Causes of episodic acidification in Alpine streams

SUMMARY 1. Despite their global importance as mountain landscapes, the effect of acid deposition on running waters in the Alps is still incompletely described. The acid–base status of 30 clearwater streams in Canton Ticino (Switzerland) was therefore assessed at low and high flow across a south/north gradient of acid deposition in 2000.
2. At low flow, no stream was acidic, alkalinity being ≥29 μeq L⁻¹ and pH ≥6.4. However, NO₃^– was present in streamwater at these flows in concentrations that suggested anthropogenic enrichment, and its concentration correlated with spatial patterns of N deposition from the atmosphere.
3. Severe loss of alkalinity occurred in most streams at high flow during snowmelt (spring) or rainstorms (spring and autumn). Autumn episodes were because of dilution of streamwater alkalinity by rainwater. By contrast, spring episodes involved dilution and NO₃^– titration, the increase in nitrate being correlated with patterns of N deposition. SO₄^2– declined during most episodes.
4. High NO₃^– leaching at low flow suggests that catchment soils in Canton Ticino are approaching saturation in N. These data are among the first to illustrate that NO₃^– can drive episodic acidification in Europe, paralleling locations in north-eastern United States.