A PALEOLIMNOLOGICAL ASSESSMENT OF ACIDIFICATION AND LIMING EFFECTS ON DIATOM ASSEMBLAGES IN A SWEDISH LAKE

In Sweden, about 6000 lakes have been limed to mitigate acidification. Lysevatten (southwestern Sweden) was limed in 1974 and 1986 and has better historical pH and fish records than most other, similar forest lakes. Historical data were compared with diatom analyses of a sediment core to assess to what extent diatom-inferred pH reflects the known pH history. The diatom-inferred pH history agrees well with the development known from documentary sources, although the sediment record is slightly smoothed. Both the historical and sediment records show that the acidification, with acid episodes that caused severe damage to lake biota, began during the 1940s and became acute in the 1960s, with pH values of 4.5-4.9. Prior to acidification, lake water pH was between 6 and 7, and the lake had a planktonic Cyclotella flora. The liming increased pH to 7.5 and resulted in the expansion of Achnanthes minutissima agg., Cymbella microcephala, and Synedra acus. The reacidification is reflected by a decrease of the abundance of these species. Neither the liming in 1974 nor that in 1986 restored the preacidification diatom flora; there was no recovery of Cyclotella.