Liming of acidified lakes: induced long-term changes [Sweden]

This study presents data concerning long-term trends after neutralization of four acidified lakes in two regions on the Swedish west coast. Neutralization was achieved by a di-Ca-silicate with 52% CaO and about 11.5% MgO. Between 61 and 74% of the spread lime product dissolved during a 5 to 7 yr period. The liming increased pH, from a range of 4.5 to 5.2 to near neutral and restored alkalinity in the range of 0.2 to 0.3 meq/l and the Ca-content became 3 to 4 times higher than before liming. In two lakes transparency decreased significantly presumably due to changed phytoplankton composition. These changes successively declined due to dilution and continuous acid loading. The changes in water chemistry and development of stocked brown trout (Salmo trutta) populations initiated biotic changes. Phyto- and zooplankton communities reacted both instantly and later with successions in species composition. Changes of benthic macroinvertebrate species occurred over several years, but some pelagic species, e.g. corixids were rapidly reduced due to predation of fish. Observed changes were predominantly due to expanding populations of species present at very low abundances even during acid state of the lakes. Some organisms found during preacid state of the lakes did not establish new populations and this process may need a prolonged time with favourable conditions. Reacidification towards the end of the study period significantly stressed the brown trout population and also favoured expansion of the filamentous alga Mougeotia sp. and Sphagnum sp. that almost vanished during the first year after liming. Decreasing concentration of total P was not influenced by neutralization and may be mostly dependent on negative changes in the soils surrounding the lakes. If generally valid, this process may be an important factor for the oligotrophication of lakes in areas where acid deposition is high.