Park Air Profiles - Isle Royale National Park

Air Quality at Isle Royale National Park

Most visitors expect clean air and clear views in parks. Isle Royale National Park (NP), Michigan, is a heavily forested, remote island in Lake Superior that experiences relatively good air quality. However, air pollution from mainland sources in Canada and the Midwest, including pollutants from industries along the Ohio River Valley, does affect the park. Air pollutants blown into the park can harm natural and scenic resources such as soils, surface waters, plants, wildlife, and visibility. The National Park Service works to address air pollution effects at Isle Royale NP, and in parks across the U.S., through science, policy and planning, and by doing our part.

Nitrogen and Sulfur

Aerial view of Locke Point
Visitors come to Isle Royale NP to explore scenic views of the remote island in Lake Superior, as well as to enjoy fishing, scuba diving, and boating.

Nitrogen (N) and sulfur (S) compounds deposited from the air may have harmful effects on ecosystem processes. Healthy ecosystems can naturally buffer a certain amount of pollution, but once a threshold is passed the ecosystem may respond negatively. This threshold is the critical load, or the amount of pollution above which harmful changes in sensitive ecosystems occur (Porter 2005). N and S deposition change ecosystems through eutrophication (N deposition) and acidification (N + S deposition). Eutrophication increases soil and water nutrients which causes some species to grow more quickly and changes community composition. Ecosystem sensitivity to nutrient N enrichment at Isle Royale National Park (ISRO) relative to other national parks is very low (Sullivan et al. 2016); for a full list of N sensitive ecosystem components, see: NPS ARD 2019. Acidification leaches important cations from soils, lakes, ponds, and streams which decreases habitat quality. Ecosystem sensitivity to acidification at ISRO relative to other national parks is high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.

From 2017-2019 total N deposition in ISRO ranged from 4.0 to 6.5 kg-N ha-1 yr-1 and total S deposition ranged from 1.6 to 2.4 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). See the conditions and trends website for park-specific information on N and S deposition at ISRO.

Boreal lakes—including Sargent and Richie—may be particularly sensitive to N enrichment, which could rapidly affect algal communities and lake biodiversity (Saros 2008; Sullivan et al. 2016).

At ISRO, thin, undeveloped soils, and low buffering capacity result in surface waterways and soils that are vulnerable to acidification (Sullivan et al. 2016). S is a concern at ISRO because it plays an essential role in the methylation of mercury, leading to toxic accumulation of methylmercury in fish and wildlife.

Epiphytic macrolichen community responses

Epiphytic macrolichens grow on tree trunks, branches, and boles. Since these lichens grow above the ground, they obtain all their nutrients directly from precipitation and the air. Many epiphytic lichen species have narrow environmental niches and are extremely sensitive to changes in air pollution. Geiser et al. (2019) used a U.S. Forest Service national survey to develop critical loads of nitrogen (N) and critical loads of sulfur (S) to prevent more than a 20% decline in four lichen community metrics: total species richness, pollution sensitive species richness, forage lichen abundance, and cyanolichen abundance.

McCoy et al. (2021) used forested area from the National Land Cover Database to estimate the impact of air pollution on epiphytic lichen communities. Forested area makes up 380 km2 (17.1%) of the land area of Isle Royale National Park.

  • N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 100% of the forested area.
  • S deposition was below the 2.7 kg-S ha-1 yr-1 critical load to protect S-sensitive lichen species richness in every part of the forested area.

For exceedances of other lichen metrics and the predicted decline of lichen communities see Appendices A and B of McCoy et al. (2021).

Additional modeling was done on 459 lichen species to test the combined effects of air pollution and climate gradients (Geiser et al. 2021). A critical load indicative of initial shifts from pollution-sensitive toward pollution-tolerant species occurred at 1.5 kg-N ha-1 yr-1 and 2.7 kg-S ha-1 yr-1 even under changing climate regimes.

