Abstract
Organic matter is the basic source of energy for consumers in ecosystems. In lotic ecosystems, most of the organic matter is allochthonous, imported as bank runoff or as aerial drift. Bank runoff is already processed soil material, particulate or dissolved. Unprocessed plant litter represents the aerial drift. The energy content of unprocessed organic matter is not readily available to consumers; it has to be processed by the microbial community. Microorganisms are most active in biofilms, comprised of fungi, bacteria, protozoa etc., and their organic excretions attached to surfaces. The colonizable surface area in sediments is negatively correlated with the grain size. Therefore, the largest amounts of organic matter are likely to occur in small grain size classes, which shows that biofilms are an important component of the organic matter pool. Most of the meiobenthic species, which play also a very important role in these processes, are closely connected to biofilms. These and their associated communities are doubtless an important food source for benthic consumers. The main energy pathway passes from organic matter (either particulate or dissolved) to the microbial community in biofilms, which transforms the organic matter and makes it available and palatable to benthic consumers. Wherever the benthic community is living, either in bed sediments or on macrophytic surfaces, the energy stored in biofilms or their associated communities is mostly used. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)