Concern and controversy over the effects of acidic deposition on low ionic strength surface waters has led to much discussion on the nature and extent of proton transformations within acid sensitive ecosystems1-4. The source of base neutralizing capacity (BNC) within acid surface waters has been attributed to atmospheric deposition of H2SO4 (or SO 2)5,6 or HNO3 (ref. 7), as well as production of soluble organic acids from soils8. Unfortunately many of these studies have failed adequately to characterize aluminium, which is often a very significant component of BNC in acidic waters9. We have evaluated the nature of short-term changes in the BNC of an acidic clearwater lake. Our results suggest that much of the variation in hydrogen ion and aluminium BNC can be attributed to changes in nitrate concentration, rather than to variations in sulphate, chloride, or organic anion concentrations.
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