Plant species response

Plants vary in their tolerance of eutrophication and acidification, and some plant species respond to nitrogen (N) or sulfur (S) pollution with declines in growth, survival, or abundance on the landscape. Horn et al. (2018) used the U.S. Forest Service national forest survey to develop critical loads of N and critical loads of S to prevent declines in growth or survival of sensitive tree species. Clark et al. (2019) used a database of plant community surveys to develop critical loads of N and critical loads of S to prevent a decline in abundance of sensitive herbaceous plant species. According to NPSpecies, Isle Royale National Park contains:

  • 2 N-sensitive tree species and 68 N-sensitive herbaceous species.
  • 6 S-sensitive tree species and 55 S-sensitive herbaceous species.

Persistent Pollutants

Park Ranger collecting dragonfly larvae
A park ranger searches for dragonfly larvae to monitor mercury at Lake Harvey in Isle Royale NP. Dragonfly larvae are good indicators of environmental mercury levels.

Airborne mercury, and other toxic air contaminants, when deposited are known to harm birds, salamanders, fish and other wildlife, and cause human health concerns. These substances enter the food chain and accumulate in the tissues of organisms causing reduced reproductive success, impaired growth and development, and decreased survival.

Power plants and other sources of air pollution on the mainland contribute to the deposition of toxics at Isle Royale NP. Mercury levels in lake sediment, fish, birds, and mammals in the Great Lakes region have been declining in recent decades in response to pollution controls on mercury discharge to surface waters and decreased air emissions (Evers et al. 2011). However, mercury pollution remains a major concern (Evers et al. 2011; Weiner et al. 2011) and air emissions are now the primary source of mercury pollution (Evers et al. 2011). Isle Royale NP is particularly sensitive to mercury pollution. The abundance of wetlands, low pH lakes, complex food webs, and predatory fish creates an environment susceptible to the bioaccumulation of toxics.

The first study documenting air toxics at the park assessed concentrations of PCBs, DDT, and more in fish in 1978. Since then, more than three decades of scientific studies at the park continue to show elevated concentrations of many contaminants—specifically mercury and PCBs—in air, precipitation, sediment, fish, and loons (Swackhamer and Hornbuckle 2004).

Mercury and toxics effects:

  • Elevated mercury and PCB concentrations and State of Michigan fish consumption advisories for fish caught in Siskiwit Lake. Learn more about fishing at Isle Royale NP.
  • Concentrations of mercury in pike at levels associated with adverse health and reproductive effects (NPS 2010; Sandheinrich et al. 2011) as well as cell damage and liver toxicity (Drevnick et al. 2008)
  • Concentrations of mercury in loon blood and feathers are high enough to affect behavior and reduce reproductive success (Sandheinrich et al. 2011; Evers et al. 2011; Evers et al. 1998; Scheuhammer and Blancher 1994);
  • Mercury detected in deer mice (Vucetich et al. 2001) and moose teeth (Vucetich et al. 2009), a sign that mercury is accumulating in the land-based food web;
  • Elevated mercury in rain and snow at monitoring sites near Isle Royale NP (Risch et al. 2012);
  • Pesticides including atrazine and cyanazine detected in rainfall at the park (Thurman and Cromwell 2000);
  • Contaminants including pesticides, PCBs, and mercury detected in herring gull eggs (Bowerman et al. 2011);
  • PCBs detected in tree bark, verifying long-range transport of persistent organic pollutants to the park (Hermanson and Hites 1990);
  • PBDEs (flame retardants) found in freshwater mussels collected from 10 inland lakes in the park (Chernyk et al. 2002).


View of water at ISRO
Clean, clear air is essential to appreciating the scenic vistas at Isle Royale NP.

Visitors come to Isle Royale NP to enjoy the spectacular remote islands in the vastness of Lake Superior, with forests, inland lakes, and opportunities to see wildlife. Park vistas are sometimes obscured by haze, reducing how well and how far people can see. Visibility reducing haze is caused by tiny particles in the air, and these particles can also affect human health. Many of the same pollutants that ultimately fall out as nitrogen and sulfur deposition contribute to this haze. Additionally, organic compounds, soot, and dust reduce visibility. Smoke from nearby forest fires also contributes to particulate matter in the region. Significant improvements in park visibility have been documented since the 2000’s. Overall, visibility in the park still needs improvement to reach the Clean Air Act goal of no human caused impairment.

Visibility effects:

  • Reduced visibility, at times, due to human-caused haze and fine particles of air pollution, including dust;
  • Reduction of the average natural visual range from about 115 miles (without pollution) to about 100 miles because of pollution at the park;
  • Reduction of the visual range to below 45 miles on very hazy days.

