The interactions between bacteria and heterotrophic protists are essential for the ecosystem ecology of freshwater. The basic objective of the study was to analyse the seasonal dynamics of changes in particular elements of the microbial loop (bacteria, flagellates and ciliates) in various types of small water bodies. The classification into particular trophic groups of ciliates and the role of the organisms in the functioning of the microbial loop was also analysed. The abundance and biomass of microbial communities has been assessed in eight reservoirs of varied origin (peat pits, clay pits and post-mining reservoirs), in order to test a hypothesis that the microbial communities differed among reservoirs and that differences could be related to fertility of habitats. Fieldwork was done from April to November 2010-2012. From each reservoirs once a month, three samples were collected with a 5-litre planktonic sampler. Samples were taken on two stations: pelagic – located in the deepest part of each reservoirs and littoral. The density and biomass of bacteria and protists significantly differed between the studied reservoirs, with the lowest numbers in the peat pits and clay pits and the highest in the post-mining reservoirs. All of the studied reservoirs were generally dominated by ciliates belonging to the orders Oligotrichida and Scuticociliatida constituted > 30% of the total numbers. In all reservoirs bacterivorous ciliates occurred in the highest numbers, while predators ones in the lowest. Generally, concentrations of nutrients (especially total phosphorus) is not a factor having the strongest effect on the abundance of bacteria and heterotrophic flagellates and the number and strength of correlations between microbial loop components. The relations demonstrated between bacteria and protists point to an important process of matter and energy flow from bacteria to higher trophic levels. In peat pits, relations between bacteria and ciliates were stronger. Only in clay pits and post-mining reservoirs the correlation between bacteria and heterotrophic flagellates were found. Consuming much of bacterial productions, protozoa become an important link between bacteria and micrometazoans.
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