The effect of chemical, physical, and biological factors on bacterial density in streams was assessed in surveys of streams throughout two ecoregions. Using both regression and path analysis, I found that algal biomass was the best predictor of bacterial density in both ecoregions. Inorganic nutrients appeared to affect bacteria indirectly through their influence on algal biomass. Dissolved organic carbon (DOC) in the water was not a good predictor of bacterial density. Likewise, algal biomass was not a strong predictor of DOC. Since algae and bacteria were not statistically related through water column DOC, bacteria may be statistically related to algae, because they utilize algal generated DOC that is entrained within periphyton mats or use algae as a substrate and habitat. I experimentally tested three hypotheses regarding the nature of algal-bacterial associations in streams under different environmental conditions. First, the bacterial carbon limitation hypothesis predicts that bacteria in streams with negligible allochthonous inputs of labile dissolved organic carbon (DOC) and adequate light will be associated with algae because they are carbon limited and require algal generated DOC. I tested this hypothesis in experiments run in Kentucky and Michigan utilizing nutrient diffusing substrates and in a mesocosm experiment. Results indicate that bacteria in streams can be carbon limited and grow independent of algae if an external carbon source is provided. Second, the nutrient competition hypothesis predicts that if bacteria are given an external source of labile DOC under low inorganic nutrient conditions, they will limit algal accrual through nutrient competition. I did not find evidence of competition when carbon and inorganic nutrients were manipulated using nutrient diffusing substrates, which was attributed to low water temperatures. However, when temperature was included as a variable in a mesocosm experiment, peak algal biomass was likely suppressed through nutrient competition with bacteria at high temperatures. The results of these experiements indicate that water temperature and allochthonous labile DOC may both be important in determining if the interaction between algae and bacteria is a trophic link or competition. These factors may ultimately contribute to the relative importance of autotrophic versus heterotrophic pathways in stream ecosystems.