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Air Resource Management Program - Air Pollutants Deposited to Ecosystems

Deposition occurs when compounds of various types of air pollution are deposited on the Earth’s surface through rain, clouds, snow, fog, or particulates. The amount of deposition is affected by the concentration of pollutants in the atmosphere and how the pollutants are deposited. General factors, such as meteorology and topography, influence how much pollution reaches the area from both local and distant sources and how much pollution lands on the Earth’s surface via the various wet and dry deposition forms. There are several types of ecosystem effects associated with deposition related to the pollutant being deposited.

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Acidic Deposition (Acid Rain)

A forest covered mountain peak.
The Colville National Forest, Washington. (USDA Forest Service Pacific Northwest Region photo)

Sulfuric and nitric acids are the primary components of acidic deposition. Sulfur and nitrogen can have acidifying effects that contribute to the degradation of stream and lake water quality by lowering the acid neutralizing capacity, which represents the water’s natural acid buffering system. As the acid neutralizing capacity decreases, the acid levels will increase. In areas such as the central and southern Appalachians, forest streams have acidified to the point where they are no longer capable of sustaining aquatic life.

The sensitivity of ecosystems to acidic deposition is often linked to natural watershed characteristics, such as hydrology and geology. Waters at the tops of their watersheds are more susceptible than others with a larger contributing watershed. Watersheds containing minerals with high levels of bases (e.g., limestone) are more resilient. In contrast, watersheds where the soils are derived from granites or sands are more vulnerable. Aquatic ecosystems in these areas may exhibit lake and stream water chemistry changes that can affect the health of native aquatic organisms that are sensitive to the acidification of their environment. Similarly, terrestrial ecosystems more vulnerable to acid deposition may show effects in soil chemistry, such as cation leaching. Cation leaching can eventually lead to deficiencies in macro nutrients important for plant growth.

Excess Nitrogen

A large wetland with lush green grass, aquatic plants, and a small forest covered mountain range in the mid-background.
The Klamath Marsh National Wildlife Refuge on the Fremont-Winema National Forest, Southern Oregon. (USDA Forest Service photo)

Nitrogen can excessively fertilize both terrestrial and aquatic ecosystems. This excessive nitrogen input (also termed nutrient enrichment, fertilization, and eutrophication) can disrupt the natural flora and fauna by allowing certain species that would not naturally occur in abundance to out-compete those that thrive in the nitrogen-limited systems commonly found in the national forests. Shifting the natural species composition and reducing biodiversity has subsequent effects on other components of ecosystem. Some of these effects include increased fire frequency, pest infestation, and declines in forest health.

Mercury

An underwater photo of approximately 15 fish swimming above an algae encrusted river bottom
A school of fish at the Alexander Springs Recreation Area within the Ocala National Forest, Florida. (USDA Forest Service photo by Kate Schaefer)

Mercury is found naturally in emissions from volcanoes. It is also produced by human activities such as coal-fired utilities, metals mining, and municipal and medical-waste incineration. Atmospheric mercury may be carried thousands of miles before entering lakes and streams through deposition.

Mercury can accumulate and magnify through the aquatic food chain in fish, humans, and other animals. Non-organic forms of mercury are converted to methylmercury primarily by sulfur-reducing bacteria in aquatic sediments. Methylmercury is a potent neurotoxin and has been shown to have detrimental health effects on human populations and behavioral and reproductive effects on wildlife. Almost every state has fish consumption advisories for mercury-contaminated fish and shellfish in lakes and streams. High concentrations of mercury have been measured in sediments and fish tissue, even in remote areas of the Arctic.

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Critical Loads

The term “critical load” describes the threshold of nitrogen or sulfur deposition where there is no known harm to sensitive resources in an ecosystem. A critical load exceedance occurs when deposition is above the critical load. When this happens, the risk of environmental harm increases. Critical loads can be developed for a variety of ecosystem responses, including shifts in microscopic aquatic species, decreases in biodiversity, increases in invasive grass species, changes in soil chemistry affecting tree growth, and lake and stream acidification to levels that can no longer support fish.

The Forest Service uses critical loads in air quality analyses for forest and project planning, wilderness stewardship, Clean Air Act-related permit reviews, and national scale assessments.

There are many reasons to define critical loads. This information can be used for purposes like:

  • Resource managers on forest lands to communicate the effects of air pollution on sensitive resources to decision-makers and air regulators.

  • To assess how some management activities may exacerbate air pollution-related problems or identify areas where mitigation may be an option for resources that have already been negatively affected.

  • In a regulatory context when consulting with and advising air regulatory agencies on the effects on forest resources resulting from new and existing sources of air pollution.

In terrestrial ecosystems, nitrogen and/or sulfur critical load exceedances can alter soil chemistry, plant community composition, species richness (biodiversity), and vegetative health, growth, and mortality. Sensitive lichen species can be used to indicate predicted ecosystem impacts from acidic atmospheric deposition and from nutrient nitrogen deposition. Lichen, herb and tree critical loads are some of the best documented critical loads for terrestrial ecosystems.

In aquatic ecosystems, the critical load for acidity is the maximum amount of total annual nitrogen and sulfur loading to a surface water or watershed from atmospheric deposition that would maintain a healthy aquatic ecosystem. These critical loads are calculated for specific waterbodies using surface water chemistry data collected at each monitoring location. Models take this data and calculate what is necessary to maintain the surface water acidity above a pre-selected acid neutralizing capacity level.

Fish swimming.
A fish on the Alexander Springs Recreation Area in the Ocala National Forest, Florida. (USDA Forest Service photo by Kate Schaefer)

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