Effects of in-lake retention of nitrogen on critical load calculations

Critical loads (CLs) for soils and surface waters and their exceedances hav e been the basis for negotiations of emission reductions in Europe and else where. In Sudbury, Canada, large reductions in sulphur emissions have resul ted in reduced critical load exceedances of many lakes in the Killarney Pro vincial Park. To achieve a more complete chemical recovery even larger redu ctions of acid deposition are necessary. We extended the FAB (First-order A cidity Balance) model to include in-lake retention of nitrogen in upstream lakes and applied it to calculate CLs for Killarney lakes. Three different approaches were compared; one-lake, big-lake and lake-system. Use of "lake- system" resulted in the highest N retention and thus highest CLs, indicatin g that lakes at the end of chains are less sensitive to nitrogen deposition than calculated by the previous version of the model. Proper description o f in-lake retention in such lake systems, as well as good data on catchment properties like land use and land cover, are important for CL-calculations used for the evaluation of future emission reduction policies.