During Fall 1996 to Winter 1998, we completed the most extensive data set yet collected on Hg°, CO₂, and water vapor fluxes measured with a micrometeorological tower-based gradient system (Figure 2) over wetlands at several Everglades sites, primarily in the Everglades Nutrient Removal Project (ENR), and in Water Conservation Areas (WCA) 2 and 3 for comparison. Elemental mercury (Hg°) fluxes over emergent macrophytes were dominated by emissions from the plant surfaces, and transpiration is now realized as an appropriate term for this phenomenon. The patterns of the emission of water vapor and Hg are clearly similar, with the latent energy flux explaining ~40% of the variance in Hg flux (r=0.62, p<0.001, n>200). On the hand, weaker but still significant correlation exists for the sawgrass data as well (r=0.4, p<0.01, n=96). These observations, and the relationships also apparent for Hg° and CO₂ emissions forms the basis for our flux modeling. Mercury fluxes appeared to be influenced by solar radiation and temperature: for Typha, mean summer daytime emission = 31±50 ng m^-2 h^-1, mean nighttime = 0.2±15 ng m^-2 h^-1; for Cladium, mean daytime = 17±29ng m^-2 h^-1, mean nighttime approached zero. Compared to the Hg flux data from open water surface, the “transpiration” of Hg° from aquatic macrophytes is the single largest flux of Hg in this ecosystem. These fluxes are comparable to a northern Spartina marsh.

Evasion averaged ~3±4 ng m^-2 h^-1, much below vegetation fluxes, and lower than fluxes measured in Swedish boreal lakes which averaged 8-9 ng m^-2 h^-1. The data suggest that evasion is marginally elevated during the summer, at both the ENR and WCA2A sites, with daytime fluxes on the order of 5 ng m^-2 h^-1. Although evasion from the water surface arises from dissolved gaseous mercury in the water column, incubation studies on sediment and lacunal gas data both suggested that the source of Hg flux from vegetation was in the sediment. Presumably roots of vegetation interact with mercury compounds in the sediments and reduce ionic mercury to elemental Hg which is then transpired and released into the atmosphere. Vegetation fluxes of H° showed some similarities with plant emissions of methane, which is known to be derived from the rooting zone in the sediment.

^*Research sponsored by the South Florida Water Management District, the Florida Department of Environmental Protection, and the Electric Power Research Institute in collaboration with the National Oceanic and Atmospheric Administration, J. Chanton at Florida State University, and G. Keeler and F. Marsik at the University of Michigan.

Reference:  Lindberg S. E., and T. P. Meyers. 2001. Development of an automated micrometeorological method for measuring the emission of mercury vapor from wetland vegetation. Invited paper for Wetland Ecology & Management 9:333-347.

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