Air Quality at Acadia National Park
Most visitors expect clean air and good visibility in parks. However, Acadia National Park (NP), Maine, is downwind from large urban and industrial areas in the states to the south and west. Polluted air coming from these areas is trapped by the park’s steep slopes and high peaks. Over 30 years of air quality monitoring has shown that Acadia NP receives some of the highest levels of pollution in the northeastern U.S. Air pollution can harm ecosystems, scenic vistas, and public health. This is one of the most important environmental issues facing the park. The National Park Service works to address air pollution effects at Acadia 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 Acadia National Park (ACAD) 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 ACAD 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 ACAD ranged from 3.0 to 3.2 kg-N ha-1 yr-1 and total S deposition ranged from 2.4 to 2.5 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). ACAD has been monitoring atmospheric N and S deposition since 1981, see the conditions and trends website for park-specific information.
Surface waters and vegetation on ACAD’s high peaks and steep slopes with shallow soils and resistant bedrock that is unable to buffer excess acids are particularly sensitive.Additional N and S Research:
- Annual average precipitation is 3 times more acidic than unpolluted rain. Measured pH ranges from 4.8 to 5.2 (NADP 2018)
- Decline in red spruce at sites with both acid fog and acid rain (Jiang and Jagels 1999)
- Acid fog limits lichen growth on maple and spruce trees (Cleavitt et al. 2011)
- Episodic acidification in park streams following precipitation events, with pH values as low as 4.7 (Kahl et al. 1992; Heath et al. 1993)
- Chronic acidification of Sargent Mountain Pond (Kahl et al 2000)
- Long-term N and S deposition has acidified some streams and a lake in the park (Kahl et al. 1992) and caused high nitrate concentrations in streams (Johnson et al. 2007; Nelson et al. 2008)
- Elevated N concentrations in some park streams (Johnson et al. 2007; Nelson et al. 2008) suggests that forest soils are saturated with N (Vaux et al. 2008)
- Algal growth and community composition in Jordan Pond and Echo Lake have not changed in response to 20th century or current N deposition (Saros et al. 2014, Daggett et al. 2015)
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 121 km2 (79.5%) of the land area of Acadia National Park.
- N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 0.2% of the forested area.
- S deposition exceeded the 2.7 kg-S ha-1 yr-1 critical load to protect S-sensitive lichen species richness in 0.4% 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, Acadia National Park contains:
- 17 N-sensitive tree species and 61 N-sensitive herbaceous species.
- 22 S-sensitive tree species and 48 S-sensitive herbaceous species.
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 ACAD 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).
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.
- Mercury concentrations in some fish sampled at Acadia NP exceeded the threshold for human consumption. Preliminary data from eight sites in the park indicate an average fish mercury concentration of 0.255 ppm ww. Mercury concentrations in 30% of fish sampled (n=123) 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 Acadia NP had mercury concentrations at moderate or higher impairment levels. Dragonfly larvae have been sampled and analyzed for mercury from 18 sites in the park; 71% of the data fall into the moderate (100-300 ng/g dw) and 29% 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).
- Other studies also found mercury in park fish, birds, and water. Mercury was detected in tree swallow chicks and golden shiner fish (Webber and Haines 2003; Longcore et al. 2007a, Longcore et al. 2007b; Mierzykowski et al. 2013). Mercury and other toxic air pollutants are elevated in aquatic and terrestrial ecosystems at Acadia NP (Peckenham et al. 2007). Mercury concentrations are elevated in park wildlife from all levels of the food chain, including fish, dragonfly larvae, salamanders, tadpoles, loons, bald eagles, river otter, and mink (Eagles-Smith et al. 2019; Eagles-Smith et al. 2020). Levels of mercury in fish exceed safe consumption thresholds for humans and fish-eating wildlife such as loons (Haines et al. 2000).
