Acid precipitation causes many observable, as well as nonobservable, effects on aquatic ecosystems. Included are changes in water chemistry and aquatic faunal and floral species. One reason for changes in surface water chemistry is the release of metals from stream or lake sediments. For example, Wright and Gjessing (1976) note that concentrations of aluminum, manganese, and other heavy metals are higher in acid lakes due to enhanced mobilization of these elements in acidified areas.
Due to the extant water chemistry and sediment characteristics, some surface water is more susceptible to changes in water chemistry than others. Several surface water sensitivity studies leading to classification schemes have been conducted. For example, Hendrey et al. (1980) analyzed bedrock geology maps of the eastern United States to determine the relationship between geological material and surface water pH and alkalinity. They verified map accuracy by examining the current alkalinity and pH of water in several test states, including Maine, New Hampshire, New York, Virginia, and North Carolina. In regions predicted to be highly sensitive, the alkalinity in upstream sites was generally low, less than 200 microe-quivalents per liter. They pinpoint many areas of the eastern United States in which some of the surface water, especially upstream reaches, are sensitive to acidification.
Acid precipitation affects microdecomposers, algae, aquatic macrophytes, zooplankton, benthos, and fish (Hendry et al. 1976). For example, many of the 2000 lakes in the Adirondack Region of New York are experiencing acidification and declines or loss of fish populations. Baker (1981) found that, on the average, aluminum complexed with organic ligands was the dominant aluminum form in the dilute acidified Adirondack surface water studied. In laboratory bioassays, speciation of aluminum had a substantial effect on aluminum and hydrogen ions, and these ions appeared to be important factors for fish survival in Adirondack surface water affected by acidification.
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