EXCEEDANCE OF CRITICAL LOADS FOR LAKES IN FINLAND, NORWAY, AND SWEDEN- REDUCTION REQUIREMENTS FOR ACIDIFYING NITROGEN AND SULFUR DEPOSITION

The main objectives of this study were to identify the regions in Fenn oscandia where the critical loads of sulfur (S) and acidifying nitroge n (N) for lakes are exceeded and to investigate the consequences for d eposition reductions, with special emphasis on the possible trade-offs between S and N deposition in order to achieve nonexceedance, in the steady-state model for calculating critical loads and their exceedance s, all relevant processes acting as sinks for N and S are considered. The critical loads of N and S are interrelated (defining the so-called critical load function), and therefore a single critical load for one pollutant cannot be defined without making assumptions about the othe r. Comparing the present N and S deposition with the critical load fun ction for each lake allows determination of the percentage of lakes in the different regions of Fennoscandia where: (1) S reductions alone c an achieve nonexceedance, (2) N reductions alone are sufficient, and ( 3) both N and S reductions are required but to a certain degree interc hangeable. Secondly, deposition reduction requirements were assessed b y fixing the N deposition to the present level, in this way analyzing the reductions required for S, and by computing the percentage of lake s exceeded in Finland, Norway and Sweden for every possible percent de position reduction in S and N, in this way showing the (relative) effe ctiveness of reducing S and/or N deposition. The results showed clear regional patterns in the S and N reduction requirements. In practicall y the whole of Finland and the northern parts of Scandinavia man-made acidification of surface waters could be avoided by reducing S deposit ion alone. In the southern parts of Sweden some reductions in N deposi tion are clearly needed in addition to those for S. In southern Norway strong reductions are required for both N and S deposition.