The diversity and composition of lake bacterial communities are driven by the interplay between local contemporary environmental conditions and dispersal of cells from the surroundings, i.e. the metacommunity. Still, a conceptual understanding of the relative importance of the two types of factors is lacking. For instance, it is unknown which sources of dispersal are most important and under which circumstances. Here, we investigated the seasonal variation in the importance of dispersal from different sources (mixing, precipitation, surface runoff and sediment resuspension) for lake bacterioplankton community and population dynamics. For that purpose, two small forest lakes and their dispersal sources were sampled over a period of 10 months. The influence of dispersal on communities and populations was determined by 454 sequencing of the 16S rRNA gene and SourceTracker analysis. On the community level direct effects of dispersal were questionable from all sources. Instead we found that the community of the preceding sampling occasion, representing growth of resident bacteria, was of great importance. On the population level, however, dispersal of individual taxa from the inlet could be occasionally important even under low water flow. The effect of sediment resuspension and precipitation appeared small.