MACROZOOBENTHOS OF LAKE-PEIPSI-PIHKVA - TAXONOMICAL COMPOSITION, ABUNDANCE, BIOMASS, AND THEIR RELATIONS TO SOME ECOLOGICAL PARAMETERS

The large but shallow (3,558 km(2), up to 15.3 m deep) lake is eutroph ic, with Chironomus plumosus and Potamothrix hammoniensis as dominatin g macroinvertebrates in the profundal. The extensive well-aerated subl ittoral with sandy bottom sediments has a mesotrophic appearance and s upports a diverse fauna with several oxyphilous species, including a v ery abundant population of Dreissena polymorpha. The phytophilous faun a is limited to small sheltered areas only. The average abundance of t he ''small'' animals of macrozoobenthos (without big molluscs) was 2,6 17 ind. m(-2), their biomass 12.34 g m(-2) (corresponding to 52.2 kJ m (-2)) in 1964-1994. The same figures for big molluscs (mostly Dreissen a) were 304 ind, m(-2) and 238 g m(-2) in 1964-1994, and even 864 ind. m(-2) and 687 g m(-2) in 1985-1988, at the time of their special mapp ing. The sublittoral zone revealed the lowest biomass of ''small'' ani mals but the highest biomass of big molluscs. The southern, shallower lake regions, more enriched with nutrients and better protected from w ind, revealed a significantly higher biomass of ''small'' macrozoobent hos in the near-shore zone than the cleaner and open northern part, wh ile no positive effect of enrichment was observed neither in the bioma ss of profundal zoobenthos nor in that of big molluscs. The production of the ''small'' macrozoobenthos was calculated as 111 and 53 kJ m(-2 ) during two annual cycles in Lake Peipsi s. s., the most productive p eriod being the autumn overturn. Lake Peipsi-Pihkva has the highest ab undance and biomass of macrozoobenthos among the large lakes of North Europe.