Critical loads of sulphur and nitrogen for freshwaters in Great Britain and assessment of deposition reduction requirements with the First-order Acidity Balance (FAB) model

The critical loads approach is widely used within Europe to assess the impa cts of acid deposition on terrestrial and freshwater ecosystems. Recent wor k in Great Britain has focused on the national application of the First-ord er Acidity Balance (FAB) model to a freshwaters dataset of 1470 lake and st ream water chemistry samples from sites across Britain which were selected to represent the most sensitive water bodies in their corresponding 10 km g rid square. A "Critical Load Function" generated for each site is compared with the deposition load of S and N at the time of water chemistry sampling . The model predicts that when catchment processes reach steady-state with these deposition levels, increases in nitrate leaching will depress acid ne utralizing capacity (ANC) below the critical threshold of 0 mu eql(-1) at m ore than a quarter of the sites sampled, i.e. the critical load of acid dep osition is exceeded at these sites. The critical load exceedances are gener ally found in upland regions of high deposition where acidification has bee n previously recognised, but critical loads in large areas of western Scotl and are also exceeded where little biological evidence of acidification has yet been found. There is a regional variation in the deposition reduction requirements for protection of the sampled sites. The FAB model indicates t hat in Scotland, most of the sampled sites could be protected by sufficient ly large reductions in S deposition alone. In the English and Welsh uplands , both S and N deposition must be reduced to protect the sites. Current int ernational commitments to reduce S deposition throughout Europe will theref ore be insufficient to protect the most sensitive freshwaters in England an d Wales.