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

摘要

The regional-scale trends (south, central and north Finland) in keyacidification parameters over the period 1990–2003 were studied in lakesused for monitoring of acidification (157 lakes), and the catchmentcharacteristics that best discriminate between lakes showing recovery(significant increase in alkalinity) and those not showing recovery (nosignificant increase) were determined. A significant decline in sulphateconcentrations occurred in 82–98% of the lakes, depending of the region.Base cation (BC) concentrations decreased for most lakes, but to a lesserextent than those of SO₄. Consequently, a significant increase in Granalkalinity occurred in 40–92% of the lakes. The recovery fromacidification has been strongest in lakes in south Finland, where bothlevels and decrease of S (and N) deposition have been higher compared toother regions. A significant increase in pH was detected in about 50% ofthe lakes in the south. Here labile aluminium concentrations also decreasedin the most acidic lakes. Recovery has occurred most strongly in lakes whichhave SO₄ as a dominant acid anion, whereas recovery has been weaker inacidified humic lakes which have organic anion as a dominant acid anion. Thenon-recovering lakes in south Finland have higher proportion of exposedbedrock in the catchment, and higher TOC and lower BC concentrations. Incentral Finland the proportion of peatland and TOC concentrations werehigher and the decrease of BC concentration was steeper in non-recoveringlakes than in recovering lakes. In north Finland, catchment characteristics,trend slopes and concentrations did not separate the recovering andnon-recovering lakes. The non-recovering lakes were also located in regionswhich are acid-sensitive based on bedrock type, soil properties, weatheringrate and runoff. These factors have resulted in lower concentrations andsteeper downward trends for base cations. Climate change may increase themineralization of soil organic matter and change the frequency and magnitude of runoff andorganic acid episodes. An increase of these types of confounding effects on pH and alkalinityrecovery may therefore be anticipated in the future.