In the early 1970s it was widely believed that lakes acidified by acid precipitation would not recover. Geological materials in the catchments of lakes were believed to be the primary source of alkalinity for neutralizing acid in rain and snow. It was thought that these would become exhausted, after which neutralization of incoming acids could not occur. Early acidification models were constructed on this belief.

This view was peculiarly at variance with limnological studies. More than 30 years earlier, G. E. Hutchinson (1941) and C. H. Mortimer (1941-1942) had published observations showing that alkalinity was produced by anoxic lake sediments, although the mechanisms by which this occurred were not elucidated.

A whole-lake experiment at Canada's Experimental Lakes Area quantified the extent of in-lake alkalinity production and revealed that when sulfuric acid was the primary strong acid added, microbial reduction of sulfate to sulfide was the most important process (Cook and Schindler, 1983; Cook et al., 1986). Subsequent whole-lake experiments with nitric acid showed that algal uptake of nitrate and microbial denitrification to N2 similarly neutralized incoming nitric acid (Rudd et al., 1990). Investigations of lakes in other regions showed that the acid-neutralizing processes are widespread in lakes (Baker et al., 1986; Rudd et al., 1986; Schindler, 1986; Brezonik et al., 1987).

Although the microbial in-lake acid neutralizing mechanisms are not 100 percent efficient, they greatly reduce the effect of acid precipitation and allow lakes to recover when acid precipitation is reduced. Limnologists have developed successful models to predict the rate of internal alkalinity generation in different lakes with different inputs of sulfuric and nitric acids (Kelly et al., 1987; Baker and Brezonik, 1988).

predict that recently imposed controls on sulfur oxide emissions will reduce acidification damage only to about half of that caused by emissions at early 1980s levels.

Limnologists have shown that the degree of biological change occurring in aquatic communities as a result of acid rain can be related clearly to the degree of acidification (Brezonik et al., 1993), but in only a few cases can the chain of cause and effect be specified in detail; these cases relate chiefly to fish of recreational and economic importance, in particular salmonids (Baker et al., 1994). Simple direct responses by individual species to changes in chemical conditions can be ruled out as the mechanisms underlying many organismal responses to acidification (Webster et al., 1992). Likewise, it has been shown that standard laboratory bioassays are of limited use in predicting organismal responses to acidification in actual ecosystems (Gonzalez and Frost, 1994). Despite expenditures of many millions of dollars on research related to acidic deposition, especially

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