Structure, succession, and function of plankton communities in temperate lakes are considered, with emphasis on the role and ecological importance of the microbial loop. Bacteria, nanoflagellates (including mixotrophs and autotrophs), ciliates, and mesozooplankton reveal distinct spatial and seasonal changes in biomass composition and in importance in the carbon budget. Constraints in growth of microbial communities include resource competition and predation by organisms occurring at several trophic levels. Daphnia species play a key role in breaking the microbial loop and establish direct routes from several microbial compartments to higher trophic levels. In highly productive temperate lakes, Daphnia can be replaced by Bosmina, which is less efficient as grazer on picoplankton. The consequences include an increase in remineralization and a decrease in the transport of particulate carbon to higher trophic levels. Conceptual and predictive models of the microbial loop are few and include at most 2-3 trophic levels. -from Authors