Acid rain

 is a silent killer, destroying forests, streams, and lakes throughout the world. As we burn fuels rich in sulfur, such as much of the coal used to generate electricity, the sulfur burns to sulfur dioxide and is emitted through the smoke-stack of the generating plant. In the air, sulfur dioxide reacts with water to form sulfuric acid, which then dissolves into the water droplets of clouds. As the drops fall as rain, snow, or sleet, they carry the sulfuric acid with them as acid rain, also called acid precipitation.

Acid precipitation damages plants in many ways. Because the cuticle on the epidermis is not absolutely impermeable, some acid slowly moves directly into the plant tissues and damages leaves, flowers, fruits, and cones. Perhaps more significantly, most of the acid enters the soil and accelerates cation exchange, causing positively charged ions to be released from the soil particles and to be washed away in the rain. The soil is left depleted of nutrients, and plants suffer from mineral deficiency. Downwind of the most heavily polluting industrial cen-ters of Germany, entire forests are dying or dead. Pollution from the United States and Canada is causing extensive damage to North American forests.

As acid rain accelerates cation ex-change, minerals are washed from the soil and enter streams, in effect fertilizing them and causing rapid growth of algae. In small quantities, this provides more food for fish, turtles, and other aquatic animals, but in many cases the algal growth is so abundant it forms a massive, impenetrable layer across the top of a lake or slowly moving river. As the algae die, their bodies sink and are attacked by decomposers, mostly bacteria. Bacterial decomposition consumes oxygen, and before long the bottom of a lake or quiet river is an anaerobic dead zone with too little oxygen to support any animal life. This process is called eutrophication, and the result is a lake or river that is basically dead, having little life other than a mat of algae at its surface.

These conifers have been damaged by acid rain. Notice that the broadleaf trees in the foreground appear healthy. The acid rain does not always destroy a forest but instead may alter the species composition and diversity. (Terraphotographics/BPS)