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) = SO₄/^2- + NO₃ + Cl^-), at rates of 3.0 to 9.0 μeq L^-1 yr^-1. Decreases in SO₄/^2-, NO₃/^-, and Cl^- (at rates up to 5.1 μeq L^-1 yr^-1, 3.2 μeq L^-1 yr^-1, and 0.6 μeq L^-1 yr^-1, respectively) occurred. The response to the decrease in deposition of S was rapid and annual decline of SO₄/^2- in lake water was directly proportional to SO₄/² 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 (CB = Ca²⁺ + Na⁺ + Mg²⁺ + K⁺) were the principal contributing factor to ANC increases in the two lakes with positive ANC, whereas decrease in C(SA) 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.