The total abundance of bacteria, the numbers of saprophytic, oligotrophic and sulfate -reducing microorganisms, the shapes of bacterial cells, the primary and bacterial production, organic matter destruction, and the rate of sulfate reduction were studied in small Karelian lakes during the stagnation period. It is shown that the uneven vertical distribution of light, temperature, oxygen and hydrogen sulfide creates ecological niches for the development of many various groups of microorganisms.