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 species of rotifers, 58 species of cladocerans, 28 species of copepodsand 1 mollusc. One rotifer species, Ploesoma peipsiense Maemets et Kutikova, has been described as new for science here. The zooplankton of L. Peipsi-Pihkva is remarkably rich in species including rarities in Estonia: 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 humic content, which support species with different ecologicalrequirements. Most of the aquatory of the lake has lately been mesotrophic, favouring the coexistence of indicators of oligo- and mesotrophic state and species preferring a higher trophic state. The occurrece of 10 species of the genus Bosmina including B. berolinensis, B. gibbera, B. lilljeborgi, B. thersites and B. crassicornis, sparse in Estonian lakes, is the most noteworthy feature of the zooplankton of L. Peipsi-Pihkva. The coexistence of B. coregoni and B. berolinensis, B. gibbera, B. lilljeborgi etc. which were earlier regarded as subspecies of B. c. coregoni proves that they are different species producing usually no hybrids. The species composition was subjected to certain changes during the years under consideration. Larvae of Dreissena were first found in zooplankton in 1962. The oligo-mesotrophic indicator Holopedium gibberum occurred in the lake in 1909-1964, but was lacking in later samples. [ABSTRACT FROM AUTHOR]