Vegetative and landscape influences on forest litter mercury at Acadia National Park

Sheehan, Katherine

Following taken from abstract: 'Mercury deposition to the landscape is typically evaluated using data from precipitation Hg deposition. Litterfall is also believed to be a major flux of Hg to soils in forested landscapes, yet much less is known about litterfall Hg flux. To quantify this component of Hg cycling in forested landscapes, we measured litterfall Hg contributions to total Hg deposition in two small research watersheds in Acadia National Park, Maine. Hadlock Brook and Cadillac Brook watersheds are of similar size, relief, and are exposed to the same climatic conditions, yet have differences in vegetative community composition due to a fire that burned the majority of the Cadillac watershed in 1947. Litterfall was collected at 39 sampling sites in 2003 and 2004 and analyzed for total Hg. A sub-set of samples were analyzed for C and N. at each collection site, vegetation class, aspect, elevation, and canopy density were determined. In addition, two digestion techniques were evaluated for total Hg analysis and a small-scale decomposition study using leached litter was conducted by deploying litter bags in the O-horizon of each watershed.' 'The four vegetation classes designated in this study were hardwoods, softwoods, mixed and scrub. The mean litter Hg concentration in softwoods (58.8 +- 3.3 ng Hg g-1) was significantly greater than in mixed (41.7 +- 2.8 ng Hg g-1) and scrub (40.6 +- 2.7 ng Hg g-1) and significantly lower than in hardwoods (31.6 +- 2.6 ng Hg g-1). In contrast, the mean weighted litter Hg flux was not significantly different among vegetation classes. The lack of a significant difference in litter Hg flux between hardwoods and softwoods was attributable to the large autumnal hardwood litter Hg flux being balanced by the higher softwood litter Hg concentrations, along with the higher chronic litterfall flux throughout the winter and spring in softwoods.' 'Landscape characteristics (i.e., aspect, elevation and canopy density) were significantly correlated with litter Hg concentrations and flux. No mechanistic linkages could be inferred from these correlations since there was a high degree of covariance among these variables and vegetation types. The direct influence of vegetation type on Hg dynamics in these forested watersheds was clear.' 'A significant negative correlation was defined between litter C:N and litter Hg concentrations suggesting that C:N could be a potential predictor of litter Hg concentrations. The correlation was believed to largely reflect the differences in litter type and the amount of woody plant tissue in the litter sample. We also found some evidence to suggest that Hg concentration increases over the winter months in decomposing litter and that Hg was strongly bound to the non-labile components of organic matter.' 'The estimated annual deposition of Hg via litterfall in Hadlock Brook watershed (10.1 ugm-2 and Cadillac Brook watershed (10.0 ugm-2) was greater than precipitation Hg deposition and similar to or greater than the magnitude of Hg deposition via throughfall. These results demonstrate that litterfall Hg flux to forested landscapes is at least as important as precipitation Hg inputs. Further research should define (a) the degree to which these results are representative of the larger forested landscapes of the eastern U.S., and (b) design options to incorporate estimates of Hg litter flux in conjunction with precipitation Hg monitoring efforts.' /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND/Cadillac Brook/Hadlock Brook/