238 - Comparing net ecosystem exchange patterns of atmospheric gaseous elemental mercury (GEM) between a deciduous and coniferous temperate forest

Vegetation uptake of gaseous elemental mercury (GEM) constitutes more than half of anthropogenic Hg emissions. However, little information is available on magnitude, seasonality and diel patterns of GEM exchange fluxes in ecosystems. Here we present time-extended (>1 year each) measurements of ecosystem–level atmosphere-surface GEM exchange using tower-based flux-gradient approaches in a coniferous forest and a deciduous forest. Annual GEM depositions amounted to 25.1 μg m−2 in the deciduous forest and 5.7 μg m−2 in the coniferous forest, yet with an over four times difference in magnitude between the two forests. Interestingly, the different GEM magnitudes correlated with similar differences in ecosystem CO2 uptake with 2.5 times smaller as well in the coniferous forest. In both forests, annual GEM uptake were driven by pronounced growing season (e.g., June to October) GEM deposition, accounting for 137% and 63%, respectively. GEM emissions occurred in springtime before vegetation was fully active, yet such emissions were much smaller in the coniferous forests likely due to the presence of canopy shading. Interesting differences existed between the GEM flux patterns. First, GEM deposition peaked during midday in the deciduous forest and daytime periods accounted for 96% of annual GEM deposition. In contrast, GEM fluxes in the coniferous forest showed significant uptake both during daytime and nighttime and nighttime deposition accounted for 43%. Flux partitioning showed forest floors served as additional GEM sinks in the deciduous forests, while it was a source of GEM in the coniferous forest partially explaining its lower annual GEM deposition. GEM deposition correlates with respective annual CO2 uptake, suggesting a strong coupling between photosynthetic CO2 uptake and GEM deposition. Diel and seasonal patterns of GEM uptake support strong stomatal GEM uptake in both forests, yet significant nighttime GEM uptake particularly in the coniferous forest also provides evidence of non-stomatal uptake of GEM.