DISRUPTION OF SULFUR CYCLING AND ACID NEUTRALIZATION IN LAKES AT LOW PH

Experimental acidification of a softwater lake to below pH 5 fundament ally changed the sulfur cycle and lowered internal alkalinity generati on (IAG). Prior to reaching pH 4.5, the balance of sulfur reduction an d oxidation reactions within the lake was in favour of reduction, and the lake was a net sink for sulfate. In the four years at pH 4.5 the b alance of reduction and oxidation reactions was in favour of oxidation , and there was a net production of sulfate (SO42-) within the lake. E vidence indicating a decrease in net SO42- reduction at pH 4.5 was als o obtained in an anthropogenically acidified lake that had been acidif ied for many decades. In both lakes, the decrease in net SO42- reducti on appeared to be linked not to a simple inhibition of SO42- reduction but rather to changes in benthic ecosystem structure, especially the development of metaphytic filamentous green algae, which altered the b alance between SO42- reduction and sulfur oxidation. At pH's above 4.5 , net SO42- reduction was the major contributor to IAG in the experime ntal lake, as it is in many previously studied lakes at pH 5 and above . At pH 4.5, the change in net annual SO42- reduction (a decrease of 1 10%) resulted in a 38% decrease in total IAG. Because of the important role of net SO42- reduction in acid neutralization in softwater lakes , models for predicting acidification and recovery of lakes may need t o be modified for lakes acidified to pH < 5.