Heterotrophic bacteria are accounted for the most important proportion of decomposers in the aquatic ecosystems, which are responsible for key process regulating the function and productivity of ecosystem through the microbial loop. Heterotrophic bacteria in the fresh water ecosystem form a major part of the food web and mediate important processes in the carbon budget. As in both fresh water and marine waters, the biomass and size distribution of bacteria are important parameters of ecosystem function.