Mercury (Hg) emissions to the atmosphere can transport at global scales and subsequently deposit anywhere, including pristine national parks. As Hg accumulates in the ecosystem it can be toxic to natural resources the National Park Service is mandated to protect. Possible deleterious effects include reduced foraging efficiency and reproductive success of wildlife, and the issuance of fish consumption advisories to protect human health. But since Hg is a naturally occurring element it can often be challenging to distinguish how much arrives from anthropogenic sources. Therefore, we measured Hg in more than 3,900 fish from 239 remote lakes and rivers (spanning 51 fish species and 56 national parks in 33 states), analyzed composite samples for Hg isotopic signatures to assess sources, and compared eastern fish Hg concentrations to health benchmarks.

Preliminary findings suggest that across all parks, sites, and species, fish Hg concentrations ranged from 7.3 to 2,663 ng/g ww with a mean of 149.6 ng/g ww. Fish Hg levels varied greatly both among and within parks, suggesting that patterns of Hg risk are driven by processes occurring at site specific, local, and continental scales. Measurements of d²⁰²Hg, D¹⁹⁹Hg, and D²⁰⁰Hg have been shown to be effective indicators of Hg source, photo-chemical processing, and atmospheric transport, respectively. Preliminary data indicate that the atmospheric tracer D²⁰⁰Hg was observed in most sites, but to a lesser extent in the east, suggesting regional Hg sources that have not undergone long range atmospheric transport. Western sites show enhanced D¹⁹⁹Hg likely tied to precipitation inputs and increased photochemical processing, whereas, lower D¹⁹⁹Hg and tightly clustered d²⁰²Hg data from the eastern sites suggests a different Hg input pathway, likely dry deposition driven by regional Hg emitters. These trends are clearer when constrained to a single fish species. Preliminary results also indicate that Hg concentrations were below EPA’s fish tissue criterion for safe human consumption in 81% of the fish sampled. However, Hg levels in individual fish at some sites from 21 of 35 eastern national parks exceeded the human health criterion. Mercury concentrations in at least one individual exceeded the most conservative fish toxicity benchmark at 45% of all sites, and the most sensitive health benchmark for fish-eating birds at 76% of all sites. Much of the mercury found in these mainly remote areas is likely the result of air pollution from outside the parks. Future targeted research and monitoring across park habitats would help identify patterns of Hg distribution across the landscape and facilitate informed management decisions aimed at reducing the ecological risk posed by Hg contamination in sensitive ecosystems.