MACROZOOBENTHOS OF LAKE-PEIPSI-PIHKVA - LONG-TERM BIOMASS CHANGES

Macrozoobenthos of the large (3,558 km(2)) Lake Peipsi-Pihkva was moni tored at 22-24 sampling spots yearly in June 1964-1994 as well as 50 s hallow-water profiles in mid-summer 1970, 1980, and 1990. No qualitati ve changes were observed in the bottom fauna, except for the introduct ion of a Baikalian gammarid species, Gmelinoides fasciatus. The averag e total biomass in June (without big molluscs) revealed considerable a nnual fluctuations, with an insignificant general tendency of increase . The increase was significant in the biomass of several Mollusca (inc luding the highly abundant big clam Dreissena polymorpha), Asellus, Ga mmaridae, and Hirudinea but not in the most abundant animal groups Chi ronomidae and Oligochaeta. The low biomass of Ephemeroptera and Hydrac hnellae decreased significantly. The mid-summer samples taken from the shallow-water zone in three different years revealed the highest biom ass in 1980 and the lowest in 1990 (the latter being probably related to the high water level). A gradual decrease in the biomass of the Eph emeroptera and Hydrachnellae as well as the Oligochaeta, Pisidiidae an d Bithynia tentaculata occurred in the shallow-water zone, while the G ammaridae increased on the account of the introduced species. No coinc idence was found with the neighbouring Lake Vortsjarv when comparing t he annual fluctuations of biomass. A general tendency to a slow increa se in total biomass and a decrease in the biomass of the most vulnerab le groups Ephemeroptera and Hydrachnellae, probably due to progressing eutrophication, were common for both lakes.