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Is mercury in a remote forested watershed of the Adirondack Mountains responding to recent decreases in emissions? Jacqueline Rebecca Gerson, and Charles T. Driscoll Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02127 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016
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Environmental Science & Technology
Is mercury in a remote forested watershed of the Adirondack Mountains responding to recent decreases in emissions? Jacqueline R Gerson1 and Charles T Driscoll2*
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Corresponding Author; Syracuse University, Department of Civil and Environmental Engineering, 151 Link Hall, Syracuse NY 13244; 732-710-1844;
[email protected] 2 Syracuse University, Department of Civil and Environmental Engineering, 151 Link Hall, Syracuse NY 13244; 315-443-3434;
[email protected] ACS Paragon Plus Environment
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Environmental Science & Technology
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Abstract
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Although there has been a decline in US mercury emissions, the effects of this change on remote
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ecosystems are not well understood. We examine decadal (2004-2015) responses of atmospheric
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mercury deposition, along with total mercury (THg) and methylmercury (MeHg) concentrations
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and fluxes, to decrease in mercury emissions at Arbutus Lake-watershed in the remote forested
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Adirondack region of New York, a biological mercury hotspot. Although wet mercury
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deposition remains constant, THg deposition has decreased through decreases in litter mercury
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inputs (17.9 to 10.8 µg/m2-yr) apparently driven by decreases in atmospheric concentrations of
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gaseous elemental mercury (Hgo). While the lake is a net sink for THg and MeHg,
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concentrations and fluxes of THg and MeHg have decreased in the inlet stream and lake water
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apparently in response to decreases in Hgo deposition. Decreases in surface water mercury have
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occurred despite decadal increases in concentrations of dissolved organic carbon. Moreover, the
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fraction of THg as MeHg at the inlet has not changed despite decadal decreases in atmospheric
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sulfate deposition and surface water concentrations of sulfate. Our results indicate that recent
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decreases in US mercury emissions have resulted in decreases in litter mercury deposition, and
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stream and lake THg and MeHg concentrations and fluxes, suggesting the first steps toward
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ecosystem recovery.
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Keywords
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Atmospheric mercury deposition, lake, decadal, mercury, methylmercury, watershed
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Environmental Science & Technology
Introduction Mercury (Hg) enters remote forested watersheds predominantly from primary
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atmospheric, secondary legacy emissions, and natural emission sources, which together supply
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Hg concentrations in bodies of water (Fitzgerald et al., 1998; Morel et al., 1998). Ionic Hg
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(Hg(II)) can be converted to methylmercury (MeHg), a potent neurotoxin that is bioaccumulated
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in food chains and biomagnified within the tissues of fish. Efforts are under way to limit Hg
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emissions both at the US national (Mercury and Air Toxics Standard, MATS) and international
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(Minamata Convention) levels (UNEP, 2013; USEPA, 2014). Although MATS has been
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challenged in the US Supreme Court, it is essential to characterize the ecosystem benefits
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associated with regional emission controls (Sunderland et al., 2016). Despite declines in US Hg
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emissions since the 1990s (Pirrone et al., 1998; Driscoll et al., 2007; Zhang et al., 2016), total Hg
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deposition is the predominant input to remote forest watersheds in the Northeast and remains
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approximately 3.5 times greater than pre-industrial values (Fitzgerald et al., 1998; Lorey and
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Driscoll, 1999; Drevnick et al., 2012). While sediment stratigraphic studies in the Northeast
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have reported decreases in Hg deposition consistent with decreases in emissions (Lorey and
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Driscoll, 1999; Marvin et al., 2004), the Adirondack region of New York has not experienced a
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change in wet Hg deposition (Weiss-Penzias et al., 2016). Moreover, studies examining long-
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term changes in MeHg concentrations in fish have found mixed patterns; some report decreases
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in concentrations (Bhavsar et al., 2010; Monson et al., 2011), while others are showing increases
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in recent decades (Hammerschmidt and Fitzgerald, 2006; Paller and Littrell, 2007; Dittman and
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Driscoll, 2009).
