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

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.

Mycorrhizal fungi community response

Many plants have a symbiotic relationship with mycorrhizal fungi (MF). Through the roots, the plants supply the fungi with carbon from photosynthesis and in exchange the MF enhance nutrient availability within soils, increase drought tolerance, and provide physical resistance to soil erosion (George et al., 1995; Cheng et al., 2021; Burri et al., 2013). Anthropogenic Nitrogen (N) deposition can disrupt this symbiotic relationship resulting in a shift from N sensitive to N tolerant mycorrhizal fungi and plant communities.

With increased N deposition to the soil, MF become less important for nutrient uptake and many plants will cease the exchange of nutrients altogether making them more vulnerable to stressors such as drought (Lilleskov et al., 2019). The CL-N for the shift in mycorrhizal community is 5-6 kg-N ha-1 yr-1 in coniferous forests and 10-20 kg-N ha-1 yr-1 broadleaf forests.

Isle Royale National Park has 42.4 km2 of coniferous forests, 54.4 km2 of broadleaf forests, and 419.7 km2 of mixed forests. Using the range in critical loads above, the minimum CL is exceeded in 11.4% of forested area and the maximum CL is exceeded in 0% of forested area based on 2019-2021 TDep Total N deposition.

Change in N and S deposition from 2000 to 2021

The maps below show how the spatial distribution of estimated Total N and Total S deposition in ISRO has changed from 2000-2002 to 2019-2021 (TDep MMF version 2022.02). Slide the arrows in the middle of the image up and down to compare N and S deposition between the two years (Yearly Data).

• Minimum N deposition decreased from 4.9 to 4.0 kg-N ha^-1 yr^-1 and maximum N deposition decreased from 7.7 to 5.9 kg-N ha^-1 yr^-1.
• Minimum S deposition decreased from 3.3 to 0.9 kg-S ha^-1 yr^-1 and maximum S deposition decreased from 4.0 to 1.2 kg-S ha^-1 yr^-1.

Estimated total nitrogen and sulfur deposition levels from 2000-2002 (top) compared to the 2019-2021 (bottom) average at ISRO. Estimated values were developed using the National Atmospheric Deposition Program - Total Deposition (TDep) approach that combines measured and modeled data. Estimated values are valuable for analyzing gradients of deposition and the resulting ecosystem risks where monitors are not present.

Persistent Pollutants

Pollutants like mercury and pesticides are concerning because they are persistent and toxic in the environment. These contaminants can travel in the air thousands of miles away from the source of pollution, even depositing in protected places like national parks. In addition, while some of these harmful pollutants may be banned from use, historically contaminated sites continue to endure negative environmental consequences.

When deposited, airborne mercury and other toxic air contaminants are known to harm wildlife like birds and fish, and cause human health concerns. Many of these substances enter the food chain and accumulate in the tissue of organisms causing reduced reproductive success, impaired growth and development, and decreased survival.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. Power plants and other sources of air pollution on the mainland contribute to the deposition of toxics at Isle Royale NP.

