Using Mesocosms to Test the Effect of Land Management Practices on Monomethylmercury Production in Freshwater Seasonal Wetlands

Managed seasonal freshwater wetlands are an ideal habitat for the production of monomethylmercury (MMHg). Wetting and drying cycles created by water management promote growth of wetland vegetation for waterfowl food during the summer growing season. After fall floodup of the fields this vegetation decomposes creating anoxic conditions and an abundant carbon source utilized by MMHg producing microbes. This results in hot moments of MMHg production after floodup and increased MMHg loads from seasonal wetlands. In this work, we determine the effect land management practices have on MMHg production. A manipulative study was conducted during the fall (October-November) of 2012 in a 20 hectacre seasonally flooded wetland located in Yolo Wildlife Area, CA. Polycarbonate cylindrical mesocosms (0.75 m diameter, 0.6 m tall), were placed over 6 differing land management treatment types: disc, simulated cattle grazed, mowed with vegetation removed, mowed with vegetation left in place, scraped with all vegetation removed, and natural undisturbed. Each treatment had n = 5 replicates. Samples were collected for determination of mercury speciation and ancillary measurements. Sampling occurred bi-weekly or weekly over a period of thirty days. Dissolved MMHg concentrations peaked in all treatments by day 16 of the experiment. Comparison of treatments was done on a mass dissolved MMHg basis. Natural and mowed treatments were similar (2.2 and 2.1 μg MMHg respectively). Simulated grazing treatments (1.4 μg MMHg ) resulted in 36% reduction in MMHg relative to the natural treatment. Cleared (0.36 μg MMHg), disced (0.51 μg MMHg), and mowed with vegetation removed (0.88 μg MMHg) produced significantly less MMHg than the natural undisturbed treatment. Our results indicate specific management practices of seasonally flooded wetlands can reduce the net production of MMHg and potentially reduce loads to surrounding water. Net mass MMHg (ug) produced by treatment type within mesocosms in a freshwater seasonal wetland.