Watershed Management And Underlying Geology In Three Lakes Control Divergent Responses To Decreasing Acid Precipitation

In northeastern North America, acidification of precipitation and freshwater ecosystems is due to increased anthropogenic emissions of sulfate (SO42-) and nitrogen oxides (NOx). Legislative changes (e.g., the 1990 Clean Air Act Amendment) have resulted in decreasing emissions and increasing precipitation pH. We hypothesized that precipitation pH increased and regional improvements in acid precipitation resulted in increasing pH levels in our study lakes - Lake Minnewaska, Lake Awosting, and Mohonk Lake, on the Shawangunk Ridge, New York, USA - but the rates of change would be dependent on underlying geology and watershed management. Precipitation pH increased from 4 in 1976 to > 5 in 2015. Our 3 study lakes had increasing pH, although at different rates from changes in precipitation pH and compared to each other. Several factors likely control the differences in lake acidity improvements. Mohonk has remained neutral because of an exposed shale inlier in the lake that buffers the acidic precipitation. The bedrock under Awosting and Minnewaska is quartz-conglomerate with little buffering capacity. Changes in Awosting seem to be closely linked to precipitation pH recovery; however, the pH in Minnewaska has increased above biologically meaningful levels because of eroding shale recreational trails around the lake, a finding supported by water chemistry trends. Across this region and around the world, the rate of recovery from acid precipitation and decrease in lake acidity in this region can affect the return of acidsensitive freshwater ecological communities.