Polyurethane artificial substrates were positioned in a substrate-associated transect through littoral, sublittoral, and profundal sediments of Douglas Lake, Michigan. Substrates were also suspended in a vertical transect within the epilimnetic open water. Cluster analyses of species composition and statistical comparisons of species number indicated that distinct protozoan communities develop on each substrate, all manifesting characteristics of interactive equilibrium. The large differences in preoverturn species numbers and compositions between littoral-sublittoral sediments, profundal muds, and open waters clearly illustrate the distinctive nature of these environments in Douglas Lake. Following the major perturbation (autumnal overturn), there was a breakdown of competitive interactions with noninteractive colonization dynamics apparently determining the nature of each assemblage.