Long-term trends in the chemistry of precipitation and lake water in the Adirondack region of New York, USA

Long-term changes in the chemistry of precipitation (1978-94) and 16 lakes (1982-94) were investigated in the Adirondack region of New York, USA. Time-series analysis showed that concentrations of SO42-, NO3-, NH4+ and basic cations have decreased in precipitation, resulting in increases in pH. A relatively uniform rate of decline in SO42- concentrations in lakes across the region (1.81+/-0.35 mu eq L-1 yr(-1)) suggests that this change was due to decreases in atmospheric deposition. The decrease in lake SO42- was considerably less than the rate of decline anticipated from atmospheric deposition. Model calculations suggest that this discrepancy in watershed S budgets may be due either: 1) underestimates of dry deposition of S or 2) internal release of S from weathering or mineralization of organic S in soil. The discrepancy in watershed S budgets cannot be explained by the release of previously adsorbed SO42- from soil. Despite the marked declines in concentrations of SO42- in Adirondack lakes, there has been no systematic increase in pH and ANC. The decline in SO42- has corresponded with a near stoichiometric decrease in concentrations of basic cations in low ANC lakes. A pattern of increasing NO3- concentrations that was evident in lakes across the region during the 1980's has been followed by a period of lower concentrations. Currently there are no significant trends in NO3- concentrations in Adirondack lakes.