Our research objectives were to (i) determine the persistence of an introduced surrogate (Cellulomonas sp. NRC 2406) for a genetically engineered microorganism in sediments, growths of Cladophora glomerata (Chlorophyta), and leaf packs, (ii) test community and ecosystem structural and functional responses to the introduced bacteria, and (iii) evaluate the utility of flowing water mesocosms as tools for assessing the fates and effects of introduced bacteria in streams. Cellulomonas sp. densities were determined using fluorescent antibodies; maxima were < or = 1% of the total bacterial community in each habitat in two experiments, and approximately 25% of total densities in leaf packs in a third experiment. Densities declined from postinoculation maxima faster in sediments than in C. glomerata growths and leaf packs. Cellulomonas sp. persisted in leaf packs at densities significantly greater than background. Cellulomonas sp. had no statistically significant effects on primary productivity, community respiration, assimilation ratios, photosynthesis/respiration (P/R) ratios, bacterial productivity, and leaf litter decomposition rates. Cellulase concentrations were positively correlated with Cellulomonas sp. densities > or = 7 X 10 to the 8th power cells/g dry mass in fresh leaf litter for 2 days following exposure. Total bacterial densities, algal biomass, and total viable biomass sometimes differed between control and experimental systems, but differences were not related to Cellulomonas sp. introduction. Mesocosms were good tools for studying bacterial population dynamics in leaf litter and physiological aspects of litter degradation, but they were less well suited to measuring losses of litter mass and cellulose because physical abrasion during storms accelerated those processes in the field.