MERCURY EXPORT FROM SMALL FORESTED WATERSHEDS IN WESTCENTRAL NOVA SCOTIA, CANADA: DEVELOPMENT OF AN ECOLOGICAL MODEL
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As an efficient filter of airborne Hg compounds, forests retain a significant portion of the Hg deposited to the land. Forested watersheds have been identified as major sources of low-concentration Hg compounds to surrounding streams and lakes. Mercury export from forests is highly variable in both space and time. It is difficult to use field surveys alone to capture the spatiotemporal variation inherent in this variable. Mathematical models are required for improved representation. The objective of this Thesis is to develop and test a monthly dynamic model that can be used to estimate seasonal Hg export from forested watersheds to low-ordered forest streams. The fully developed model consists of four model components: (i) a forest hydrology component, to simulate variation in soil temperature, soil moisture, and stream discharge for input to the other model components; (ii) a forest nutrient cycling and biomass growth component, to simulate forest growth and litter production; (iii) a forest litter decomposition component, to simulate seasonal production of dissolved organic carbon (DOC); and (iv) a monthly DOC and Hg export component to simulate the translocation of DOC and Hg from forested watersheds to low-ordered streams. The Hg-export component incorporates an Hg-to-DOC binding coefficient estimated from a one-time stream survey of Hg and DOC concentrations. Simulations of in-stream Hg concentrations show two main trends: (i) an annual trend, associated with the seasonal (monthly) dynamics of forest litter production, decomposition, and DOC production and export, and (ii) a multiple-year trend, associated with forest harvesting and re-growth patterns of regenerating forests. This study demonstrates that (i) wetland- and conifer-dominated watersheds release a greater amount of Hg to aquatic ecosystems than upland- and deciduous species-dominated watersheds, and (ii) forests nearing maturity, export more Hg than young forests.