Acidification in European mountain lake districts:: A regional assessment of critical load exceedance

High mountain lakes are sensitive to environmental change and the effects of air pollution and lake acidification have been recorded in many countries. The EU funded EMERGE programme included a pan-European assessment of the extent of acidification in mountain lakes located above the tree-line. A static critical loads model, the First-order Acidity Balance (FAB) model, was used to assess (1) the extent of critical load exceedance in 300 lakes in nine European lake districts and (2) the relative importance of sulphur and nitrogen deposition in contributing to acidification. The regional sensitivity of FAB to the choice of critical acid neutralising capacity (ANC: 0 or 20 mu eq L-1) was explored. With a critical ANC value of 0 mu eq L-1 only four lake districts had sites showing exceedance of critical loads; Piedmont Ticino, the Pyrenees, the Retezat Mountains and the Tatras. When a more stringent critical ANC of 20 mu eq L-1 was used, all nine lake districts showed critical load exceedance in one or more lakes. For two lake districts, the Retezat Mountains of Romania and the Rila Mountains of Bulgaria, critical load exceedance is recorded here for the first time. Nitrogen is a more important agent of acidification in some areas such as the Pyrenees and Piedmont Ticino, and its relative importance is likely to increase elsewhere as pan-European measures to reduce sulphur deposition continue to take effect. Given the coarse scale deposition data used and potentially underestimated loads at high altitudes, the extent of the acidification problem may be under-represented here.