Is Mercury in a Remote Forested Watershed of the Adirondack Mountains Responding to Recent Decreases in Emissions?

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/m²-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.