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

摘要

Four seasonal sampling campaigns were carried out in the Florida Everglades to measure elemental Hg vapor (Hg°) fluxes over emergent macrophytes using a modified Bowen ratio gradient approach. The predominant flux of Hg° over both invasive cattail and native sawgrass stands was emission; mean day time fluxes over cattail ranged from ～20 (winter) to ～40 (summer) ng m~(-2)h~(-1). Sawgrass fluxes were about half those over cattail during comparable periods. Emission from vegetation significantly exceeded evasion of Hg° from the underlying water surface (～ 1-2 ng m~(-2)h~(-1)) measured simultaneously using floating chambers. Among several environmental factors (e.g. CO_2 flux, water vapor flux, wind speed, water, air and leaf temperature, and solar radiation), water vapor exhibited the strongest correlation with Hg°flux, and transpiration is suggested as an appropriate term to describe this phenomenon. The lack of significant Hg° emissions from a live, but uprooted (floating) cattail stand suggests that a likely source of the transpired Hg°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.