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Since gaseous elemental mercury (Hgo) has a relatively long atmospheric residence time (~0.5 years), it can be transported to and deposited in remote areas far removed from emission
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Environmental Science & Technology
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sources; in contrast, emissions of oxidized forms (Hg(II), reactive gaseous mercury (RGM),
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particulate mercury (HgP)) have short atmospheric residence times and are deposited near
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emission sources (Swain et al., 1992; Fitzgerald et al., 1998; Lorey and Driscoll, 1999; Driscoll
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et al., 2007). Once deposited in a watershed, Hg(II) can be methylated predominately by sulfate-
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or iron-reducing bacteria (SRB, IRB) in reducing environments such as wetlands and sediments
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(Morel et al., 1998; Ullrich et al., 2001; Kerin et al., 2006; Tjerngren et al., 2012; Hsu-Kim et al.,
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2013; Podar et al., 2015). In the northeastern United States, rates of methylation are highest in
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the summer, when warmer temperatures allow for increases in microbial activity and lower
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discharge increases hydraulic residence time (Babiarz et al., 1998; Bowles et al., 2003; Galloway
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and Branfireun, 2004; Selvendiran et al., 2009). The Adirondack Mountains of New York State
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have been shown to be a biological Hg hotspot characterized by high concentrations found in
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fish and aquatic birds due to abundant forest and wetland cover, as well as unproductive surface
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waters impacted by acid deposition (Driscoll et al., 1994; Evers et al., 2007; Yu et al., 2013).
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Although a relatively small fraction of watershed Hg inputs are exported by drainage
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waters, the quantity and quality of fluvial Hg losses are a critical consideration since these
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pathways drive the transport and transfer of Hg to aquatic foodchains and ultimately lead to
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human and wildlife exposure. Watersheds leaching high concentrations of dissolved organic
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matter (DOM) are particularly effective at transporting Hg through the landscape, as thiol groups
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on dissolved organic carbon (DOC) strongly bind to Hg(II) (Bishop et al., 1995; Scherbatskoy et
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al., 1998; Gabriel and Williamson, 2004; Mast et al., 2005; Gilmour, 2011; Chiasson-Gould et
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al., 2014). In northern watersheds, there is a widespread pattern of increases in DOC
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concentrations in surface waters (Driscoll et al., in press; Monteith et al., 2007); researchers
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Environmental Science & Technology
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have speculated that increases in fish Hg reported in Scandinavia are linked to increases in the
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mobility of DOM (Åkerblom et al., 2012; Hongve et al., 2012).
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Many studies have been conducted to understand the fate and transformations of Hg in
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watersheds and freshwater ecosystems (Branfireun et al., 1998; Shanley et al., 2005; Watras et
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al., 2005). There have also been reports of long-term changes in Hg emissions, deposition, and
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concentrations in fish (Pirrone et al., 1998; Hammerschmidt and Fitzgerald, 2006; Paller and
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Littrell, 2007; Dittman and Driscoll, 2009; Bhavsar et al., 2010); however, little focus has been
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given to long-term changes in watershed fluxes of Hg. Given the inconsistency of the literature
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on fish Hg response to changes in emissions and deposition, there is a critical need for coupled
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time-series measurements of atmospheric Hg deposition and surface water Hg losses at the
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watershed level to understand the landscape response to decreases in atmospheric emissions.
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In this study, we evaluate a 10+-year record (2004-2015) of Hg deposition and losses and
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ancillary characteristics in Arbutus Lake-watershed in the Adirondacks to quantify the response
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of Hg to decreases in Hg emissions within a remote forest lake-watershed. Decadal
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measurements consisted of wet and litter Hg inputs, stream and lake discharge, and Hg
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concentrations. Specifically, we seek to answer the questions: 1) Are there decadal trends in
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THg and MeHg entering and being exported from Arbutus Lake? 2) What hydrochemical
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factors drive decadal changes in THg and MeHg concentrations and fluxes? We utilized one-
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way ANOVA and regression analyses to gain insights on data from the inlet and outlet of
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Arbutus Lake. We also evaluated Mann-Kendall trends of Hg and ancillary characteristics, as
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well as Hg fluxes entering and leaving the lake.
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Experimental
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Site Description
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Arbutus Lake is located in the Huntington Wildlife Forest (HWF) in the Adirondack
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Mountains of New York State (43°59’N, 74°14’W) (SI Figure 1). The watershed is 3.52 km2,
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with a mean slope of 11%, and total relief of 227 m (McHale et al., 2000). Previous studies in
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the HWF have examined atmospheric Hg concentrations and deposition, as well as the
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biogeochemistry of Hg and other elements in the Arbutus watershed uplands, wetlands, and lake
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(Mitchell et al., 1998; McHale et al., 2000; Bischoff et al., 2001; Choi et al., 2008a; Selvendiran
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et al., 2008; Blackwell et al., 2014).
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Arbutus watershed is predominately forested, containing northern hardwood species,
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including American beech (Fagus grandifolia), sugar maple (Acer saccharum), eastern hemlock
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(Tsuga canadensis), red spruce (Picea rubens), and balsam fir (Abies balsamea) (Bischoff et al.,
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2001). More than three quarters of the landscape contains hardwood species. Soil depth is
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generally
