ZOOPLANKTON OF LAKE-PEIPSI-PIHKVA IN 1909-1987

164 taxa were identified in the net zooplankton of the pelagial of L. Peipsi-Pihkva in 1909-1987, including 3 species of protozoans, 74 spec ies of rotifers, 58 species of cladocerans, 28 species of copepods and 1 mollusc. One rotifer species, Ploesoma peipsiense Maemets et Kutiko va, has been described as new for science here. The zooplankton of L. Peipsi-Pihkva is remarkably rich in species including rarities in Esto nia: Limnosida frontosa, Drepanothrix dentata, Bythotrephes longimanus , B. cederstroemi etc. Due to its large surface area, L. Peipsi-Pihkva provides a large scale of biotopes of a diverse trophic state and hum ic content, which support species with different ecological requiremen ts. Most of the aquatory of the lake has lately been mesotrophic, favo uring the coexistence of indicators of oligo- and mesotrophic state an d species preferring a higher trophic state. The occurrece of 10 speci es of the genus Bosmina including B. berolinensis, B. gibbera, B. lill jeborgi, B. thersites and B. crassicornis, sparse in Estonian lakes, i s the most noteworthy feature of the zooplankton of L. Peipsi-Pihkva. The coexistence of B. coregoni and B. berolinensis, B. gibbera, B. lil ljeborgi etc. which were earlier regarded as subspecies of B. c. coreg oni proves that they are different species producing usually no hybrid s. The species composition was subjected to certain changes during the years under consideration. Larvae of Dreissena were first found in zo oplankton in 1962. The oligo-mesotrophic indicator Holopedium gibberum occurred in the lake in 1909-1964, but was lacking in later samples.