Visit the NPS air quality conditions and trends website for park-specific visibility information. Isle Royale NP has been monitoring visibility since 1988. Explore scenic vistas of Lake Superior and other sites in the Great Lakes via live webcams, and explore air monitoring »

Ground-Level Ozone

Butterfly on milkweed plant
Milkweed is one of the ozone sensitive species found at Isle Royale NP.

At ground level, ozone is harmful to human health and the environment. Ground-level ozone does not come directly from smokestacks or vehicles, but instead is formed when other pollutants, mainly nitrogen oxides and volatile organic compounds, react in the presence of sunlight.

Over the course of a growing season, ozone can damage plant tissues making it harder for plants to produce and store food. It also weakens plants making them less resistant to disease and insect infestations. Some plants are more sensitive to ozone than others. Ozone-sensitive plants in Isle Royale NP include Apocynum androsaemifolium (Spreading dogbane), Ascelpias syriaca (Common milkweed), and Prunus serotina (Black cherry). A risk assessment that considered ozone exposure, soil moisture, and sensitive plant species concluded that plants in Isle Royale NP were at low risk of damage to plant leaves (see network report: Kohut 2004). Ozone injury to plants has not been documented in regions near Isle Royale NP (Swackhamer and Hornbuckle 2004). Search for more ozone-sensitive plant species found at Isle Royale NP.

Visit the NPS air quality conditions and trends website for park-specific ozone information.

Bowerman, W., Moore, L., Leith, K., Drouillard, K., Sikarskie, J., Best, D., Allan, T., Garvon, J., Scharf, W., Perlinger, J., and Romanski, M. 2011. Concentrations of Environmental Contaminants in Herring Gull Eggs from Great Lakes Colonies in Michigan, 2002–2006. MI/DEQ/WRD—12/007. Michigan Department of Environmental Quality: Lansing, MI. 68 pp.

Chernyk, S., Hickey, J., and Benoche, I. 2002. PBDEs in Great Lakes Biota. Proceedings from Society of Environmental Toxicology and Chemistry: North America. Salt Lake City, UT: 16–20.

Clark, C.M., Simkin, S.M., Allen, E.B. et al. Potential vulnerability of 348 herbaceous species to atmospheric deposition of nitrogen and sulfur in the United States. Nat. Plants 5, 697–705 (2019).

Drevnick P. E., Roberts, A. P., Otter, R. R., Hammerschmidt, C. R. Klaper, R., and Oris, J. T. 2008. Mercury toxicity in livers of northern pike (Esox lucius) from Isle Royale, USA. Comparative Biochemistry Physiology Part C 147: 331–338.

Evers, D. C., Kaplan, J. D., Meyer, M. W., Reaman, P. S., Braselton, W. E., Major, A., and Burgess, N., Scheuhammer, A. M. 1998. Geographic trend in mercury measured in common loon feathers and blood. Environmental Toxicology & Chemistry 17 (2): 173–183.

Evers, D. C., Wiener, J. G., Driscoll, C. T., Gay, D. A., Basu, N., Monson, B. A., Lambert, K. F., Morrison, H. A., Morgan, J. T., Williams, K. A., and Soehl, A. G. 2011a. Great Lakes Mercury Connections: The Extent and Effects of Mercury Pollution in the Great Lakes Region. Biodiversity Research Institute. Gorham, Maine. Report BRI 2011—18. 44 pp. Available at

Evers, D. C., Williams, K. A., Meyer, M. W., Scheuhammer, A. M., Schoch, N., Gilbert, A., Siegel, L., Taylor, R. J., Poppenga, R. and Perkins, C. R. 2011b. Spatial gradients of methylmercury for breeding common loons in the Laurentian Great Lakes region. Ecotoxicology 20: 1609–1625.

Geiser, Linda & Nelson, Peter & Jovan, Sarah & Root, Heather & Clark, Christopher. (2019). Assessing Ecological Risks from Atmospheric Deposition of Nitrogen and Sulfur to US Forests Using Epiphytic Macrolichens. Diversity. 11. 87. 10.3390/d11060087.

