Transpiration of gaseous elemental mercury through vegetation in a subtropical wetland in Florida

Four seasonal sampling campaigns were carried out in the Florida Everglades to measure elemental Hg vapor (Hgdegrees) fluxes over emergent macrophytes using a modified Bowen ratio gradient approach. The predominant flux of Hgdegrees over both invasive cattail and native sawgrass stands was emission; mean day time fluxes over cattail ranged from similar to20 (winter) to similar to40 (summer) ng m(-2)h(-1). Sawgrass fluxes were about half those over cattail during comparable periods. Emission from vegetation significantly exceeded evasion of Hgdegrees from the underlying water surface (similar to1-2 ng m(-2)h(-1)) measured simultaneously using floating chambers. Among several environmental factors (e.g. CO2 flux, water vapor flux, wind speed, water, air and leaf temperature, and solar radiation), water vapor exhibited the strongest correlation with Hgdegrees flux, and transpiration is suggested as an appropriate term to describe this phenomenon. The lack of significant Hgdegrees emissions from a live, but uprooted (floating) cattail stand suggests that a likely source of the transpired Hgdegrees is the underlying sediments. The pattern of Hg' fluxes typically measured indicated a diel cycle with two peaks, possibly related to different gas exchange dynamics: one in early morning related to lacunal gas release, and a second at midday related to transpiration; nighttime fluxes approached zero. Published by Elsevier Science Ltd.