Winter contribution to annual throughfall inputs of mercury and tracer ions at Acadia National Park, Maine.

Nelson, Sarah

'This dissertation examined major ion and mercury (Hg) atmospheric deposition and winter Hg mobility in forested watersheds at Acadia National Park. Throughfall fluxes of Hg and major ions were estimated using several methods in paired research watersheds with contrasting vegetation, and compared the results to the literature. Conifer forested sites had greater snow Hg concentrations (19.5 +- 7.7 ng/L) and deposition (27.8 +- 11.7 ug/m2/yr) than deciduous (concentration: 5.85 +- 2.6 ng/L, deposition 9.77 +- 6.4 ug/m2/yr) or non-vegetated (concentration: 4.33 +- 2.1 ng/L, deposition: 6.39 +- 4.3 ug/m2/yr) sites. The ratios of Hg in throughfall to open deposition in snowpack (ratio = 2.1) and snow events (ratio = 3.8) were greater than the throughfall to wet deposition ratio (1.8) during growing season as reported in the literature. This unexpected result indicates that enhancement of dry deposition by forest canopies is at least as important during winter months as during the growing season. However, estimates of Hg in snow throughfall deposition varied by up to a factor of two depending on the frequency of sampling and collection method. Individually sampled snow events had the greatest Hg deposition (3.11 +- 0.54 ug/m2/cold season), and snow left open to the atmosphere for the entire winter ('seasonal' snow) had the least (1.16 +- 0.21 ug/m2/cold season). Using labeled Hg tracer additions, we found that snowpack containing Hg contributed from underlying litter and soils, and its Hg burden was intermediate (2.38 +- 0.68 ug/m2/cold season) between seasonal and event snow Hg. These findings suggest that previous research at other sites has documented low Hg burdens in snow because of un-measured losses to the atmosphere. To place these winter Hg findings in context, I developed a conceptual model explaining throughfall flux at Acadia using marine tracers, coherence analysis, and spatial autocorrelation. In winter, throughfall chemistry showed a strong marine air mass signal (wet-only precipitation sulfate: chloride = 1.36), which contrasted dramatically with the summer signal sulfate: (chloride = 10.2). Spatial analysis indicated that precipitation amount was more variable and had a different spatial pattern in winter than in summer. The coherence analysis indicated that vegetation type was the primary influence on concentration and throughfall flux of most analytes during the growing season.' Thesis is divided into the following chapters: 1. Executive summary, 2. Literature review; 3 Deposition, re-emission, and evasion: mercury mass fluxes in winter at Acadia National Park, Maine, USA; 4. Characteristics of snow mercury concentration and deposition in eastern temperate North America; 5. Seasonal controls on the temporal coherence of mercury and major ion throughfall flux; 6. Implications and future work. Appendices include: A. Canon National Parks Science Scholars Program proposal, 2003; B. Detailed project methods; C. Cluster analysis report; D. Upland/watershed mercury data for Maine, USA; E. Throughfall chemistry, 1999-2005, Acadia National Park, Maine, USA; F. Coordinates and characteristics for throughfall collector sites at Acadia National Park, Maine, USA; G. Public summary of snow research. Research recommendations: Further study is needed 'to assess the precise processes that result in the translocation of soil-bound Hg to the overlying snowpack...' In addition: 'Future research on snow Hg deposition in this region should employ standardized field and laboratory methods, make Hg speciation measurements, and examine interannual and spatial variability.' 'The next steps in Hg research desired by National Park Service management are to document effects of this contaminant on biota, and to continue long-term monitoring to assess changes in atmospheric deposition as global and regional Hg emissions souces change through time (Manski, pers. comm.).' (Abstract continued in Notes)