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-alkalinity Adirondack Park lakes based on the species composition of scaled chrysophytes (Chrysophyceae, Synurophyceae) in stratigraphie intervals from 210Pb-dated sediment cores. Approximately 80% of the lakes acidified since preindustrial times. Four categories of lake response to acidic deposition were identified: (i) lakes that showed little or no evidence of acidification since preindustrial times, (ii) lakes with preindustrial 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 ca. 1930–50. Lakes that acidified ca. 1900 were generally smaller, higher elevation lakes with lower preindustrial pH values than lakes in category i or iv. These patterns are consistent with the acidic deposition hypothesis of recent lake acidification. Our results indicated that "critical" sulfate deposition loads for Adirondack lakes that started 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 small and variable, suggesting that low-alkalinity Adirondack lakes have been relatively unresponsive to the post-1970 declines in sulfate deposition.