Long-term phytoplankton changes in acid and limed lakes in SW Sweden

Phytoplankton development in the acidified Lakes Ostra Nedsjon and Ommern, situated in SW Sweden, was followed during 1973-92. The former lake was first limed in 1971-73 which increased the pH value from ca 5.3 to 6, without affecting the plankton markedly. In Lake Ommern, acidification lowered the pH value from ca 6 to 5.3 during 1976-1981, which caused oligotrophication and reduced number of species. In 1981/82 both lakes were limed, and during the following period treatments were regularly made, resulting in average pH levels of 7.2 and 6.9, respectively. The concentrations of humic compounds and phosphorus increased and the distribution of the major plankton groups was markedly changed. In both lakes the cyanophycean volume decreased, whereas diatoms became quantitatively important. Especially in relation to the acidic period 1979-81, there was increased species richness in both lakes. However, in contrast to the development in La ke Ommern, the desmids were eliminated in Lake O.Nedsjon, where the pre-acidification phytoplankton was not restored.