A mechanism for bimodal emission of gaseous mercury from aquatic macrophytes

We performed intensive sampling campaigns of Hg0 fluxes over emergent macrophytes in the Florida Everglades to find a surrogate for Hg fluxes from water and vegetation to identify the mechanisms of emission. We measured daytime lacunal and sediment gas concentrations of Hg0, which suggested that the lacunal space acts as temporary storage for Hg0 and CH4. The absence of detectable Hg0 fluxes measured over uprooted (floating) plants and sediment incubation experiments suggest that the Hg0 emitted from emergent macrophytes such as Typha and Cladium originates in the sediment. HgII in the rhizosphere is reduced to Hg0 in these sediments by various processes, and is then transported by the plants to the atmosphere by two separate processes. At night, Hg0 and CH4 formed in the sediment accumulate in the lacunal space after crossing the root-sediment barrier. At sunrise, a form of pressurized through-flow purges the lacunal space of Typha into the atmosphere forming an early morning emission pulse for both gases, and coincidental peaks of CH4 and Hg0 suggest that the same lacunal gas transport mechanism is involved. Later in the day while the release of methane continues to deplete the lacunal pool, the Hg0 flux increases again to form a second peak in the afternoon when the CH4 emission has decreased. This peak parallels that of transpiration, and is presumably due to xylem transport of Hg0 from continued production of Hg0 in the rhizosphere, perhaps in response to release of root exudates. A mass balance for this ∼1500 ha wetland suggested that the total transpiration of Hg0 is ∼1 kg yr−1, or ∼20 times the amount evaded from the water surface.