Identifying the potential for wetland vegetation management as a strategy to decrease methymercury production.

Wetlands are often associated with elevated concentrations of methylmercury (MeHg) because the conditions are favorable to methylating microbes, such as stagnant/low oxygen water and higher nutrient and organic carbon concentrations, when compared to other aquatic systems (i.e., rivers, lakes). Wetlands can become dominated by exotic/invasive species such as reed canarygrass (Phalaris arundinacea).  Reed canarygrass is distributed globally and is a fast-growing and high-density plant species that can produce large quantities of organic matter and can out compete native vegetation.  The objective of this study was to determine if wetland plant species influence MeHg production and if removing reed canarygrass and replacing it with native species can decrease MeHg production.  Controlled experiments were performed in mesocosms containing different individual wetland plant species. In addition to reed canarygrass, common/soft rush (Juncus effusus), Sitka willow (Salix sitchensis), daggerleaf rush (Juncus ensifolius), and Baltic rush (Juncus balticus) were planted in triplicate mesocosms containing mercury-contaminated sediments.  These species were selected as they differed in capability to affect sediment carbon storage, nitrogen fixation, effect on sediment redox conditions and microbial methylation.  In addition, field measurements from wetlands dominated by reed canarygrass were compared to areas that have been restored to native vegetation.  Samples of sediment and porewater from the mesocosms and field were analyzed for total-Hg (THg) and MeHg concentrations as well as common ancillary parameters associated with methylation (i.e., major anions, sulfide, reduced iron, oxidation-reduction potential, dissolved oxygen). Results show that wetland plant community species composition can have a significant impact on MeHg production.  The results also show that wetlands which have reed canarygrass removed and replaced with lower density native species have significantly lower THg and MeHg in the porewater as well as lower dissolved organic carbon and a higher oxidation-reduction potential. Overall, these results highlight the potential to utilize wetland vegetation management as a strategy to decrease sediment MeHg production.

Impact/Purpose

Wetlands downstream from Mercury mines often have high levels of methylmercury (MeHg) because the conditions are favorable to methylating microbes.  This is because wetlands often have conditions such as stagnant/low oxygen water and higher nutrient and organic carbon concentrations.  Methymercury bioaccumulates through the food chain in water bodies proximal to these wetlands and can result in exposure to Mercury if contaminated fish are consumed.  Often wetlands become dominated by exotic/invasive species such as reed canarygrass (Phalaris arundinacea) that produce large quantities of organic matter and out compete native vegetation.  This injection of organic matter fuels the microbes that methylate the Mercury.  This study was conducted to determine if wetland plant species influence MeHg production and if removing reed canarygrass and replacing it with native species can decrease MeHg production.  Experiments were performed in wetland mesocosms to assess the effects of removing reed canarygrass and planting native plants instead. These native plants used were selected as they differed in capability to affect sediment carbon storage, nitrogen fixation, effect on sediment redox conditions and microbial methylation.  The results of this study show that wetland plant community species composition can have a significant impact on MeHg production.  The results also show that wetlands which have reed canarygrass removed and replaced with lower density native species have significantly lower THg and MeHg in the porewater as well as lower dissolved organic carbon and a higher oxidation-reduction potential. Overall, these results highlight the potential to utilize wetland vegetation management as a strategy to decrease sediment MeHg production.

Citation

Johnson, M., C. Eckley, T. Luxton, M. Bollman, R. Wilkin, J. Goetz, C. Eagles-Smith, B. Thomas, AND J. Crawford. Identifying the potential for wetland vegetation management as a strategy to decrease methymercury production. 16th International Conference On Mercury as a Global Pollutant, Cape Town, SOUTH AFRICA, July 21 - 26, 2024.