Uptake of Gaseous Elemental Mercury by a Rainforest: Insights from a Tropical Glasshouse Used as a Dynamic Flux Chamber

Vegetation uptake of gaseous elemental mercury (Hg⁰) is the main deposition pathway to terrestrial environments. However, the fluxes and processes of forest-atmosphere Hg⁰ exchange remain ill-constrained, especially in rainforests. To help address this, we used the 1 ha Masoala Rainforest hall of the Zoo Zurich as a dynamic flux chamber to measure net rainforest Hg⁰ fluxes and even calibrate Hg⁰ deposition velocities with turbulence measurements. The net Hg⁰ flux correlated well with CO₂ assimilation, showing peak Hg⁰ uptake at noon. The interquartile range of Hg⁰ uptake spanned from 1.69 to 3.45 ng m^-2 h^-1 during the day and from 0.01 to 0.44 ng m^-2 h^-1 at night. The study results revealed a Hg⁰-specific canopy resistance (<i>R</i>_c = 1000 s m^-1), which underlined the importance of stomatal uptake as a dominant Hg⁰ deposition pathway in the rainforest. Even though a dynamic flux chamber, however large, is only an approximation of a real rainforest, our findings underline the value of whole-rainforest flux studies both for constraining Hg exchange with the atmosphere and resolving the role of specific mechanisms.