Laura Elizabeth Flucke

USGS2025

Application of mercury stable isotopes to examine sources and hydrologic factors impacting mercury bioaccumulation and cycling in invertebrates of a model saline lake

Invertebrates, such as brine shrimp and brine flies, are key prey items for millions of resident and migratory birds that utilize saline lakes such as Great Salt Lake (GSL). Elevated methylmercury (MeHg) in invertebrate and waterfowl species of GSL has been assumed to be linked to elevated MeHg in GSL’s anoxic Deep Brine Layer (DBL) where aqueous concentrations can exceed 30 ng/L. Here, we leverage mercury (Hg) concentration and stable isotope measurements on brine flies ( Ephydra hians and Ephydra cinerea ), brine shrimp ( Artemia franciscana ), and spider (western spotted orbweaver [ Neoscona oaxacensis ]) to examine temporal changes in Hg concentrations and sources during periods of DBL presence and absence. Mercury concentrations in brine flies were inversely correlated with lake level and directly correlated with salinity, possibly resulting from factors such as enhanced Hg bioaccumulation due to osmoregulatory stress and stunted growth and/or elevated salinities impacting composition, abundance, and Hg concentrations of food sources. DBL presence did not correspond to higher invertebrate Hg concentrations, highlighting that the DBL is not the primary source of MeHg to biota. Hg stable isotope signatures (Δ 199 Hg and δ 202 Hg) in brine shrimp varied seasonally and indicated greater cumulative photochemical Hg loss from the water column in late summer and fall months. Co-located brine fly and western spotted orbweaver samples show equivalent Δ 199 Hg and δ 202 Hg signatures, supporting Hg transfer from the aquatic to terrestrial food webs. Furthermore, Hg isotope results (Δ 200 Hg) indicate that the majority of Hg accumulating in GSL invertebrates is of atmospheric origin. This study highlights temporal controls on Hg bioaccumulation within GSL, which will help assess Hg cycling within the system in response to management actions and declining lake levels.

Utah↗

USGS2023

National-scale assessment of total gaseous mercury isotopes across the United States

With the 2011 promulgation of the Mercury and Air Toxics Standards by the U.S. Environmental Protection Agency, and the successful negotiation by the United Nations Environment Program of the Minamata Convention, emissions of mercury (Hg) have declined in the United States. While the declines in atmospheric Hg concentrations in North America are encouraging, linking the declines to changing domestic and global source portfolios remains challenging. To address these research gaps, the U.S. Geological Survey initiated the first national-scale effort to establish a baseline of total gaseous mercury stable isotope values at 31 sites distributed across the United States. Results indicated that unique Hg sources, such as Hg evasion from an elemental Hg contaminated site or free tropospheric intrusions in high altitude sites, were distinguishable from background atmospheric values. Minor gradients were observed across the nation, with regions of heavy industrial activity demonstrating lower δ 202 Hg, but no consistent changes in other isotopes such as Δ 199 Hg and Δ 200 Hg were observed. Furthermore, δ 202 Hg was impacted by foliar uptake and senescence but trends varied between forested regions in the northeastern and midwestern United States. These data demonstrate regional emission sources and other environmental variables can impact total gaseous Hg (TGM) isotope values, highlighting the need to characterize atmospheric Hg isotopes over larger geographical areas to evaluate changes related to national and international Hg regulations.

JGR Atmospheres↗