The role of terrestrial vegetation in mercury deposition: fate of stable mercury isotopes applied to upland and wetland forest canopies during the METAALICUS experiment (Invited)
Abstract
Methylmercury (MeHg) is an organic, neurotoxic form of mercury (Hg) that is responsible for fish consumption advisories in North American freshwaters. It is generally believed that increases in anthropogenic Hg emissions have resulted in high MeHg concentrations of fish. However, a direct relationship between deposition of inorganic Hg(II) and concentrations of MeHg in fish has been difficult to demonstrate because of our inability to distinguish newly-deposited Hg from Hg accumulated historically in ecosystems. The Mercury Experiment to Assess Atmospheric Loading In Canada and the US (METAALICUS) increased atmospheric inputs of mercury (Hg) to a small lake and its watershed to levels comparable to those in more industrialized regions. Between 2001 and 2006, three different enriched stable isotopes of Hg (spikes) were loaded to the watershed, one each to the surface of the lake (200Hg), the wetland (198Hg) and the forested upland (202Hg) areas of the catchment to determine the relative contribution of these sources to fish MeHg concentrations. Terrestrial vegetation often represents the first landscape compartment that new atmospheric Hg contacts upon deposition, and plants act as conduits of atmospheric Hg to the landscape. We will present pools and fluxes of spike Hg within upland and wetland canopy and ground vegetation compartments. Our Geographical Information Systems-based modeling approach to calculating spike pools used aircraft spray tracks, regressions between spike application rate and concentrations of spike in vegetation, a LiDAR-derived Leaf Area Index (LAI) map and relationships between LAI and canopy biomass. We observed that 30-50% of spike Hg applied to the upland and wetland was initially intercepted by the forest canopy. Average half lives of spike Hg on deciduous (110±30 days) and coniferous (180±40 days) forest canopy and ground vegetation (890±620 days) indicated that retention of new atmospheric Hg(II) on terrestrial vegetation delays downward transport of new atmospheric Hg(II) into the soil profile and delivery of this Hg(II) to methylating zones in wetlands and lakes. Measurements of re-emission of spike Hg from tree foliage using an Hg(0) flux chamber suggested that 40-70% of the spike initially retained in the forest canopy was photoreduced and re-emitted to the atmosphere. ~20% of the initial canopy spike pool was deposited in throughfall over the course of the growing season and a similar proportion was accounted for in litterfall. Spike Hg was still detectable on coniferous foliage the following spring (~2-8% of the initial pool), indicating that wet deposited Hg may contribute to foliar Hg concentrations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.H53K..02G
- Keywords:
-
- 0461 BIOGEOSCIENCES / Metals