How Does Acid Rain Affect Lakes?

Acid rain is a product of the burning of fossil fuels and poses a threat to the ecosystems of streams and lakes. In particular, when certain levels of acidity are reached, fish and other aquatic life will die. Since all flora and fauna in a given ecosystem are interdependent, the effects of acidic water are far-reaching. Acid rain is worse in areas with dense populations and heavy industry, particularly the eastern United States.

Acid Rain

Atmospheric pollutants, particularly nitrogen oxide and sulfur dioxide from the burning of fossil fuels, cause precipitation to become acidic. These gases mix with water vapor and oxygen in the atmosphere to form nitric and sulfuric acids, which cause acid rain. This acidic precipitation lowers the pH level of water in streams and lakes, creating adverse effects. Acidic waters will also more readily absorb aluminum leeched from soil runoff, and the combination of aluminum and acidic water is especially harmful to many aquatic species.

Lakes and Acidity

Water pH is the amount of free hydrogen ions in a given sample, and the scale ranges from 0 to 14, with numbers below 7 being acidic. Each level down the scale represents a tenfold increase in hydrogen ions, so a pH of 5 has ten times more ions than a pH of 6. Normally, lakes will be neutral, pH levels between 6 and 8, but lakes with pH below 5 are considered acidic. In addition to acid rain, acidic rainfall leeching through soils that lack acid-buffering capability will eventually wind up in streams and lakes. Normally, lake water contains bicarbonates and other compounds that have the ability to buffer their pH levels, but acid rain in particular disrupts this ability.

Acid Rain and Aquatic Life

When an influx of acid rain reaches a certain threshold in a lake, it overcomes the lake’s ability to buffer acids. The lower pH levels in the water have a direct and primary effect on many of the various creatures that live within the lake ecosystem. Many species of fish, shellfish and other lake species such as snails and salamanders are adversely affected by acidic water. For example, a pH of 5 will kill fish eggs, and lower levels will kill adult fish. Not only does the low pH affect aquatic life, but it also affects the lives of terrestrial species and birds that feed on aquatic life.

Most Sensitive Regions

Given the adverse effects of acid rain on lake ecosystems, it is important to understand which areas are most at risk. Lakes in the northeast United States, Appalachian highlands, upper Midwest and the mountains of western North America are most prone to having pH levels below 5. In 2006, the National Atmospheric Deposition Program noted that acidic precipitation lower than pH 5 falls downwind of heavy industrial areas, and it is worst in the Ohio River Valley region of the upper Midwest. The pH of precipitation in this region can be at dangerously low levels below 4.3. The entire region east of the Mississippi River in the United States is especially vulnerable.

The Advantages of Acid Rain

Acid rain comes in the form of rain, fog, smog and dry depositions, and it harms forests, kills fish and erodes rocks and buildings. It is caused by excessive emissions of sulfur dioxide and nitrogen oxide compounds from industrial and natural sources. About two-thirds of sulfur dioxide and one-quarter of nitrogen oxides come from fossil-fuel-burning power plants. Some environmental studies indicate that the effects of acid rain are complex and include positive effects on global warming and improvement of forests.

Global Warming

Methane and carbon dioxide are major greenhouse gases that cause global warming. Methane, compared to carbon dioxide, is more potent. When compared by weight, methane has 20 times more of an impact on climate change than carbon dioxide does over a 100-year period, according to the U.S. Environmental Protection Agency. Methane is emitted from agricultural and industrial processes, but the largest source of methane entering the atmosphere is wetlands.

Suppressing Methane Production

About 22 percent of the human-enhanced greenhouse gas effect is due to methane. Microbes feeding on hydrogen- and acetate-containing vegetation in wetlands are natural contributors to total atmospheric methane. In a study of wetlands in Europe and North America, researchers at the Open University, U.K., found that sulfur compounds deposited by acid rain inhibited methane production by wetlands. The researchers found that methane production was significantly suppressed because sulfur-eating bacteria in wetland areas out-competed methane-emitting microbes. Sulfur compound deposition from acid rain can reduce methane production by up to 30 percent. In another study, the effects of acid rain on rice paddies were mimicked by adding small amounts of sulfate to the soil. Up to 24 percent of the methane was prevented from entering the atmosphere.

