WHEN DID ACID-SENSITIVE ADIRONDACK LAKES (NEW-YORK, USA) BEGIN TO ACIDIFY AND ARE THEY STILL ACIDIFYING

A transfer function was used to reconstruct pH values of 20 low-alkali nity Adirondack Park lakes based on the species composition of scaled chrysophytes (Chrysophyceae, Synurophyceae) in stratigraphic intervals from Pb-210-dated sediment cores. Approximately 80% of the lakes acid ified since preindustrial times. Four categories of lake response to a cidic deposition were identified: (i) lakes that showed little or no e vidence of acidification since preindustrial times, (ii) lakes with pr eindustrial pH values between 5 and 6 that began to acidify ca. 1900, (iii) ''naturally'' acidic lakes that acidified even further ca. 1900, and (iv) lakes with preindustrial pH values around 6 that acidified c a. 1930-50. Lakes that acidified ca. 1900 were generally smaller, high er elevation lakes with lower preindustrial pH values than lakes in ca tegory i or iv. These patterns are consistent with the acidic depositi on hypothesis of recent lake acidification. Our results indicated that ''critical'' sulfate deposition loads for Adirondack lakes that start ed to acidify ca. 1900 and ca. 1950 are between 5-10 and 20-25 kg.ha(- 1)yr(-1), respectively. Post-1970 trends in lake water pH have been sm all and variable, suggesting that low-alkalinity Adirondack lakes have been relatively unresponsive to the post-1970 declines in sulfate dep osition.