Many factors contribute to the complex patterns of ecological conditions in streams. Nutrient and light availability are 2 factors that directly affect benthic algal assemblages and thereby indirectly affect stream faunas. A null model of the major changes and determinants of benthic algal assemblage structure and function along resource (nutrient and light) gradients is proposed. This model predicts that low-resource stress, herbivory, competition, and colonization of habitats by new algal frowth forms are the dominant determinants of benthic algal ecology at successively higher resource levels. It also predicts major changes in algal assemblages in resource conditions at which regulation of communities changes from one dominant determinant to another. In many streams, diatoms dominate low-density biofilms in low resource conditions where resource stress and herbivory regulate periphyton. Above a herbivory threshold, a resource condition in which algal accumulation rates equal grazing rates, diatoms should accrue to densities that are regulated by competition for light and nutrients. Another resource threshold can occur where increasing nutrient or light regimes make the habitat available for colonization by different algae, such as Cladophora or other filamentous greens, that have growth forms very different from those of diatom-dominated assemblages. A concept of ecological sustainability is also proposed which quantifies the changes in environmental conditions that an ecosystem can withstand without "significantly" changing ecological integrity. Linkage between the resource threshold hypothesis and ecological sustainability helps to define environmental criteria at points along resource gradients where significant changes in ecological processes are expected.