Air Quality at Voyageurs National Park
Most visitors expect clean air and clear views in parks. Given its remote location on the northern edge of Minnesota’s border with Canada, Voyageurs National Park (NP) experiences relatively good air quality. However, the park lies downwind of pollution from sources in the Midwest and Canada, as well as nearby industrial sources like paper mills. 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 Voyageurs NP, and in parks across the U.S., through science, policy and planning, and by doing our part.
Nitrogen and Sulfur
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 Voyageurs National Park (VOYA) relative to other national parks is 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 VOYA relative to other national parks is very high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.
From 2017-2019 total N deposition in VOYA ranged from 4.5 to 5.5 kg-N ha-1 yr-1 and total S deposition ranged from 1.0 to 1.3 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). VOYA has been monitoring atmospheric N and S deposition since 2000, see the conditions and trends website for park-specific information.
In boreal lakes, increased N can affect biodiversity, algal communities, and water clarity (Sullivan et al. 2016; Kallemeyn et al. 2003; Wiener et al. 2006; Swackhamer and Hornbuckle 2004).
VOYA’s thin, undeveloped soils, underlying granitic rock, and low buffering capacity result in surface waterways and soils at high risk from acidification by atmospheric N and S (Sullivan et al. 2016). S is a concern at VOYA because it plays an essential role in the methylation of mercury, leading to toxic accumulation of methylmercury in fish and wildlife. Also, S is a strong driver of acidification in the VOYA’s vulnerable lakes and streams.
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 368 km2 (44.4%) of the land area of Voyageurs 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, Voyageurs National Park contains:
- 14 N-sensitive tree species and 79 N-sensitive herbaceous species.
- 23 S-sensitive tree species and 65 S-sensitive herbaceous species.
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.
While Voyageurs NP is in a remote location on the Minnesota-Canada border, nearby and distant sources contribute to the deposition of toxics at the park, including mercury. 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. 2011a). However, mercury pollution remains a major concern (Evers et al. 2011a; Weiner et al. 2011) and air emissions are now the primary source of mercury pollution (Evers et al. 2011a). Voyageurs NP is particularly sensitive to mercury pollution. The abundance of wetlands, low pH lakes, complex food webs, and predatory fish create an environment susceptible to the bioaccumulation of toxics.
Mercury and toxics effects:
- Widespread mercury in the park’s aquatic ecosystems, originating primarily from coal-fired power plants and taconite processing emissions. Elevated concentrations have been documented in fish as well as water, lake sediments, zooplankton, aquatic plants, benthic organisms, fish-eating birds, bald eagles, and river otters (Kallemeyn et al. 2003; Wiener et al. 2006; Swackhamer and Hornbuckle 2004);
- High mercury concentrations in fish from nearly all of Voyageur NP’s 30 lakes (Sorensen et al. 2001). The average mercury level in fish exceeds the State of Minnesota fish consumption advisories. This is a concern since approximately 70% of visitors fish in the park (Kallemeyn et al. 2003);
- Mercury levels in walleye, pike, bass, and other fish from lakes in the park exceed thresholds known to damage fish health (Sandheinrich et al. 2011; NPS 2010), and are at levels also known to harm fish-eating loons (Sorensen et al. 2001);
- Concentrations of mercury in loon blood are high enough to reduce reproductive success (Evers et al. 2011a; Evers et al. 1998), and adult loon feather mercury concentrations are above a level associated with toxic effects (Scheuhammer and Blancher 1994);
- Detectable levels of contaminants including mercury, PCBs, DDE, and dieldrin in feathers of bald eagle nestlings at the park (Pittman 2010). Mercury concentrations in nestling feathers have declined from the mid-1980s to the late 1990s, but recent samples suggest an increasing trend (Pittman et al. 2011);
- Elevated concentrations of toxic elements including mercury, cadmium, and chromium in lichens (Bennett 1997);
- PFOS, a by-product in the manufacture of fabric protectors, firefighting foams, and other chemicals, detected in water samples at the park (Simcik and Dorweiler 2005).
Visitors come to Voyageurs NP to enjoy the spectacular “North Woods”, a wilderness of interconnected waterways, and the northern lights. 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. Organic compounds, soot, and dust reduce visibility as well. 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.
- Reduced visibility, at times, due to human-caused haze and fine particles of air pollution;
- 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 50 miles on very hazy days.
Visit the NPS air quality conditions and trends website for park-specific visibility information. Voyageurs 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 »
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 Voyageurs 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 Voyageurs 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 Voyageurs NP (Swackhamer and Hornbuckle 2004). Search ozone-sensitive plant species found at Voyageurs NP.
Visit the NPS air quality conditions and trends website for park-specific ozone information. Voyageurs NP has been monitoring ozone since 1987. View live ozone and meteorology data, and explore air monitoring »
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Sorenson, J. A., Rapp Jr., G., Glass, G. E. 2001. The effect of exotic rainbow smelt (Osmerus mordax) on nutrient/trophic pathways and mercury contaminant uptake in the aquatic food web of Voyageurs National Park, a benchmark study of stable element isotopes. NPS Final Report. 52 pp.
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