This paper summarizes the results of the studies of biota structural organization and dynamics of small rivers of the Karelian White Sea coast. The chemical features and the structure of biological communities in the watercourses were investigated. Analysis of the species composition, abundance and biomass of phytoperiphyton, zooplankton, and zoobenthos was carried out. Variations in the structure of aquatic communities primarily are due to specific hydrographic and hydrological characteristics of river stretches. The paper discusses the main principles of the changes in a structure and functioning of hydrobiont communities in rivers. The trophic status of the rivers, their saprobiological state, and the significance of individual communities and biotic indices for bioindication of the ecological state of the rivers,are estimated. We concluded that the consideration of three groups of organisms enables a more comprehensive and reliable monitoring than the assessment based on a single group.