Park Air Profiles - Isle Royale National Park

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.

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.

Mercury and toxics

Park ranger searching for larval dragonflies on Lake HarveyA 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).

Nitrogen and sulfur

Nitrogen and sulfur compounds deposited from the air may have harmful effects, including acidification on soils, lakes, ponds, and streams. Thin, undeveloped soils, and low buffering capacity result in surface waterways and soils that are vulnerable to acidification in the park (Sullivan et al. 2011a; Sullivan et al. 2011b). Also, some plants are sensitive to acidification, search for acid-sensitive plant species found at Isle Royale NP.

Excess nitrogen can also lead to nutrient enrichment, a process that changes nutrient cycling and alters plant communities. Boreal lakes—including Sargent and Richie—may be particularly sensitive to nitrogen enrichment, which could rapidly affect algal communities and lake biodiversity (Saros 2008; Sullivan et al. 2011c; Sullivan et al. 2011d). A study at Isle Royale NP investigated the fertilization effects of excess nitrogen loading, including changes to the species composition of sensitive aquatic communities, and determined that lakes in the park are vulnerable to increasing nitrogen deposition compared to other national parks (Saros 2008; Saros 2014).

Healthy ecosystems can naturally buffer a certain amount of pollution, but as nitrogen and sulfur accumulate, a threshold is passed where the ecosystem is harmed. “Critical load” is a term used to describe the amount of pollution above which harmful changes in sensitive ecosystems occur (Porter 2005). Nitrogen deposition exceeds the critical load for one or more park ecosystems (NPS ARD 2018).

Sulfur emissions and sulfur levels in precipitation have significantly declined in recent decades due to air pollution controls (Lehmann and Gay 2011). However, sulfur remains a concern at Isle Royale NP because it plays an essential role in the methylation of mercury, leading to toxic accumulation of methylmercury in fish and wildlife.

Visit the NPS air quality conditions and trends website for park-specific nitrogen and sulfur deposition information.

Ground-level ozone

Milkweed plant and butterfly 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.


Scenic view of water and Hill Islands 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 »

Bennett, J. P. 1995. Abnormal Chemical Element Concentrations in Lichens of Isle Royale National Park. Environmental and Experimental Botany 35 (3): 259–277.

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.

Caine, A., Morgan, J. T., and Brooks, N. 2011. Mercury policy in the Great Lakes states: past successes and future opportunities. Ecotoxicology 20: 1500–1511.

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.

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.

Gorski, P. R., Cleckner, L. B., Hurley, J. P., Sierszen, M. E., and Armstrong, D. E. 2003. Factors affecting enhanced mercury bioaccumulation in inland lakes of Isle Royale National Park, USA. Science of the Total Environment 304 (1–3): 327–348.

Gotelli, N. J. and Ellison, A. M. 2002. Nitrogen deposition and extinction risk in the northern pitcher plant, Sarracenia purpurea. Ecology 83: 2758–2765.

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

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

Kohut R.J. 2007. Ozone Risk Assessment for Vital Signs Monitoring Networks, Appalachian National Scenic Trail, and Natchez Trace National Scenic Trail. NPS/NRPC/ARD/NRTR—2007/001. National Park Service. Fort Collins, Colorado. Available at

Lehmann C. M. B. and Gay,D. A. 2011. Monitoring Long-Term Trends of Acidic Wet Deposition in U.S. Precipitation: Results from the National Atmospheric Deposition Program. PowerPlant Chemistry 13 (7): 386–393.

Lehmann, C. M. B., Gay, D. A., and Bowersox, V. C. (in preparation). Trends in NADP/NTN Precipitation Chemistry and Wet Deposition, 1985–2009. Illinois State Water Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, IL.

[MDNR] Michigan Department of Natural Resources. 2011. 2011–2012 Michigan Fish Advisory. Available at,5885,7-339-71548_54783_54784_54785_58671-296074--,00.html.

[NADP] National Atmospheric Deposition Program. 2010. Inorganic nitrogen wet deposition from nitrate and ammonium, 2009.

[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.

Saros, J. E., Clow, D. W., Blett, T., and Wolfe, A. P. 2010. Critical nitrogen deposition loads in high-elevation lakes of the western U.S. inferred from paleolimnological records. Water, Air, and Soil Pollution 216 (1–4): 193–202.

Sullivan, T. J., McDonnell, T. C., McPherson, G. T., Mackey, S. D., Moore, D. 2011a. Evaluation of the sensitivity of inventory and monitoring national parks to nutrient enrichment effects from atmospheric nitrogen deposition: main report. Natural Resource Report NPS/NRPC/ARD/NRR—2011/313. National Park Service, Denver, Colorado. Available at

Sullivan, T. J., McDonnell, T. C., McPherson, G. T., Mackey, S. D., Moore, D. 2011b. Evaluation of the sensitivity of inventory and monitoring national parks to nutrient enrichment effects from atmospheric nitrogen deposition: Great Lakes Network (GLKN). Natural Resource Report NPS/NRPC/ARD/NRR—2011/313. National Park Service, Denver, Colorado. Available at

Sullivan, T. J., McPherson, G. T., McDonnell, T. C., Mackey, S. D., Moore, D. 2011c. Evaluation of the sensitivity of inventory and monitoring national parks to acidification effects from atmospheric sulfur and nitrogen deposition: main report. Natural Resource Report NPS/NRPC/ARD/NRR—2011/349. National Park Service, Denver, Colorado. Available at

Sullivan, T. J., McPherson, G. T., McDonnell, T. C., Mackey, S. D., Moore, D. 2011d. Evaluation of the sensitivity of inventory and monitoring national parks to acidification effects from atmospheric sulfur and nitrogen deposition: Great Lakes Network (GLKN). Natural Resource Report NPS/NRPC/ARD/NRR—2011/349. National Park Service, Denver, Colorado. Available at

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, Colorado. Available at

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.

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., 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.

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.

Wiener, J. G., Evers, D. C., Gay, D. A., Morrison, H. A., and Williams, K. A. 2012. Mercury contamination in the Laurentian Great Lakes region: introduction and overview. Environmental Pollution 161: 243–251.

Part of a series of articles titled Park Air Profiles.

Isle Royale National Park

Last updated: September 27, 2018