HISTORY OF INVESTIGATIONS ON THE ECOSYSTEM OF LAKE PEIPSI

The investigations of the biota in L. Peipsi, one of the largest lakes of Europe, started already one and a half centuries ago, complex studies were initiated in the 1930s. The paper gives a brief survey of research into the hydrochemistry, macrophytes, bacterio-, phyto- and zooplankton, zoobenthos, fishes, and fisheries management of the lake. Special attention is paid to the main findings. Intensive anthropogenic eutrophication of L. Peipsi started in the 1970s. The nutrient content reached its maximum in the 1980s and decreased thereafter. An expansion of reeds, increase in the similarity between different lake parts, and decline of rare macrophyte species have occurred. Regarding phytoplankton, L. Pihkva has become strongly eutrophic, whereas L. Peipsi s.s. has eutrophied to a smaller extent, its northern part has maintained a mesotrophic appearance. In 1965- 99, the number but not the biomass of zooplankton increased. The number of small-sized zoo- plankters (especially rotifers) increased. No essential changes in the composition and distribution of macrozoobenthos in connection with eutrophication of the lake were observed. Dreissena polymorpha, introduced accidentally in the lake in the 1930s, has become the most significant animal population in L. Peipsi. Considering annual fish catches L. Peipsi surpasses all large lakes in North Europe. The main commercial fishes are smelt, perch, pikeperch, roach, bream, pike, and until the 1990s also vendace. At present it is changing into a smelt-bream-pikeperch lake.