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 (SO₄ ²⁻) within the lake. Evidence indicating a decrease in net SO₄ ²⁻ 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 SO₄ ²⁻ reduction appeared to be linked not to a simple inhibition of SO₄ ²⁻ reduction but rather to changes in benthic ecosystem structure, especially the development of metaphytic filamentous green algae, which altered the balance between SO₄ ²⁻ reduction and sulfur oxidation.

At pH's above 4.5, net SO₄ ²⁻ reduction was the major contributor to IAG in the experimental lake, as it is in many previously studied lakes at pH 5 and above. At pH 4.5, the change in net annual SO₄ ²⁻ reduction (a decrease of 110%) resulted in a 38% decrease in total IAG. Because of the important role of net SO₄ ²⁻ reduction in acid neutralization in softwater lakes, models for predicting acidification and recovery of lakes may need to be modified for lakes acidified to pH <5.