DEVELOPMENT OF ZOOPLANKTON IN RELATION TO LIME TREATMENT IN 2 ACIDIFIED LAKES

Zooplankton has been studied in Lakes Ostra Nedsjon and Ommem (1974-94 ), two acidified lakes in South-Western Sweden. The former lake was fi rst limed in 1971-73, and secondly in 1982. The first liming increased the ps-level from ca. 5.3 to 6, and the second one from ca. 5.8 to 7- 7.5. The pH-value in Lake Ommem was about 6.1 in 1973, but decreased t o 5.3 in 1981, when liming raised the value to 6.5-7. After the first liming in Lake O. Nedsjon, the rotatorians represented in avenge 1% of the total volume of zooplankton. Among the copepods, which made up ca . 228 of the volume, Cyclops was frequent, and both Eudioptomus gracil is and Heterocope appendiculata were recorded After the second liming on the other hand the rotatorians increased to in avenge 28% of the to tal volume, while the volume of copepods was only ca. 6%. Heterocope d isappeared, and Eudiaptomus and Cyclops, usually favoured by liming, w ere rare. Consequently de supply of larger forms of crustaceans, suita ble as fish food was severely reduced In Lake Ommem the effects of aci dification within the zooplankton were moderate. The species richness was about the same as in unacidified lakes. After liming the developme nt of zooplankton was similar to that in most other limed lakes, i.e. increased frequency of rotatorians, cladocerans and Cyclops spp.. In L ake O. Nedsjon, however, the small zooplankton volumes and the elimina tion of copepods was in contrast to the zooplankton development in oth er limed lakes and in unacidified ones.