Recent re-establishment of the key species Daphnia longispina and cladoceran community changes following chemical recovery in a strongly acid-stressed region in southern Norway

Abstract

The present investigation aimed to study how pelagic cladocerans in formerly acidified lakes responded to liming and natural improvements in water quality. Contrasting regional composition of bedrock geology and Quaternary deposits have given rise to aquatic sites with very different acidification histories in an area strongly affected by acidification in southern Norway. A number of lakes situated on base-poor, granitic bedrock and shallow Quaternary deposits have been limed since the mid-1980s, with main liming activities from the 1990s. The majority of poorly buffered lakes were barren of Daphnia spp. during the peak anthropogenic acidification until liming was initiated. The present zoogeographical study showed that the key species D. longispina inhabited most limed lakes, even sites exceeding 100 m depth. Limed lakes with high density of Eurasian perch (Perca fluviatilis) or other efficient planktivorous species, had low abundances or were devoid of D. longispina altogether. Unlimed lakes with constant low pH around 4.4-4.8 were still barren of daphnids, irrespective of their geographical distribution. Many investigated lakes harboured prior to liming numerous Daphnia ephippia in deeper sediment strata, from where the new pelagic populations most probably originated. The source of the Daphnia populations, the recovery processes, and the ecological structures of the cladoceran communities during and after chemical recovery are discussed. The present study forms a useful test what may happen to the cladoceran communities and D. longispina over large areas in Scandinavia and in similar acidified regions during natural recovery.