The significance of European high mountain lakes in critical load distributions at the EMEP grid scale

The integrated assessment models that drive atmospheric pollutant emission reduction policies within Europe, under the Convention on Long-range Transboundary Air Pollution, work at the scale of the EMEP grid (50 x 50 km). Critical loads are used to link anthropogenic deposition to adverse effects for individual ecosystems within each grid cell. The critical loads have two aspects, ecosystem area and the ecosystem specific critical load. The presence of highly acid-sensitive ecosystems within these grid cells can lead to very low critical loads of acidity which provide emission targets that are difficult to achieve: these cells are known as "binding grid squares". Since mountain lakes are highly sensitive to environmental change, their inclusion in critical loads assessments might be expected to result in lower EMEP scale critical loads. The combination of mountain lakes data with those from other ecosystems already included in the integrated modelling shows unexpected results. The average accumulated exceedance of critical loads of acidity may increase with either the presence of significant mountain lake catchment areas within grid cells or the presence of highly acidified clusters of sites. These circumstances occur in only a small number of grid cells. Furthermore, the relatively low sensitivity of mountain lakes to acidification in some regions means that their inclusion in EMEP scale models would lead to a relaxation of emission abatement requirements because of an increase in the area of ecosystems not exceeding critical loads and a decrease in AAE. Hence the potential effects of the inclusion of mountain lake critical loads of acidity in EMEP scale models vary greatly on a regional basis. The assumption that mountain lakes data could increase the incidence of "binding grid squares" is found to be unlikely.