Effects of site selection strategy on freshwater critical load exceedancesin Wales

Critical loads are used in international negotiations to reduce acid deposi tion resulting from emissions of sulphur and nitrogen compounds within Euro pe. For freshwater ecosystems, the First-order Acidity Balance (FAB) model is used to generate national maps of critical loads and exceedances for bot h sulphur (S) and nitrogen (N). In Wales, two survey datasets have been use d to calculate critical loads and exceedances; one based on water bodies se lected to be "most-sensitive" to acidification within a 10 km grid and the other based on a random selection of standing waters. Both datasets indicat e that critical loads were exceeded in 1990 in a significant proportion of Welsh lakes and streams; 36% of sites in the grid-based survey and 31% of s ites in the random survey. However, implementation of the Gothenburg Protoc ol would protect all but 6% of sites in the grid-based survey and ail sites in the random survey. Assessment of the relative success of the Gothenburg Protocol in protecting Welsh freshwater ecosystems therefore depends on th e site selection strategy employed.