Bacterial community dynamics was examined in an actual biological activated carbon (BAC) process for four consecutive seasons, using quantitative polymerase chain reaction and pyrosequencing. The BAC stably removed organic carbons for the period, although the water temperature substantially varied over the study period. Neither the population density nor community organization was correlated with time and temperature. However, the similarity degree between communities significantly reduced with time and temperature differences. Community analyses indicated that the community evolved over time, resulting in four distinct groups, and that the abundances of particular bacteria were significantly correlated with time and temperature, as well as their interaction. Additionally, backwashing did not affect the BAC bacterial population, community organization (diversity, evenness, and richness), or composition, although backwashing dislodged a large number of bacteria from the BAC (a parts per thousand 10(15) center dot m(-3)). These results suggest that water temperature is an important factor driving community dynamics and that backwashing is a harmless management option for biomass control.