Acid Rain and Its Effects on Our Aquatic Ecosystems

Acid Rain and its Effects On Our Aquatic Ecosystems

In the past century, one of the greatest threats to North America's aquatic ecosystem has been the widespread acidification of hundreds of thousands of waterways. Acid rain has effected plant and animal life within aquatic ecosystems, as well as microbiologic activity by affecting the rates of decomposition and the accumulation of organic matter.

What causes this poisonous rain, and what can be done to improve North America's water quality and prevent future catastrophes?

To answer these questions, we must first examine the cause and formation of acid rain, as well as understand ways to decrease or prevent its formation.

Formation of acid rain.

Acid deposition, more commonly known as acid rain, occurs when emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) react in the atmosphere with water, oxygen, and oxidants to form acidic compounds. This mixture forms a mild solution of sulfuric and nitric acid which then falls to the earth in either wet (rain, snow, sleet or fog) or dry (gas and particles) form. Approximately one-half of the atmosphere's acidity falls back to earth through dry deposition in the form of particles and gases, and are then spread hundreds of miles by winds where they settle on surfaces of buildings, cars, homes, and trees. When acid rain falls, the dry deposited gases and particles are sometimes washed from buildings, trees and other surfaces making the runoff water combine with the acid rain more acidic than the falling acid rain alone. This new combination is referred to as acid deposition. The runoff water is then transported by strong prevailing winds and public sewer systems into lakes and streams. Although some natural sources such as volcanic eruptions, fire and lightening contribute to the emissions of sulfur dioxide and nitrogen oxides in the atmosphere, more than 90% is the result of human activities such as coal burning, smelting of metals such as zinc, nickel and copper, and the burning of oil, coal and gas in power plants and automobiles.

When does rain become acidic?

Scientists determine whether rain or lake water is acidic by measuring its pH (the measure of acidity or alkalinity of a solution on a scale of 0 to 14). A value of 7 is considered neutral, whereas values less than 7 are acidic and values over 7 are alkaline or basic. A change of one unit on the pH scale represents a factor of ten in acidity; for example, a solution with a pH of five is ten times as acid as one with a pH of six . Normal or clean rainfall--without pollutants--is slightly acidic due to carbon dioxide, a natural gas in the air that dissolves in water to form weak carbonic acid. But rain, snow, or other moisture is not called "acid rain" until it has a pH value below 5.6 . Rainfall in eastern North America is often acidic with a pH of 4 to 5.

Why is North America greatly at risk?

Acid rain is more common in the Eastern U.S. and Canada than in the Western U.S. because emissions rise high into the atmosphere and are carried by prevailing winds from the west, falling out with precipitation in the east. Some areas in the U.S. where acid rain is most common include the New York Adirondacks, mid-Appalachian highlands, and the upper Midwest. Canada shows an even greater threat with half of its acid deposition caused by a large amount of metal smelting industries in Ontario and the other half attributed to pollution from combustion in U.S. factories in Ohio, Indiana, Pennsylvania, Illinois, Missouri, West Virginia, and Tennessee. Most lakes have a pH between 6 and 8; however, some are naturally acidic even without the effects of acid rain. Lakes and streams become acidic (pH value goes down) when the water itself and its surrounding soil cannot buffer, or shield, the acid rain enough to balance its pH level. In areas such as the northeastern United States and parts of Canada where soil buffering is poor, many lakes now have a pH value of less than 5. One of the most acidic lakes reported is Little Echo Pond in Franklin, New York, which has a pH of only 4.2. In New York's Adirondack region, acid deposition has affected hundreds of lakes and thousands of miles of headwater streams, while 300,000 lakes in eastern Canada are now vulnerable to acid deposition.

How does Acid Rain effect Aquatic Ecosystems?

As lakes and streams become more acidic, the amount of fish, aquatic plants and animals that live in these waters decrease. Although some plants and animals can survive acidic waters, others are acid-sensitive and will die as the pH declines. Plants and animals living within an ecosystem are highly interdependent. If acid rain causes the loss of acid-sensitive plants and animals, organisms at all trophic levels within the food chain may be affected which then causes a disruption to the entire ecosystem. In New York's Adirondack region, the diversity of life in these acidic waters has been greatly reduced. Fish population has disappeared and loons and otters have moved to other lakes where they can find food. In Canada, over 14,000 lakes have been acidified to the point where they have lost significant amounts of fish.

There are two patterns that contribute to the disappearance of fish from acidic bodies of water. The first pattern is known as "acid shock", which is a sudden drop in pH. These pH shocks usually occur in early spring when melting snow releases acidic elements accumulated during the winter into a lake or stream causing a rapid decrease in pH level, which in turn causes fish to die. A second pattern is the gradual decrease in pH level over a prolonged period of time interfering with fish reproduction; therefore, causing decrease in fish population, and a change in size and age of the population.

Other animals are affected by acidic water as well. For example, low pH will often stunt the growth of frogs, toads and salamanders. Changes in pH level have caused alterations in the structure of the aquatic plant life involved in primary production. Reducing the diversity of the plant communities in lakes and streams and disrupting primary production will most likely reduce the supply of food; therefore, the energy flow within the ecosystem will decrease. Changes in these communities also reduce the supply of nutrients. These factors limit the number of organisms that can exist within the ecosystem. In addition to affecting the plant and animal life, microbiological activity is also reduced affecting the rate of decomposition and accumulation

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