highly toxic form of mercury that affects the nervous system. Moreover, methylmercury can accumulate in fish, potentially affecting their health and the health of fish-consuming humans and wildlife.

The U.S. Geological Survey’s Ecologically-Driven Exposure Pathways Science Team tested the efficacy of a wetland management strategy designed to address the dual goals of reducing the amount of methylmercury exported from wetlands to downstream habitats and reducing methylmercury bioaccumulation in fish within the wetlands themselves. 

The team’s approach was to modify the physical structure of existing seasonal wetlands to enhance the naturally occurring processes that have been shown to reduce methylmercury in other surface waters. These naturally occurring processes include particle settling, microbial degradation, and photo-demethylation. The team established four treatment and four control wetlands at the Cosumnes River Preserve in California. The control wetlands received traditional wetland management. In contrast, treatment wetlands had open- and deep-water pools constructed at the downstream end in an attempt to remove methylmercury from surface waters.

During 2015–17, 3,000 (1,000 per year) western mosquitofish were tagged, weighed, and then caged for 30 days each season at specific locations within the control and treatment wetlands to determine changes in methylmercury concentrations. Water was sampled within 1 week of fish deployment and again within 1 week of fish retrieval.

Methylmercury concentrations in water were successfully decreased within treatment wetlands only during the third year of study, but treatment cells were not effective for reducing mercury concentrations in wetland fish. The scientists concluded there was limited support for the use of open- and deep-water treatment pools at the downstream end of seasonal wetlands to reduce methylmercury concentrations in water. Scientists suggest it may take more than 3 years to see decreases in fish mercury concentrations because wetland construction activities likely resulted in a short-term increase in methylmercury concentrations in fish. Therefore, it may take longer term studies to fully observe reductions in methylmercury concentrations in fish and water.  

This research was funded by the U.S. Geological Survey Environmental Health Program (Contaminants Biology and Toxic Substances Hydrology) of the U.S. Geological Survey Ecosystems Mission Area, U.S. Environmental Protection Agency, California Department of Fish and Wildlife, and U.S. Geological Survey's San Francisco Bay-Delta Priority Ecosystems Science Program.

a constructed wetland

Graphical depiction of a seasonal wetland engineered to have a deeper pool and more open water on the downstream side of the wetland. This management strategy was tested during a 3-year period to determine if it resulted in a reduction of methylmercury in caged fish and surface water.

(Public domain.)