• Mercury concentrations in some fish sampled at Isle Royale NP exceeded the threshold for human consumption. Preliminary data from five sites in the park indicate an average fish mercury concentration of 0.246 ppm ww. Mercury concentrations in 30% of fish sampled (n=162) exceeded the US EPA threshold established for human consumption (0.3 ppm ww) (Eagles-Smith et al. 2019). However, the data may not reflect the risk at other unsampled locations in the park. Fish consumption advisories may be in effect for mercury and other contaminants (NPS 2022).
• Some dragonfly larvae sampled at Isle Royale NP had mercury concentrations at moderate or higher impairment levels. Dragonfly larvae have been sampled and analyzed for mercury from five sites in the park; 58% of the data fall into the moderate (100-300 ng/g dw) and 8% fall into the high (300-700 ng/g dw) impairment categories for potential mercury risk. An index of moderate impairment or higher suggests some fish may exceed the US EPA benchmark for protection of human health (Eagles-Smith et al. 2018; Eagles-Smith et al. 2020). Haro et al. 2013 similarly detected mercury in dragonfly larvae from the park and identified a positive correlation with mercury concentrations in fish.
• Mercury in wet deposition in the Great Lakes region has been monitored by various long-term studies since the 1990’s (Evers et al. 2011a; Weiner et al. 2011; Weiss-Penzias et al. 2016). The data show that declining trends in mercury deposition generally occurred until 2008, after which relatively flat or even positive trends occurred. Elevated levels of mercury were found in rain and snow at monitoring sites near Voyageurs NP (Risch et al. 2012).
• Contaminants and pesticides were found in park rainfall and surface water. A study in four Midwestern U.S. national parks found more chemicals and higher concentrations were found at parks with greater urban influences than at the more remote parks. Atrazine and DEET were detected in park surface water samples (Elliott and VanderMeulen 2017). Contaminants were also found in elevated concentrations in tributaries to the Great Lakes (Elliot et al. 2017; Thomas et al. 2017).
• Over three decades of studies continue to show elevated concentrations of many contaminants—specifically mercury and PCBs—in park air, vegetation, precipitation, sediment, fish, mussels, loons, gull eggs, mice, and moose teeth (Bowerman et al. 2011; Cain et al. 2011; Chernyk et al. 2002; Drevnick et al. 2008; Evers et al. 2011a; Evers et al. 2011b; Evers et al. 1998; Gorski et al. 2003; Hermanson and Hites 1990; NPS 2010; Route 2011; Sandheinrich and Drevnick 2016; Sandheinrich et al. 2011; Scheuhammer and Blancher 1994; Swackhamer and Hornbuckle 2004; Thurman and Cromwell 2000; Vucetich et al. 2009; Vucetich et al. 2001; Wiener et al. 2022; Wiener et al. 2016; Wiener et al. 2012; Wiener et al. 2006).

The NPS Air Resources Division reports on park conditions and trends for mercury. Visit the webpage to learn more.

Visibility

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

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. Isle Royal National Park may be affected by high levels of ozone formation over the Great Lakes (Stroud et al. 2020, Pierce et al. 2023)

Over the course of a growing season, ozone can damage plant tissues making it harder for plants to grow and store carbon. Ozone causes leaf injuries like bleaching or dark spots on some sensitive plants.  A park risk assessment concluded that plants in Isle Royale NP were at low risk of injury to leaves (Kohut 2004). There are 10 plants that may display ozone injury at Isle Royale National Park. Search ozone-sensitive plant species found at Isle Royale National Park.

US Environmental Protection Agency and NPS found in ozone exposure experiments that ozone slowed tree seedling growth. NPS uses W126 values from averaged seedling responses in those experiments to describe park condition in terms of Vegetation Health. Ozone affects actively growing plants, so the W126 metric weights a sum of ozone concentrations during daylight hours over three months in the growing season.

A recent re-analysis of the seedling experiments established critical levels of ozone protective of each tree species tested (Lee et al. 2022). The ozone critical levels are W126 values that will prevent 5% or greater deficit in tree seedling biomass. Air Quality Conditions and Trends reports a 5-year average of W126 for each park. In 2018-2022, the average W126 value for Isle Royale National Park was 3.9 ppm-h. Based on this ozone level, trees present in the park (NPSpecies) are at low risk of ozone effects:

• Tree species red maple (Acer rubrum), sugar maple (Acer saccharum), quaking aspen (Populus tremuloides) and eastern white pine (Pinus strobus) are at low risk from ozone despite their known sensitivity. Recent ozone levels in the park are below critical levels that protect these trees from 5% biomass deficit.

Ozone critical levels are for tree seedlings, which represent the regenerative capacity and long-term stability of sensitive species within a forest. These tree species are also known to be sensitive to ozone as adults (Bell et al. 2020), but critical values for seedling growth do not predict ozone effects on mature trees. Air Resources Division is currently working with collaborators to establish critical levels for mature trees using data from forest monitoring plots.

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

Explore Other Park Air Profiles

There are 47 other Park Air Profiles covering parks across the United States and its territories.

References

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