Cyanobacterial blooms pose a serious threat to the water quality of freshwater lakes because of their scums, toxins, and odors. Synergistic interactions between eutrophication and climate change may be causing cyanobacterial blooms to increase worldwide, which will have substantial consequences for aquatic food webs and nutrient concentrations in lakes. In particular, the trophic state of a lake may be an important determinant of how blooms affect ecosystem functioning. In this dissertation, I used a combination of literature reviews, field surveys, field experiments, and laboratory experiments to examine the causes and effects of cyanobacterial blooms in both oligotrophic and eutrophic freshwater lakes. My research shows that increased nutrients are an important driver of the global increase in cyanobacterial blooms, and future climatic and hydrological conditions may interact to favor cyanobacterial dominance. My experiments with G. echinulata, a cyanobacterium that is increasing in the northeastern United States, demonstrate that high densities of G. echinulata can increase nitrogen and phosphorus concentrations in low nutrient lakes. In these systems, G. echinulata can play an important role structuring food webs by increasing the biomass of small-sized phytoplankton. G. echinulata's interactions with phytoplankton are fairly complex, iii however, as I observed that its positive effects were mediated by both trophic interactions (zooplankton biomass) and trophic state (nutrient concentrations), highlighting the context-dependency of the effect of this species on other plankton. I found that nutrients play an important role mediating G. echinulata's effects on phytoplankton: first, increasing nutrients in the water column may be the mechanism by which G. echinulata stimulate other phytoplankton in oligotrophic systems, and second, nutrient concentrations may alter the direction of G. echinulata's effect (i.e., stimulatory or inhibitory) on other plankton. Finally, my data demonstrate that G. echinulata blooms in oligotrophic lakes may have important consequences for water quality. As cyanobacteria continue to increase, it is vitally important to understand how both oligotrophic and eutrophic systems will respond. iv