Microcosms were used in this research to assess the environmental risk associated with two types of stressors: genetically-engineered microorganisms (GEMs) and toxic chemicals. Approaches used to evaluate the potential environmental impact of these two stressors are fundamentally different, and arise mainly from the fact that the GEMs are living and capable of replication. Risk assessment for genetically-engineered Erwinia carotovora strain L-864 included investigations of its persistence and effect on structure and function of aquatic microcosms. Densities of genetically-engineered and wild type E. carotovora declined at the same rate in water or in sediment, falling in 32 days below the level of detection by viable counts. Selective media, antibiotic resistance, and most probable number analysis were used to enumerate genetically-engineered E. carotovora in environmental samples. This technique was able to detect as few as 1 to 10 target cells/10 g soil. In thermally-perturbed aquatic microcosms, genetically-engineered E. carotovora persisted significantly longer than in unperturbed microcosms, suggesting the vulnerability of stressed ecosystems to colonization by GEMs. Competition study showed that the genetically-engineered E. carotovora did not displace the wild type strain. Effects of genetically-engineered and wild type strain on indigenous bacteria belonging to specific functional groups important in nutrient cycling were similar: inoculation of either strain caused a temporary increase in densities of total and proteolytic bacteria, while it did not affect amylolytic and pectolytic bacteria. Treatment with engineered bacteria did not change biomass values of the receiving community, but caused a transitory increase in its metabolic activity. The inability of genetically-engineered E. carotovora to persist, displace resident species, and affect metabolic activity of a community indicates a low risk of adverse ecological effects in aquatic systems. Microcosms were also used to assess environmental risk for toxic chemicals. A study was conducted to assess the fate of sediment-associated copper and to investigate its effects on the structure and function of the aquatic community. Most of the added copper was bound to sediment particles. In microcosms containing 100 µg Cu/kg sediment, chlorophyll a content and respiration significantly decreased compared to the control. Addition of 1000 µg Cu/kg sediment caused a decrease in production, respiration, respiration/biomass ratio, ATP, and chlorophyll a. The last study compared responses of Simocephalus exspinosus (Daphniidae) to copper during a single-species test to responses of S. exspinosus populations in a microcosm test. Responses of S. exspinosus were similar in both test systems: there was an increased production of young at 30-46 µg Cu/l, while the organisms did not survive exposure to concentrations > 100 µg Cu/l. In these studies, microcosms showed a potential to predict fate and effects of chemical and biological contaminants released into the biosphere.