This project builds on ongoing, large-scale experiments where for six years mercury has been added to whole watersheds including upland forests, wetlands and their associated lakes. The goal is to understand trace metals cycling and to assess how newly deposited substances move through the watershed into wetlands and lakes. In this experiment, additions were originally made to the uplands, wetlands and lakes using different enriched stable isotopes, so that now the pathways of biogeochemical cycling within and among these ecosystems can be determined. Methylmercury levels in fish responded rapidly to the mercury added directly to the lake. But almost all of the mercury applied to the watershed has thus far remained there, through biogeochemical mechanisms of internal ecosystem recycling and retention that remain unknown. Researchers have a unique opportunity to determine the biogeochemical mechanisms responsible for the high degree of retention. Controls over the uptake, storage, transformation and movement of mercury among the vegetation, mineral soil, and soil and sediment organic components will be determined. The results will reveal the time scales at which trace metals are cycled in different types of ecosystems, a major unknown variable critical to modeling future dynamics. Knowing the time scale at which lakes respond to increases in deposition is especially important for the formulation of environmental policy. This interdisciplinary project will have educational impacts through support of graduate students and post-doctoral associates.
Normal 0 false false false EN-US JA X-NONE Methylmercury contamination of fisheries from centuries of industrial atmospheric emissions negatively impacts humans and wildlife worldwide. The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. The Mercury Experiment to Assess Atmospheric Loading in Canada and the United States (METAALICUS) project is a whole-watershed mercury loading experiment being carried out at the Experimental Lakes Area (ELA) in northwestern Ontario. Begun in 2001, the study was designed to directly test the timing and magnitude of ecosystem response to an increase in mercury deposition, at a watershed scale. METAALICUS is being accomplished by a multidisciplinary team of researchers from the US and Canada. From 2001 to 2007, the METAALICUS team increased the Hg load to boreal Lake 658 and its watershed roughly five times over ambient wet deposition. During this loading phase, mercury additions to upland, wetland and lake were made using different enriched stable isotopes. The isotopes allowed the team to distinguish the experimentally applied mercury and the mercury already present in the ecosystem, and to examine bioaccumulation of mercury deposited to different parts of the watershed. Fish methylmercury concentrations responded rapidly to increased mercury loaded directly to the lake. However, almost all of the mercury applied to the watershed either remained there after seven years or was returned to the atmosphere, and did not accumulate in the lakes aquatic food web. Watershed retention of Hg therefore delays the timing of a complete ecosystem response to changing Hg loads, and this lag time remained unknown. Beginning in 2007, the project shifted to a recovery phase. The major goals of the recovery phase of the METAALICUS study were to follow the fate of the Hg spikes to the L658 ecosystem through time after loading stopped in 2006/2007, and conduct biogeochemical research to advance understanding of the processes controlling their fate. This award specifically supported the fate and biogeochemistry of Hg isotopes in upland soils and lake sediments, with the context of the larger METAALICUS study. Five years after Hg loading was stopped, recovery of L658 from the Hg load to the lake is nearly complete. The rate of recovery occurred at about the same rate as the response to increased loads. Concentrations of lake spike Hg in water and small fish were nearly undetectable by 2012. The concentration of MeHg produced from lake spike in aquatic sediments has also steadily declined. There is some "memory" of the lake spike in sediments, where methylmercury continues to be produced from lake spike Hg, but now at a much-reduced rate. The bioavailability of Hg deposited to lake sediments for microbial methylation decreased through time, through changes in complexation, partitioning to sediments and finally burial. Associated laboratory research demonstrated that Hg bioavailability for microbial methylation is impacted by the rate of formation of particulate mercury sulfide, a process that is slowed in the presence of dissolved organic matter. The results suggest that declines in Hg bioavailability over time in nature depend on the concentration and character of DOC, and therefore the rate permanent removal of Hg from actively methylated pools in sediments will depend on sediment chemistry. The L658 watershed was labeled over many years with atmospherically-relevant loads of traceable, enriched stable Hg isotopes, and it provides a one-of-a-kind chance to study the fate of Hg in watersheds. By following the Hg isotope applied to the upland over the decade, the timing of delivery of Hg deposited to watersheds to receiving water bodies was roughly estimated, and the processes modeled. The timing of the lag in Hg flux depends on watershed variables such as hydrologic connectivity, soil structure and type. Importantly, a portion of Hg deposited to watershed accumulates in mineral soil horizons in near-permanent storage. Roughly 45 % of the Hg deposited to the watershed is lost to re-emission, while roughly 25 % is now retained in very long-term storage in soil mineral horizons. The portion of Hg that was reemitted to the atmosphere was larger prior estimates for terrestrial ecosystems. The remaining 30% may be released to the lake or moved into storage in soil mineral horizons during transport. The time frame for transport of this fraction appears to be roughly 1-2 decades. Since watershed export is now declining, the final total stream export of Hg deposited to the watershed is likely be roughly 15%. The METAALICUS study has provided the only direct experimental information on magnitude and timing of response of watersheds to changing atmospheric mercury loads. The results of the study continue to have significant impact on environmental policy formulation.