Geiser, Linda & Root, Heather & Smith, Robert & Jovan, Sarah & Clair, Larry & Dillman, Karen. (2021). Lichen-based critical loads for deposition of nitrogen and sulfur in US forests. Environmental Pollution. 291. 118187. 10.1016/j.envpol.2021.118187.

Hermanson, M. and Hites, R. 1990. Polychlorinated biphenyls in tree bark. Environmental Science & Technology 24: 666–671.

Horn KJ, Thomas RQ, Clark CM, Pardo LH, Fenn ME, Lawrence GB, et al. (2018) Growth and survival relationships of 71 tree species with nitrogen and sulfur deposition across the conterminous U.S.. PLoS ONE 13(10): e0205296.

Kohut, R. 2004. Assessing the Risk of Foliar Injury from Ozone on Vegetation in Parks in the Great Lakes Network. Available at

McCoy K., M. D. Bell, and E. Felker-Quinn. 2021. Risk to epiphytic lichen communities in NPS units from atmospheric nitrogen and sulfur pollution: Changes in critical load exceedances from 2001‒2016. Natural Resource Report NPS/NRSS/ARD/NRR—2021/2299. National Park Service, Fort Collins, Colorado.

[NADP] National Atmospheric Deposition Program. 2018. NTN Data. Accessed January 20, 2022. Available at

[NPS] National Park Service, Inventory & Monitoring Program. 2010. Monitoring Persistent Contaminants at Isle Royale. Great Lakes Network Resources Brief. Available at:

Porter, E., Blett, T., Potter, D.U., Huber, C. 2005. Protecting resources on federal lands: Implications of critical loads for atmospheric deposition of nitrogen and sulfur. BioScience 55(7): 603–612.[0603:PROFLI]2.0.CO;2

Risch M. R., Gay, D. A., Fowler, K. K., Keeler, G. J., Backus, S. M., Blanchard, P., Barres, J. A., Dvonch, J. T. 2012. Spatial patterns and temporal trends in mercury concentrations, precipitation depths, and mercury wet deposition in the North American Great Lakes region, 2002–2008. Environmental Pollution 161: 261–271.

Sandheinrich, M. B., Bhavsar, S. P., Bodaly, R. A., Drevnick, P. E., and Paul, E. A. 2011. Ecological risk of methylmercury to piscivorous fish of the Great Lakes region. Ecotoxicology 20: 1577–1587.

Saros, J. E. 2008. Determine critical nitrogen loads to boreal lake ecosystems using the response of phytoplankton. NPS Implementation Plan. 10 pp.

Scheuhammer, A. M. and Blancher, P. J. 1994. Potential risk to common loons (Gavia immer) from methylmercury exposure in acidified lakes. Hydrobiologia 279/280: 445–455.

Sullivan, T. J. 2016. Air quality related values (AQRVs) in national parks: Effects from ozone; visibility reducing particles; and atmospheric deposition of acids, nutrients and toxics. Natural Resource Report NPS/NRSS/ARD/NRR—2016/1196. National Park Service, Fort Collins, CO.

Swackhamer, D. L. and Hornbuckle, K. C. 2004. Assessment of Air Quality and Air Pollutant Impacts in Isle Royale National Park and Voyageurs National Park. NPS Report. Available at

Thurman, E. M. and Cromwell, A. E. 2000. Atmospheric Transport, Deposition, and Fate of Triazine Herbicides and their Metabolites in pristine areas at Isle Royale National Park. Environmental Science and Technology 34 (15): 3079–3085.

Vucetich, L. M., Outridge, P. M., Peterson, R. O., Eide, R., and Isrenn, R. 2009. Mercury, lead and lead isotope ratios in the teeth of moose (Alces alces) from Isle Royale, U.S. Upper Midwest, from 1952 to 2002. Journal of Environmental Monitoring 11 (7): 1352–1359.

Vucetich, L. M., Vucetich, J. A., Cleckner, L. B., Gorski, P. R., and Peterson, R. O. 2001. Mercury concentrations in deer mouse (Peromyscus maniculatus) tissues from Isle Royale National Park. Environmental Pollution 114 (1): 113–118.

Part of a series of articles titled Park Air Profiles.

Isle Royale National Park

Last updated: December 5, 2022