The structural assimilative capacity (ability to maintain structural and functional integrity under stress) of protozoan communities from nine lakes in the area of UMBS and six stations at Smith Mountain Lake, Va. were studied to determine: (1) if the communities from lakes of differing trophic state differ in their ability to assimilate various amounts of copper sulfate; and, (2) to explore the possible influence of average density of individuals and/or qualitative differences in the types of species present on any observed differences in assimilative capacity. In both the northern Michigan and Smith Mountain Lake studies, a trend in response was demonstrated along the eutrophic-oligotrophic gradient; eutrophic communities had a greater structural assimilative capacity than did oligotrophic communities. Both mean species density and community composition appear to be important factors in the ability to maintain structural integrity. A laboratory study was conducted to determine: (1) whether a sublethal dose of CuSO4 would affect the colonization rate of freshwater protozoan communities established on polyurethane foam units; and, (2) if a correlation exists between the length of initial colonization of an artificial substrate epicenter in the lake and the extent of the toxicant’s effects on the colonization process in the laboratory. Observed colonization rates for the systems under stress were compared to those of the control systems at the same stage of community development. A sublethal dose of copper sulfate significantly decreased the colonization rate of both the mature and immature communities (p < 0.05); however, the effects were greater on the immature systems.