Recovery from acidification of Finnish lakes: regional patterns and relations to emission reduction policy.

The regional-scale response of Finnish headwater lakes to changes in acidifying deposition loads was studied using data from a national deposition monitoring network (19 stations), acidification monitoring lakes (163 lakes) and results of a statistically based national lake survey (873 lakes). Data from 1990 to 1999 were used for statistical trend analysis. A deposition model was used to assess changes in S and N deposition for the year 2010, assuming emission reductions according to two international agreements. The deposition of sulfate and H(+) showed statistically significant (Kendall-tau, P<0.05) decreasing trends at nearly all deposition stations. For N compounds, nearly all slopes were negative, but rarely statistically significant. Sulfate concentrations have declined in all types of small lakes throughout Finland in the 1990s (significant decline in 64-85% of the lakes in three different lake regions), indicating a clear response to S emission reductions and declined sulfate deposition. Base cation concentrations decreased in both deposition and lake water, especially in southern Finland, but to a lesser extent than sulfate concentrations. The median slope of the trend for Gran alkalinity in lakes ranged between 0.98 and 2.1 microeq l(-1) a(-1). Some 1400 (27%) of Finnish headwater lakes of size 4-100 ha were estimated to show statistically significant increases in Gran alkalinity (recovery). No large changes were observed in the lake water TOC concentrations. The reduction in S deposition is the main driving factor for the lake acidification recovery process in Finland. Deposition model calculations showed that further large reductions in S deposition beyond the 1999 level are not likely to occur by the year 2010, particularly for southeastern Finland. The mean estimated S deposition change by 2010 for the three lake regions in Finland was only between -0.9 and -6.6% for the two policy scenarios (UN/ECE Gothenburg protocol, EU NEC-directive), respectively. A slower acidification recovery of the lake ecosystems is, therefore, anticipated in the future.