Is Mercury in a Remote Forested Watershed of the Adirondack Mountains Responding to Recent Decreases in Emissions? Jacqueline R. Gerson* ,† and Charles T. Driscoll Department of Civil and Environmental Engineering, Syracuse University, 151 Link Hall, Syracuse, New York 13244, United States * S Supporting Information ABSTRACT: Although there has been a decline in U.S. mercury emissions, the effects of this change on remote ecosystems are not well understood. We examine decadal (2004- 2015) responses of atmospheric mercury deposition, along with total mercury (THg) and methylmercury (MeHg) concentrations and fluxes, to decrease in mercury emissions at Arbutus Lake-watershed in the remote forested Adirondack region of New York, a biological mercury hotspot. Although wet mercury deposition remains constant, THg deposition has decreased through decreases in litter mercury inputs (17.9 to 10.8 μg/m 2 - yr) apparently driven by decreases in atmospheric concentrations of gaseous elemental mercury (Hg o ). While the lake is a net sink for THg and MeHg, concentrations and fluxes of THg and MeHg have decreased in the inlet stream and lake water apparently in response to decreases in Hg o deposition. Decreases in surface water mercury have occurred despite decadal increases in concentrations of dissolved organic carbon. Moreover, the fraction of THg as MeHg at the inlet has not changed despite decadal decreases in atmospheric sulfate deposition and surface water concentrations of sulfate. Our results indicate that recent decreases in U.S. mercury emissions have resulted in decreases in litter mercury deposition, and stream and lake THg and MeHg concentrations and fluxes, suggesting the first steps toward ecosystem recovery. ■ INTRODUCTION Mercury enters remote forested watersheds predominantly from primary atmospheric, secondary legacy emissions, and natural emission sources, which together supply Hg concen- trations in bodies of water. 1,2 Ionic Hg (Hg(II)) can be converted to methylmercury (MeHg), a potent neurotoxin that is bioaccumulated in food chains and biomagnified within the tissues of fish. Efforts are under way to limit Hg emissions both at the U.S. national (Mercury and Air Toxics Standard, MATS) and international (Minamata Convention) levels. 3,4 Although MATS has been challenged in the U.S. Supreme Court, it is essential to characterize the ecosystem benefits associated with regional emission controls. 5 Despite declines in U.S. Hg emissions since the 1990s, 6-8 total Hg deposition is the predominant input to remote forest watersheds in the Northeast and remains approximately 3.5 times greater than pre-industrial values. 2,9,10 While sediment stratigraphic studies in the Northeast have reported decreases in Hg deposition consistent with decreases in emissions, 9,11 the Adirondack region of New York has not experienced a change in wet Hg deposition. 12 Moreover, studies examining long-term changes in MeHg concentrations in fish have found mixed patterns; some report decreases in concentrations, 13,14 while others are showing increases in recent decades. 15-17 Since gaseous elemental mercury (Hg o ) has a relatively long atmospheric residence time (∼0.5 years), it can be transported to and deposited in remote areas far removed from emission sources; in contrast, oxidized forms (Hg(II), reactive gaseous mercury (RGM), and particulate mercury (HgP)) have short atmospheric residence times and are deposited near emission sources. 2,7,9,18 Once deposited in a watershed, Hg(II) can be methylated predominately by sulfate- or iron-reducing bacteria (SRB, IRB) in reducing environments such as wetlands and sediments. 1,19-23 In the northeastern United States, rates of methylation are highest in the summer, when warmer temperatures allow for increases in microbial activity and lower discharge increases hydraulic residence time. 24-28 The Adirondack Mountains of New York state have been shown to be a biological Hg hotspot characterized by high concentrations found in fish and aquatic birds due to abundant forest and wetland cover, as well as unproductive surface waters impacted by acid deposition. 29-31 Although a relatively small fraction of watershed Hg inputs are exported by drainage waters, the quantity and quality of fluvial Hg losses are a critical consideration since these pathways drive the transport and transfer of Hg to aquatic foodchains and ultimately lead to human and wildlife exposure. Watersheds leaching high concentrations of dissolved organic matter (DOM) are particularly effective at transporting Hg through the landscape, as thiol groups on dissolved organic carbon (DOC) strongly bind to Hg(II). 32-37 In northern watersheds, there is a widespread pattern of increases in DOC concentrations in surface waters; 38,39 researchers have specu- Received: April 28, 2016 Revised: September 14, 2016 Accepted: September 20, 2016 Published: September 20, 2016 Article pubs.acs.org/est © 2016 American Chemical Society 10943 DOI: 10.1021/acs.est.6b02127 Environ. Sci. Technol. 2016, 50, 10943-10950 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.