Sessile bacteria: An important component of the microbial population in small mountain streams 1

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Abstract
Direct counts by epifluorescence microscopy demonstrated that epilithic bacteria were numerically more important than free‐floating bacteria in unpolluted mountain streams. Concentrations of sessile bacteria associated with the upper surfaces of submerged rocks coincided with fluctuations in epilithic algal biomass. Electron microscopy of thin‐section preparations of natural samples showed that the attached algae provided a surface for bacterial colonization. This close physical relationship between the bacteria and algae was mediated by a network of fibers which, in mass, appeared as a layer of slime covering the streambed. The relationship between sessile bacterial concentratons and epilithic algal biomass in one tributary suggested that, in some cases, factors other than epilithic primary production governed the attached microbial population.