Trends in the chemistry of acidified Bohemian lakes from 1984 to 1995: I. Major solutes

Temporal changes in major solute concentrations in six Czech Republic lakes were monitored during the period 1984-1995. Four chronically-acidic lakes had decreasing concentrations of strong-acid anions (C-SA = SO42- + NO3- + Cl-), at rates of 3.0 to 9.0 mu eq L-1 yr(-1). Decreases in SO42-, NO3-, and Cl- (at rates up to 5.1 mu eq L-1 yr(-1), 3.2 mu eq L-1 yr(-1), and 0.6 mu eq L-1 yr(-1) respectively) occurred. The response to the decrease in deposition of S was rapid and annual decline of SO42- in lake water was directly proportional to SO42- concentrations in the acidified lakes. Changes in NO3- concentrations were modified by biological consumption within the lakes. The decline in C-SA was accompanied in the four most acidic lakes by decreases in Al-T, increases in pH at rates of 0.011 to 0.016 pH yr(-1), and decreases of Ca2+ and Mg2+ (but not Na+) in three lakes. The acid neutralizing capacity (ANC) increased significantly in all six lakes. Increases in base cation concentrations (C-B = Ca2+ + Na+ + Mg2+ + K+) were the principal contributing factor to ANC increases in the two lakes with positive ANC, whereas decrease in CSA was the major factor in ANC increases in the four chronically-acidic lakes. The continued chemical recovery of these lakes depends on the uncertain trends in N deposition, the cycling of N in the lakes and their catchments, and the magnitude of the future decrease in S deposition.