Acid rain can affect streams through many ways, including rainfall, soils washed into streams, and urban area runoff. Acid rain alters lake/stream chemistry by lowering the pH. Most healthy lakes and streams have a pH between six and eight, while acid rain has a pH less than five. As the rain falls or particulates are deposited, many lake ecosystems become less able to buffer this acid. Many lakes and rivers become more acidic as time goes on, as their neutralisation abilities are compromised. This in turn affects the ecosystem as a whole.
Healthy freshwater ecosystems have a diverse number of species, such as zooplankton, fish, and aquatic birds like loons that depend on the freshwater environment for life. As the pH falls below six, these ecosystems will begin to see a decline in the number of aquatic species, and their populations.