- Other contaminants have also been found in Acadia NP, such as DDT in bald eagles (Matz et al. 1998). Park streams and springs contain elevated levels of trace metals associated with vehicle exhaust. These metals include aluminum, zinc, copper, molybdenum, and arsenic (Peckenham et al. 2006).
- Reduction of the average natural visual range from about 110 miles (without the effects of pollution) to about 90 miles because of pollution at the park
- Reduction of the visual range from about 70 miles to below 50 miles on high pollution days;
- Human-caused haze frequently impairs scenic vistas at the park
Acadia National Park has been monitoring visibility since 1998. 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. Search ozone-sensitive plant species found at Acadia.
Ozone effects on vegetation:
- Dogbane and big-leaf aster plants are showing signs of ozone injury (Eckert et al. 1997)
- White pines experience reduced growth, as measured by tree rings (Bartholomay et al. 1997)
- Ground-level ozone at Acadia NP sometimes exceeds standards set by the U.S. Environmental Protection Agency (EPA) to protect public health and vegetation
Visit the NPS air quality conditions and trends website for park-specific ozone information. Acadia National Park has been monitoring ozone since 1995. Check out the live ozone and meteorology data from Acadia, NP and explore air monitoring »
Explore Other Park Air Profiles
Bank, M.S., Burgess, J., Evers, D. and Loftin, C. 2007a. Mercury contamination of biota from Acadia National Park, Maine: a review. Environmental Monitoring and Assessment 126(1–3): 105–115.
Bank, M.S., Crocker, J., Connery, B. and Amirbahman, A. 2007b. Mercury bioaccumulation in green frog (Rana clamitans) and bullfrog (Rana catesbeiana) tadpoles from Acadia National Park, Maine, USA. Environmental Toxicology and Chemistry 26(1): 118–125.
Bartholomay, G.A., Eckert, R.T. and Smith, K.T. 1997. Reductions in tree-ring widths of white pine following ozone exposure at Acadia National Park, Maine, U.S.A. Canadian Journal of Forest Research 27: 361–368.
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). https://doi.org/10.1038/s41477-019-0442-8.
Cleavitt, N.L., Ewing, H.A., Weathers, K.C., and Lindsey, A.M. 2011. Acidic atmospheric deposition interacts with tree type and impacts the cryptogamic epiphytes in Acadia National Park, Maine, USA. Bryologist 114:570-582. https://doi.org/10.1639/0007-2745-114.3.570.
Daggett, C.T., Saros, J.E., Lafrancois, B.M., Simon, K.S., and Amirbahman, A. 2015. Effects of increased concentrations of inorganic nitrogen and dissolved organic matter on phytoplankton in boreal lakes with differing nutrient limitation patterns. Aquat Sci 77: 511-521. http://dx.doi.org/10.1007/s00027-015-0396-5.
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Johnson, K.B., Haines, T. A., Kahl, J.S., Norton, S.A., Amirbahman, A. and Sheehan, K.D. 2007. Controls on mercury and methylmercury deposition for two watersheds in Acadia National Park, Maine. Environmental Monitoring and Assessment 126: 55–67.
Kahl, J.S., Manski, D., Flora, M. and Houtman, N. 2000. Water Resources Management Plan, Acadia National Park. National Park Service. 103 pp.
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Longcore, J.R., Dineli, R. and Haines, T.A. 2007a. Mercury and Growth of Tree Swallows at Acadia National Park, and at Orono, Maine, USA. Environmental Monitoring and Assessment 126 (1–3): 117–127. https://irma.nps.gov/DataStore/Reference/Profile/664081
Longcore, J.R., Haines, T.A. and Halteman, W.A. 2007b. Mercury in Tree Swallow Food, Eggs, Bodies, and Feathers at Acadia National Park, Maine, and an EPA Superfund Site, Ayer, Massachusetts. Environmental Monitoring and Assessment 126 (1–3): 129–143. Available at: https://irma.nps.gov/DataStore/Reference/Profile/664082
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