Landscape Structure and Watershed Mercury Sensitivity in Boreal Headwater Regions

Aquatic mercury (Hg) contamination caused by industrial Hg emissions, atmospheric transport and deposition to sensitive ecosystems is an ongoing concern in many parts of the world. Boreal ecosystems are particularly sensitive to Hg deposition, and large soil-Hg burdens in these regions may prolong recovery of Hg impacted surface waters for many decades. Four studies were undertaken to examine interactions between watershed characteristics, hydro-climatic variability and terrestrial-aquatic export of key chemical parameters linked to watershed Hg sensitivity. Two new quantitative techniques, hydrogeomorphic edge detection and characteristic morphology analysis, were developed to explicitly map and characterize the spatial distribution, geomorphic form and hydro-biogeochemical function of forested wetlands using airborne Light Detection and Ranging (LiDAR) surveys. The results demonstrate the critical contribution of forested wetlands and upland-wetland interactions to the production and mobilization of dissolved organic carbon (DOC) and methyl-mercury (MeHg) - respectively - to downstream surface waters. Results of strategic, event-based hydrochemical sampling also demonstrate the critical contribution of summer high-flow periods to terrestrial-aquatic MeHg export. Finally, an analysis of historical monitoring databases of streamflow volume, hydrochemistry and Hg concentrations in yearling perch in two contrasting headwater lake basins was conducted. The results indicate strong potential for short-term, hydrologically-driven shifts in terrestrial-aquatic coupling and watershed Hg sensitivity, but only for the wetland-dominated, humic lake that exhibited consistent, summertime hypolimnetic anoxia. These various findings suggest that accurate characterization of watershed structure can help researchers identify first-order limitations on whole-watershed methylation efficiency, particularly in relation to hydro-climatic drivers of terrestrial-aquatic coupling in Boreal headwater regions.Ph