A SIMPLE MODEL OF REGIONAL ACIDIFICATION FOR HIGH MOUNTAIN LAKES: APPLICATION TO THE PYRENEAN LAKES (NORTH-EAST SPAIN)

In a sample of 102 lakes in the Pyrenees, 68% were sensitive to acidification in varying degrees. A model which describes the regional alkalinity distribution by using random variables to represent lake alkalinity, chemical weathering and biological alkalinity generation, is proposed to calculate the alkalinity loss due to acid deposition, and the critical and target acid loads. This model is kept simple to reduce the input requirements and make the calibration procedures feasible on a regional scale. The relative simplicity of high mountain catchments lying above the tree line, in particular the lack of well developed soils, makes such a model appropriate for these environments. The loss in alkalinity since the start of large scale industrial emissions was estimated to be 35 μeq l−1 on average. Assuming a scenario of increasing nitrogen deposition, we estimate that a critical level of 10% of lakes with alkalinities below 20 μeq l−1 would be reached for a nitrogen deposition of 70–75 μeq l−1. According to the model, a 50% reduction of both nitrogen and sulphur deposition would be necessary to ensure that all lakes are above the critical level.