Hysteresis in Reversal of Central European Mountain Lakes from Atmospheric Acidification

Abstract

Extremely high emissions of S and N compounds in Central Europe (both ∼280 mmol m-2 yr-1) declined by ∼70and ∼35%, respectively, during the last decade. Decreaseddeposition rates of SO4 -2, NO3 -, and NH4 + in the region paralleled emission declines. The reduction in atmospheric inputs of S and N to mountain ecosystemshas resulted in a pronounced reversal of acidification in the Tatra Mountains and Bohemian Forest lakes. Between the 1987–1990and 1997–1999 periods, concentrations of SO4 -2 and NO3 - decreased (average ± standard deviation) by 22±7 and 12±7 μmol L-1, respectively, in theTatra Mountains, and by 19±7 and 15±10 μmol L-1, respectively, in the Bohemian Forest. Their decrease was compensated in part (1) by a decrease in Ca2+ + Mg2+ (17±7 μmol L-1) and H+ (4±6 μmol L-1), and an increase in HCO3 -(10±10 μmol L-1) in the Tatra Mountains lakes, and (2) by a decrease in Al (7±4 μmol L-1), Ca2+ + Mg2+ (9±6 μmol L-1), and H+ (6±5 μmol L-1), in Bohemian Forest lakes. Despite the rapid decline in lake water concentrations of SO4 -2 and NO3 - in response to reduced S and N emissions, their present concentrations in some lakes are higher than predictionsbased on observed concentrations at comparable emission rates during development of acidification. This hysteresis in chemical reversal from acidification has delayed biological recovery of the lakes. The only unequivocal sign of biological recovery hasbeen observed in Cerne Lake (Bohemian Forest) where a cladoceran species Ceriodaphnia quadrangular has recentlyreached its pre-acidification abundance.