Forests

A 20-year-long study by scientists at Michigan Technological University School on the hardwood forests of Michigan found that acid rain, together with modest increases in temperature, makes forests more productive. Researchers measured nitrogen deposition by acid rain and found that trees store more carbon when the soil contains more nitrogen compounds, as long as there is enough moisture. Therefore, the increased growth rate and greater ability of trees to store carbon dioxide may offset some of the negative effects of acid rain on forests, such as damage to tree foliage.

Complex Effects of Acid Rain

Acid rain clearly has negative effects on the environment, but the beneficial effects of acid rain must enter into future predictions of its role in environmental damage and global warming. A computer model, created at NASA's Goddard Space Flight Center, predicts that the sulfur dioxide in acid rain will continue to suppress methane production by wetlands well into 2030. Although nitrogen oxide deposition by acid rain may have positive effects on forest trees, highly polluted acid rain damages trees and harms some animal species.

Natural Causes of Acid Rain

Acid rain is precipitation with a pH of less than 5.6. Its principal constituents are nitric and sulfuric acids formed by chemical reactions of nitrogen and sulfur oxides with water vapor in the atmosphere. Although associated mostly with industrial, vehicle and power generation emissions, these and other gases that react with water to make acid rain are also released by natural phenomena such as volcanic eruptions and biological processes.

Atmospheric Composition

The pH of rainwater in unpolluted regions is 5.6, making it a weakly acidic solution. A neutral solution has a pH of 7. Rainwater acidity is caused by the presence of carbon dioxide, nitric oxide and sulfur dioxide in the atmosphere. Carbon dioxide concentrations in the atmosphere are 335 parts per million compared with 0.01 ppm for nitric oxide and up to 0.01 ppm for sulfur dioxide. Carbon dioxide reacts with water vapor to produce carbonic acid. However, in pristine environments the natural rainwater pH may become more acidic, ranging between 4.5 and 5.6, because of the variability of naturally occurring sulfur and nitrogen oxides in the atmosphere.

Volcanic Eruptions

Volcanic eruptions release water vapor and gases such as carbon dioxide and monoxide, sulfur dioxide, hydrogen chloride, hydrogen sulfide, hydrogen fluoride, ammonia, methane and silicon tetrafluoride. These gases react with water vapor to precipitate as acid rain. Hydrochloric and hydrofluoric acids, together with other fluoride and chloride compounds, which build up in large concentrations in the vicinity of the eruption area, are poisonous to plants and animals, cause skin and eye irritations and can damage machinery. Sulfur dioxide from a volcano, together with aerosols, can rise 19 to 32 kilometers (12 to 20 miles) into the atmosphere and cause a global cooling effect by blocking solar radiation.

Lightning Strikes

Lightning is one of the major sources of localized atmospheric nitrogen, sulfur and carbon oxides. Lightning bolts reach temperatures of about 30,000 degrees Celsius (54,000 degrees Fahrenheit) and can transmit a current of up to 20,000 amps. This is powerful enough to cause nitrogen and oxygen in the atmosphere to react and create nitrogen oxide and nitrogen dioxide. The latter dissolves in water vapor to precipitate as weak nitric acid. Lightning also triggers wildfires that emit carbon and sulfur oxides into the atmosphere that in turn dissolve in water vapor and precipitate as carbonic and sulfuric acids.

Emissions from Vegetation

Terrestrial vegetation produces carbon dioxide as part of the transpiration process and when rotting on the ground. Aquatic plants and bacteria that exist in anaerobic -- oxygen-free -- conditions emit methane. Tropical and temperate forests emit isoprene, a biogenic volatile organic compound. Isoprene is oxidized in the troposphere and reacts with nitrogen oxides to produce ozone, organic nitrates and organic acids such as methacrylic and acrylic acids. These acids contribute to the acidification of rainwater.