Long-term recovery of lakes in the Adirondack region of New York to decreases in acidic deposition

After years of adverse impacts to the acid-sensitive ecosystems of the eastern United States, the Acid Rain Program and Nitrogen Budget Program were developed to control sulfur dioxide (SO ₂) and nitrogen oxide (NO _x) emissions through market-based cap and trade systems. We used data from the National Atmospheric Deposition Program's National Trends Network (NTN) and the U.S. EPA Temporally Integrated Monitoring of Ecosystems (TIME) program to evaluate the response of lake-watersheds in the Adirondack region of New York to changes in emissions of sulfur dioxide and nitrogen oxides resulting from the Acid Rain Program and the Nitrogen Budget Program. TIME is a long-term monitoring program designed to sample statistically selected subpopulations of lakes and streams across the eastern U.S. to quantify regional trends in surface water chemistry due to changes in atmospheric deposition. Decreases in wet sulfate deposition for the TIME lake-watersheds from 1991 to 2007 (-1.04 meq m ^-2-yr) generally corresponded with decreases in estimated lake sulfate flux (-1.46 ± 0.72 meq m ^-2-yr), suggesting declines in lake sulfate were largely driven by decreases in atmospheric deposition. Decreases in lake sulfate and to a lesser extent nitrate have generally coincided with increases in acid neutralizing capacity (ANC) resulting in shifts in lakes among ANC sensitivity classes. The percentage of acidic Adirondack lakes (ANC <0 μeq L ^-1) decreased from 15.5% (284 lakes) to 8.3% (152 lakes) since the implementation of the Acid Rain Program and the Nitrogen Budget Program. Two measures of ANC were considered in our analysis: ANC determined directly by Gran plot analysis (ANC _G) and ANC calculated by major ion chemistry (ANC _calc  C _B - C _A ). While these two metrics should theoretically show similar responses, ANC _calc (+2.03 μeq L ^-1-yr) increased at more than twice the rate as ANC _G (+0.76 μeq L ^-1-yr). This discrepancy has important implications for assessments of lake recovery and appears to be due to compensatory increases in concentrations of naturally occurring organic acids coincident with decreases in lake concentrations of strong acid anions, as evidenced by increases in concentrations of dissolved organic carbon.