Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 12 2 2008 10.5194/hess-12-465-2008 http://www.hydrol-earth-syst-sci.net/12/465/2008/ http://www.hydrol-earth-syst-sci.net/12/465/2008/hess-12-465-2008.html http://www.hydrol-earth-syst-sci.net/12/465/2008/hess-12-465-2008.pdf 465 478 2008-03-05 Recovery of acidified Finnish lakes: trends, patterns and dependence of catchment characteristics J. Vuorenmaa jussi.vuorenmaa@ymparisto.fi M. Forsius Finnish Environment Institute, P.O. Box 140, 00251 Helsinki, Finland 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. 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