Zoological Institute, Russian Academy of Science, Universitetskaya nab. 1, St.-Petersburg 199034, Russia

Abstract

Experimental studies were carried out in weakly eutrophic lake Anninskoe, Pskovskaya region. It was shown that 50-90% of phytoplankton production passes through microbial organisms. Depending on development of microbial "loop" production of metazoan plankton includes from 5 to 20 % of primary production. Changes of structure of plankton community were studied under hard, weak and moderate press of large zooplankton. It was shown that bacteria and ciliata development were inversely dependent on the ratio of zooplankton to algae biomass. This ratio, reflecting integrative regulative impact of biotic factors to the microbial community from the "top" and from the "bottom", is proposed by the authors as an index of biotic relationships in plankton. The importance of Asplanchna priodonta in development of structure and abundance of phytoplankton and protozoa is discussed.