Recovery of acidified Finnish lakes: trends, patterns and dependence of catchment characteristics

The regional-scale trends (south, central and north Finland) in key acidification parameters over the period 1990–2003 were studied in lakes used for monitoring of acidification (157 lakes), and the catchment characteristics that best discriminate between lakes showing recovery (significant increase in alkalinity) and those not showing recovery (no significant increase) were determined. A significant decline in sulphate concentrations occurred in 82–98% of the lakes, depending of the region. Base cation (BC) concentrations decreased for most lakes, but to a lesser extent than those of SO4. Consequently, a significant increase in Gran alkalinity occurred in 40–92% of the lakes. The recovery from acidification has been strongest in lakes in south Finland, where both levels and decrease of S (and N) deposition have been higher compared to other regions. A significant increase in pH was detected in about 50% of the lakes in the south. Here labile aluminium concentrations also decreased in the most acidic lakes. Recovery has occurred most strongly in lakes which have SO4 as a dominant acid anion, whereas recovery has been weaker in acidified humic lakes which have organic anion as a dominant acid anion. The non-recovering lakes in south Finland have higher proportion of exposed bedrock in the catchment, and higher TOC and lower BC concentrations. In central Finland the proportion of peatland and TOC concentrations were higher and the decrease of BC concentration was steeper in non-recovering lakes than in recovering lakes. In north Finland, catchment characteristics, trend slopes and concentrations did not separate the recovering and non-recovering lakes. The nonrecovering lakes were also located in regions which are acidsensitive based on bedrock type, soil properties, weathering rate and runoff. These factors have resulted in lower concentrations and steeper downward trends for base cations. Climate change may increase the mineralization of soil organic Correspondence to: J. Vuorenmaa ([email protected]) matter and change the frequency and magnitude of runoff and organic acid episodes. An increase of these types of confounding effects on pH and alkalinity recovery may therefore be anticipated in the future.