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Deposition of mercury in forests along a montane elevation gradient Bradley D Blackwell, and Charles T. Driscoll Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505928w • Publication Date (Web): 30 Mar 2015 Downloaded from http://pubs.acs.org on April 7, 2015
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Deposition of mercury in forests along a montane elevation gradient
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Bradley D. Blackwell* and Charles T. Driscoll
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Department of Civil and Environmental Engineering, Syracuse University, Syracuse, NY 13244
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*corresponding author
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ABSTRACT Atmospheric mercury (Hg) deposition varies along elevation gradients and is influenced
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by both orographic and biological factors. We quantified total Hg deposition over a two year
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period at 24 forest sites at Whiteface Mountain, NY, USA that ranged from 450-1450 m above
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sea level and covered three distinct forest types: deciduous/hardwood forest (14.1 µg/m2-yr),
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spruce/fir forest (33.8 µg/m2-yr), and stunted growth alpine/fir forest (44.0 µg/m2-yr).
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Atmospheric Hg deposition increased with elevation, with the dominant deposition pathways
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shifting from litterfall in low-elevation hardwoods to throughfall in mid-elevation spruce/fir to
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cloud water in high-elevation alpine forest. Soil Hg concentrations (ranging from 69 to 416 ng/g
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for the Oi/Oe and 72 to 598 ng/g for the Oa horizons) were correlated with total Hg deposition,
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but the weakness of the correlations suggests that additional factors such as climate and tree
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species also contribute to soil Hg accumulation. Meteorological conditions influenced Hg
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deposition pathways, as cloud water Hg diminished in 2010 (dry conditions) compared to 2009
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(wet conditions). However, the dry conditions in 2010 led to increased Hg dry deposition and
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subsequent significant increases in throughfall Hg fluxes compared to 2009. These findings
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suggest that elevation, forest characteristics, and meteorological conditions are all important
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drivers of atmospheric Hg deposition to montane forests.
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INTRODUCTION
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Mercury (Hg) contamination is a major environmental issue for remote ecosystems that
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receive elevated deposition from long-range atmospheric transport. Anthropogenic activities
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have increased the global pool of available Hg by approximately a factor of three.1 Although Hg
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emissions have decreased in North America and Europe, activities in Asia have resulted in
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overall increases in global Hg emissions.1,2 Mercury deposition is difficult to estimate on a 2
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global basis because many factors can influence its land-atmosphere exchange. Land cover has
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been shown to impact Hg deposition as forested areas enhance Hg deposition processes.3-6
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Differences in forest type also impact Hg deposition.7-9 Additionally, geographic features such
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as latitude, longitude, and elevation have been correlated to Hg deposition.10-12
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Previous studies have shown that high-elevation forests receive elevated atmospheric
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deposition of anthropogenic pollutants.13-15 Mercury concentrations in soils increase with
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increases in elevation,16,17 and two montane areas of the northeastern United States (the
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Adirondack and Catskill Mountains) are considered to be “biological Hg hotspots”.18 The steep
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elevation gradients found in mountainous regions create abrupt changes in climate, which in turn
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lead to shifts in forest communities from low-elevation hardwood forest to high-elevation conifer
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forest. As both meteorological and biological factors influence Hg deposition, there is potential
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for marked variability in Hg deposition processes along mountain gradients, even over relatively
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small distances. Mountainous areas are also likely to experience more severe changes in
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meteorological conditions driven by human-induced climate change than low-elevation areas.19
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Mountainous areas have shallow soils, harbor sensitive plant and wildlife species, and serve as
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recharge areas for down gradient water resources. As a result, there is a critical need to
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characterize and quantify atmospheric deposition to montane landscapes.
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The purpose of this study was to quantify Hg deposition over a 1000+ m elevation
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gradient in the Adirondack Mountains of New York State, USA and to evaluate the role of
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physical factors (precipitation quantity, solar radiation, etc.) and biological factors (forest type)
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in Hg deposition processes. We estimated deposition through both wet and dry processes and
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measured Hg concentrations in organic soil horizons. As this study spanned two growing
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seasons, we also examined effects of meteorology on Hg deposition by comparing Hg deposition 3
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calculations from a wet, cool, predominantly overcast growing season (2009) with Hg deposition
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calculations from a warm, dry, and predominantly sunny growing season (2010). In addition,
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this study could serve as a baseline for monitoring both reductions in Hg emissions in North
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America and long-term variation in Hg deposition as a result of climate change.
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MATERIALS & METHODS
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Study Site and Experimental Design
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Whiteface Mountain is located in the northeast portion of the Adirondack Park near
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Wilmington, NY (summit location 44.37°N, 73.90° W) (Figure 1). The mountain has a
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prominence of over 1,000 meters, with a base elevation of approximately 400 m above sea level
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and a summit elevation of 1483 m. Whiteface is home to the State University of New York
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Atmospheric Science Research Center (ASRC) and has atmospheric monitoring stations located
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at approximately 610 m and at the summit. There is also a monitoring site for the National
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Atmospheric Deposition Program National Trends Network (NADP NTN) and a site for cloud-
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chemistry monitoring operated by the NY Department of Environmental Conservation. Mercury
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Deposition Network (MDN) stations are located approximately 50 km (Huntington Forest, NY;
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NY20) and 80 km (Underhill, VT; VT99) from the summit of Whiteface.
