Ecological Effects of Acidic Deposition

Acidic deposition is comprised of sulfuric and nitric acids, and ammonium derived from atmospheric emissions of sulfur dioxide, nitrogen oxides, and ammonia, respectively. These compounds are released to the atmosphere largely by the burning of fossil fuels and agricultural activities. Atmospheric deposition delivers acids and acidifying compounds to the Earth’s surface. The adverse ecological effects of acidic deposition on forests, streams, and lakes largely occur in eastern North America, northern and central Europe, and southwestern China. Acidic deposition has altered, and continues to alter, forest soil by accelerating the leaching of calcium and magnesium and increasing concentrations of dissolved inorganic aluminum in soil waters. At high concentrations, dissolved inorganic aluminum can hinder the uptake of water and essential nutrients by tree roots. The alteration of soils by acid deposition has serious consequences for acid-sensitive forest ecosystems. Soils that are compromised by acidic deposition are less able to neutralize additional inputs of strong acids, and provide poorer growing conditions for plants and delay the recovery of surface waters. Acidic deposition has impaired, and continues to impair, the water quality of lakes and streams in eastern North America and Europe in three important ways: lowering pH levels (i.e., increasing the acidity); decreasing acid-neutralizing capacity; and increasing aluminum concentrations. Many surface waters in acid-sensitive regions impacted by acidic deposition exhibit chronic and/or episodic (i.e., short-term) acidification. Moreover, elevated concentrations of dissolved inorganic aluminum have been measured in acid-impacted surface waters. High concentrations of aluminum and increased acidity have reduced the species diversity and abundance of aquatic life in many lakes and streams draining acid-sensitive regions. Fish have received the most attention to date, but entire food webs are often negatively affected. For example, in a survey of lakes in the Adirondack region of New York, 346 lakes (24% of the total) did not contain fish. These fishless lakes had significantly lower pH and higher concentrations of dissolved inorganic aluminum when compared to those lakes with fish.