The causes of lake acidification, with special reference to the role of acid deposition

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Abstract
Acid surface waters (pH < 5.5) occur throughout western and northern Europe. The claim that many of these waters have been acidified in recent decades and that the acidification results from acid deposition has been well-substantiated by palaeolimnological studies. At almost all sites acidification post-dates 1800 A. D.; it is accompanied by increases in the concentration of trace metals and carbonaceous particles and the spatial pattern of acidified lakes coincides with areas of high acid deposition (greater than 0.5 g sulphur m -2 a -1 ). Very sensitive sites (Ca 2 + < 50 ueq 1 -1 ) in areas of low acid deposition are not acidified. Palaeolimnological tests to evaluate the contribution of other factors suggest that leaching and paludification processes are important on a post-glacial time scale but imperceptible over the last 200 years, and that alterations to catchment burning and grazing regimes over this time scale have little or no effect. Only the afforestation of sensitive catchments in areas of high sulphur deposition appears to be significant, an effect attributed to the enhanced sulphur-scavenging efficiency of the forest canopy rather than to the direct effect of forest growth.