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.