In the current conceptual framework of aquatic detritus food webs several aspects of carbon and nitrogen dynamics remain still unresolved, in particular the issue of nitrogen enrichment during the course of detritus degradation. Perhaps one of the reasons for this is the use of complex detritus originating from plant sources in most previous studies. Using nitrogen-free pure cellulose as a simple and defined model substrate, we have addressed the detritus nitrogen problem by following the changes in microbial and chemical parameters during its degradation in a semi-continous flow of fresh North Sea water for a period of 12 weeks. Initial colonization and growth of heterotrophic bacteria and associated mucus was followed by heterotrophic protozoa. Closely attached or loosely bound cells that were embedded in voluminous mucus constituted the bacterial community. Protozoa comprised heterotrophic nanoflagellates, and large euglenoid flagellates. The euglenoids produced plentiful faecal pellets 3 to 4 micrometer in diameter; a novel source of microscopic detrital particles. By about 12 weeksŽ time, patches of cellulose fibres had been totally converted to microbial biomass and associated mucus. The carbon content decreased from 40% to 25% of the initial cellulose weight, while nitrogen increased from 0% to 0.5% as a result of net uptake of dissolved nitrogen from the ambient water. Elutriation experiments with dilute acetic acid showed that, after 12 weeks, 4.2 % of the total nitrogen in degrading cellulose was present as amino acids presumably weakly bound in some way to the mucus secreted by microorganisms, indicating a potential for sequestering capability of microbial slime that is ubiquitous in the aquatic environment. Results suggest that degrading detritus may gain an increasing pool of labile nitrogen compounds other than microbial protein which may be of considerable importance for animal nutrition.