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Forest communities of Whiteface Mountain are segregated into three distinct zones with
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small transition zones between. Lower elevations (hardwood zone) are characterized by northern
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hardwood forest dominated by sugar maple (Acer saccharum), yellow birch (Betula
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alleghaniensis), red maple (Acer rubrum), and American beech (Fagus grandifolia) and range
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from 400 m to approximately 900 m. Canopy heights in the hardwood forest range from 10-20
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m on average, and the understory is relatively open. Mid-elevations (spruce/fir zone) are thick
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stands of balsam fir (Abies balsamea) and red spruce (Picea rubens) with interspersed paper 4
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birch (Betula papyrifera) and range from approximately 1000 m to 1300 m. The canopy heights
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of the mid-elevations ranges from 5-10 m, and the understory ranges from open to extremely
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dense growths of young evergreens. The third zone is an alpine forest zone that is comprised
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almost entirely of sparse, stunted-growth balsam fir with occasional red spruce, paper birch, and
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mountain ash (Sorbus americana). The alpine zone ranges from approximately 1350 m to the
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summit. The canopy height in this area rarely exceeds 2 m, and much of the “canopy” is less
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than 1 m in height. This study was designed to estimate atmospheric Hg deposition along the entire elevation
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gradient of Whiteface Mountain. This goal was accomplished by establishing transects along
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both the southwest and northeast sides of Whiteface (Figure 1). Each transect contained 12 study
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plots that spanned a range of elevations and forest types. For both transects, four plots were
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established at regular elevation intervals in each of the three major vegetation zones (hardwood,
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spruce/fir, alpine). Samples were collected from the transects between 1 June 2009 and 6 June
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2011.
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Sample and Data Collection
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After selecting plots, we recorded latitude and longitude using a handheld global
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positioning system (GPS). Elevation was estimated using a combination of topographic maps,
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handheld GPS, and a digital elevation model of the Adirondack Park. Throughfall collectors
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were installed at each site, and throughfall was collected every 10-30 days during the growing
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season. Green canopy foliage was collected from high canopy trees in late September of both
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2009 and 2010. Senesced litter was collected in plastic crates at each site throughout the year.
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Organic soil samples were collected from each plot. Two subsamples were taken from the
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horizons of each soil core. The Oi and Oe horizons, which are made up of slightly to 5
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intermediately decomposed leaf litter and organic matter, were combined into a single sample.
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The Oa horizon, made up of highly decomposed organic matter, was also collected.
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addition, a cloud water collector was installed at the summit to estimate deposition via cloud
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water. Detailed information about the methods used to collect samples can be found in the
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supporting information.
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In
Meteorological data, which included air temperature, wind speed, cloud frequency, and
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precipitation quantity, were measured at the summit of Whiteface and are included in the
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supporting information (Table S1).
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Laboratory Analysis
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All solid samples (foliage, litter, soil) were freeze dried for at least 72 hours to remove
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moisture and were then analyzed for total Hg using EPA Method 7473
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(http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/7473.pdf). A subsample of solid
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samples was also analyzed for total carbon and total nitrogen content. Aqueous samples
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(throughfall, cloud water) were fixed with hydrochloric acid and stored at 4° C before analysis
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for total Hg using EPA Method 1631e
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(http://water.epa.gov/scitech/methods/cwa/metals/mercury/upload/2007_07_10_methods_metho
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d_mercury_1631.pdf). Throughfall samples collected in 2010 were also analyzed for NO3-,
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SO42-, and dissolved organic carbon (DOC). Detailed information on laboratory analysis,
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including quality control, is included in the supporting information.
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Deposition calculations
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Sampling years were designated as beginning June 1 of each year to simplify flux
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calculations. Sampling year 2009 was defined as 1 June 2009 to 31 May 2010, and sampling
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year 2010 was defined as 1 June 2010 to 31 May 2011. Throughfall Hg fluxes were calculated 6
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by multiplying throughfall concentrations by throughfall quantities. During the growing season,
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both components were measured directly at each plot. For periods when samples were not
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collected directly, wet Hg deposition was estimated at each plot using the NTN and MDN data
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and multiplying those data by the average precipitation and Hg enrichment factors for each plot.
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Please see the supporting information for further explanation of this method.
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Litterfall Hg fluxes were calculated by multiplying litterfall mass per unit area by foliar
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Hg concentrations. Cloud fluxes were estimated by multiplying mean measured cloud Hg
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concentrations with a modified cloud water quantity model for Whiteface Mountain outlined in
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Miller et al.14 This model was optimized specifically for Whiteface Mountain and predicts annual
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cloud water quantity at different elevations. Mercury deposition fluxes were calculated for
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litterfall, throughfall, and cloud water at each plot. These deposition pathways were assumed to
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be independent of each other, and total Hg fluxes were assumed to be the sum of the three
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components. Deposition of Hg in each forest zone was estimated by averaging calculated fluxes
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from all eight plots within each zone. Refer to supporting information for more detailed
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explanations of calculation of deposition estimates.
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Statistical Analysis
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Regression analysis was used to examine patterns in elevation with measured chemistry
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variables and estimated Hg deposition. One-way ANOVA was used to determine statistically-
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significant differences among the different forest zones. Data that were not normally distributed
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were log-transformed before analysis. Statistical significance was defined at α < 0.05. All data
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analysis was performed using IBM SPSS Statistics 19.0.
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RESULTS
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Meteorological Variables 7
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Weather patterns in 2009 were markedly different from weather patterns in 2010, as 2009
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was generally cool and overcast with regular rain showers, while 2010 was warmer with less
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cloud cover and precipitation. A substantially greater proportion of the rainfall in 2010 was the
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result of storm events. Although it was not possible to collect meteorological data at each site,
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data was collected at the summit and is summarized in Table S1 of the supporting information.
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Throughfall and Cloud Hg Concentrations
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Concentrations of Hg in throughfall varied among the different forest types, with the
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lowest concentrations found in the alpine zone and highest concentrations found in the spruce/fir
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zone (Figure 2). Results of one-way ANOVA of log-transformed throughfall Hg concentrations
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indicate that the differences among forest types were significant (F(2,140) = 10.3, p
