Abstract
In a study to determine the relative importance of various apparent molecular weight fractions to the metabolism (heat output) of attached microorganisms in rivers, a marked lack of response to gross changes in the external organic matter supply was noted over the course of several hours. Such a response by a mixed autotrophic-heterotrophic community could be attributed to the autotrophs being the sole organic energy source for the heterotrophs. However, substantial metabolic activities were noted in attached microbial communities grown in darkness (algae free), thus indicating that riverborne organic matter was a major energy source. This paradox was attributed to a proposed lag in the availability of dissolved and colloidal organic matter to the heterotrophic microorganisms.