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Aquatic Effects of Acidic Deposition

Acid deposits in water systems, such as lakes, streams and rivers, are almost entirely due to what has become known as "acid rain." Acid rain is precipitation that contains high amounts of the strong acids nitric acid and sulfuric acid. It occurs when the pollutants sulfur dioxide and oxides of nitrogen enter the atmosphere and react with water and oxygen to form acids. Such acidic rain eventually makes its way into various aquatic systems where it can have far-reaching effects.

Physical and Chemical Changes
- The main effect of acidic deposition via acid rain falling into aquatic environments is a drop in the pH of the water. Essentially, as expected, the water becomes more acidic. Normally, the water in lakes and rivers has a relatively neutral pH between 6 and 8, but this can drop drastically due to acid rain. Little Echo Pond in Franklin, New York, for example, has a pH of 4.2. Some lakes which contain minerals such as calcium have a built-in buffering capacity and can tolerate some acid rain without a major change in pH, whereas other lakes do not have this alkaline buffer effect. Acid rain run off into water can also dissolve aluminum from adjacent soils and significantly increase the level of this metal in the water.
Effects on Plant Life
- The lowered pH associated with acidic waters can have a variety of negative effects on aquatic plantlife. Some plants simply cannot tolerate a pH much below 6 and will die off. Low pH has also been shown to inhibit the process of photosynthesis. As well, increased aluminum content can bind with phosphates that plants require as a nutrient and thus stunt plant growth.
Effects on Fish and Other Aquatic Life
- Low pH also has many negative effects on fish. Most fish eggs will not hatch at a pH below 5, and low levels of pH will also kill many species of adult fish. Increased levels of aluminum also are deadly to fish; aluminum can clog the gills of fish and interfere with their normal metabolism of salt. Some acidified lakes become "wet deserts" with no living fish at all. Water pH levels even slightly below neutral are also harmful to other life forms, including bacteria and fungi that normally decompose organic matter in lakes and rivers. Amphibians associated with the water system will also be adversely affected; salamanders and toads, for example, will show signs of stunted growth.
Effects on the General Ecosystem
- Other life associated with the general ecosystem of the water will also be affected by acidification. Insects, frogs and birds that feed nearby can ingest harmful levels of aluminum as the metal is concentrated up the food chain. High aluminum levels in birds can result in soft eggshells and fewer offspring.

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