Decreasing Methylmercury Export from a Managed Wetland: A Landscape Manipulation Experiment

Abstract:

Wetlands are important areas for methylmercury (MeHg) production and export to adjacent water bodies, and yet are also critical for wildlife and healthy ecosystem function. Thus, preserving or restoring wetland habitat, while limiting MeHg export, provides a unique challenge to wetland managers. This study examined how wetland design and the use of strategically located deep cells could reduce MeHg export. The study area consisted of eight ~10 hectare leveed fields, annually flooded (4-20 cm) during mid-September thru late April to promote waterfowl habitat. In four ‘treatment’ fields, 20% of the area (~2 hectare) at the outflow end was excavated to a depth of ~1 m, while no elevation adjustments were made in the four remaining ‘control’ fields. We examined 3 potential mechanisms of MeHg removal in the deep cell areas of the treatment fields (particulate flux to the benthos, benthic demethylation, and photo-demethylation), and compared MeHg export between control and treatment fields. Particulate depositional flux was high across all deep cells (3.5-8.9 g particles/m²/d dry wt.; 25-75% quartile, n=211), as assessed with deposition pads deployed prior to and retrieved after flooding. This compared well to three separate short-term (24 hr) deposition experiments (1.6-4.7 g particles/m²/d dry wt.; 25-75% quartile, n=48) used to assess particulate MeHg depositional flux (7.6-25.5 ng MeHg/m²/d dry wt.; 25-75% quartile, n=48). With additional results pending, this presentation will describe the relative contribution of the above three MeHg removal processes, the MeHg export from treatment vs control fields, and the overall effectiveness of this wetland management option using deep water cells for reducing MeHg export from managed wetlands to adjacent water bodies.