Watershed Responses to Changes In Mercury Loading: Results from the Terrestrial Aspects of the METAALICUS Project
Abstract
The METAALICUS project is a whole-ecosystem experiment specifically designed to quantify the magnitude and timing of the watershed response to a change in mercury (Hg) loading. A decade ago, when the US and many other countries began to consider Hg-specific emission regulations, many doubted the effectiveness of such actions since terrestrial soils and aquatic sediments were already ubiquitously contaminated, and would potentially sustain mercury loading to aquatic indefinitely. To address this question, a multi-national team of scientists was formed to devise a whole-ecosystem, Hg-dosing study, whereby Hg would be deliberately added to an entire watershed - The Mercury Experiment to Assess Atmospheric Loadings in Canada and the US (METAALICUS) project, conducted at the Experimental Lakes Area (ELA), northwestern Ontario, Canada. Whole-ecosystem manipulation studies have a distinct advantage over small-scale (lab scale) studies, in that all the ecosystem components, natural processes and complexities within watersheds are accounted for. This paper focuses on the terrestrial (upland forest and wetland) aspects of METAALICUS project, with particular emphasis on assessing the long-term response to a change in Hg loading. A very unique aspect of the METAALICUS project called for the use of three different stable isotopes to be applied to the lake, forests and wetland, respectively. This experimental approach not only allowed for the direct observation of responses (time and space) to changes in loading, but also provides never before realized resolution of pathways and processes of mercury in a watershed. Mercury additions to the terrestrial components of the watershed were initiated in 2001 and continued through 2006, at a rate of about 5x measured current atmospheric deposition. Since this time, METAALICUS has become a mercury reduction experiment, and thus we have been monitoring of the response to a Hg load cessation. During the loading phase of the project, the majority of the terrestrially applied Hg isotopes were distributed approximately equally among three major compartments: soils, plants (tree canopy and ground vegetation), and losses to emissions from soil and plant surfaces. Much less (about 1%) isotope was measured in runoff that flowed into the downstream lake. With each successive annual dose the isotope pool in soils steadily increased, while the forest canopy and emission fluxes pools had comparatively low carryover from year to year. Within two years of loading cessation, isotope concentrations in canopy and emission fluxes were negligible and about half of the total isotope load was found in soils. Isotope concentrations in runoff gradually increased during the loading phase, and continued to increase for 1-2 years after loading ceased, suggesting significant translocation from compartments above the forest floor. Three years subsequent to cessation, isotope concentrations in soils have remained steady, and generally reflect the Hg abundance of the overall soil pool. Models calibrated to these results will be very useful for scenario testing to evaluate long-term recovery of lakes with significant watershed Hg contributions.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B23K..05K
- Keywords:
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- 0330 ATMOSPHERIC COMPOSITION AND STRUCTURE / Geochemical cycles;
- 0399 ATMOSPHERIC COMPOSITION AND STRUCTURE / General or miscellaneous;
- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry