The first and most obvious effects of acid deposition were observed in freshwater ecosystems. Tens of thousands of lakes and streams in North America and Europe are more acidic than they were a few decades ago as a result of acid SULFUR DIOXIDE AND ACID RAIN 73 C04 08/23/2011 12: 74 deposition. These freshwaters are in sensitive areas with hard bedrock, thin acid soils, and little acid-neutralizing capacity, and their acidification has resulted in losses of fish and other aquatic organisms.
The evidence showing that strong mineral acidity has caused ecological changes has come from many sources, including (1) historical changes in chemistry and biology of freshwaters, (2) experimental manipulations, and (3) measurements made in aquatic ecosystems receiving acid deposition. For example, analyses of historical changes in alkalinity of lakes in the Adirondack Mountain region of New York state showed highly significant acidification of a large number of those lakes during recent decades.
Approximately 80% of 274 lakes studied became acidified during the past 50 to 60 years. These changes were attributed to acid deposition and were not explainable by other factors such as changes in land use. In the Adirondacks, the Department of Environmental Conservation has found that fish populations are endangered in more than half of all the lakes and ponds in the region and more than 200 lakes have become totally fishless.