Acidification of freshwater ecosystems from external sources has historically been a problem in northeastern North America due to increased anthropogenic production of sulfur, nitrogen, and acidity in precipitation. Acidification of lakes results in changes in chemistry and a loss of sensitive taxa including fish and macroinvertebrates. The Daniel Smiley Research Center, located on Mohonk Lake within the Shawangunk Ridge in southeast New York, is one of the oldest research centers on the East Coast and has kept an extensive record of rainfall, precipitation pH, and pH of three nearby lakes: Minnewaska, Mohonk, and Awosting. We used this comprehensive data to look at the changing precipitation pH throughout the last 41 years. We hypothesized that precipitation pH would show an increase over the past 41 years; especially after 1994 when amendments to the Clean Air Act were implemented. We also hypothesized that this decrease in the acidity of precipitation would be the mechanism driving changes in the acidity of Mohonk, Awosting, and Minnewaska.
Results/Conclusions
Precipitation pH has been increasing over the past 41 years resulting in less acidic conditions. Most acid precipitation pH observations ranged between 3-4 in 1976, 4-5 in 1994, and 4.5-5.5 in 2013. Our three study lakes displayed increasing pH, although at different rates. Mohonk had a slightly increasing rate of 2.71x10-3 pH units per year. Awosting had a greater increasing rate of 3.11x10-2 pH units per year. Minnewaska had the largest increase with a rate of 7.19x10-2 pH units per year. There are several mechanistic factors controlling the differences in rates of change. The underlying bedrock is an erosion resistant quartz-conglomerate with little buffering. Mohonk has been able to remain neutral due to an exposed shale outlier in the lake that buffers the acidic precipitation. Changes in Awosting appear to be closely related to changes in precipitation pH. However, the pH in Minnewaska has been changing at a faster rate than the other two lakes. We hypothesize that this is caused by erosion of shale from walking paths that are present along the edges of the lake. The recovery of precipitation and decrease in lake acidity in this region has pervasive implications for the ecological communities.