Disruption of sulfur cycling and acid neutralization in lakes at low pH

Abstract

Experimental acidification of a softwater lake to below pH 5 fundamentally changed the sulfur cycle and lowered internal alkalinity generation (IAG). Prior to reaching pH 4.5, the balance of sulfur reduction and 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 balance of reduction and oxidation reactions was in favour of oxidation, and there was a net production of sulfate (SO42−) within the lake. Evidence indicating a decrease in net SO42− reduction at pH 4.5 was also obtained in an anthropogenically acidified lake that had been acidified for many decades. In both lakes, the decrease in net SO42− reduction 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 balance between SO42− reduction and sulfur oxidation.