Methods for quantifying the influence of different ions on depressions in ANC, based on those used by Molot et al. (1989), have been employed on streams in the Adirondack and Catskill Mountains of New York Streams were intensively monitored during the Episodic Response Project of the US EPA. Baseflow values were determined for each variable, and for each low-ANC sample the proportion of ANC depression (relative to baseflow) contributed by each ion was calculated In the Catskill streams Ca2+ dilution and NO; increases were major causes of ANC depression; SO42- dilution and Al mobilisation increased ANC. In the Adirondack streams Ca2+ and Na+ dilution,and NO3- and SO42-. increases, all contributed to ANC depressions. Inter-stream differences in results were linked to differences in stream acidity, in both regions Ca2+ dilution dominated ANC depressions in circumneutral streams, whereas NO; increases were dominant in acidic streams. Organic anions contributed more to ANC depressions in acidic streams. Al buffering, was negligible in circumneutral streams, but more than halved ANC depressions in the most acidic stream. Individual base cation behaviour differed widely, suggesting that caution should be used when treating them as a uniform group.
