The frequency and severity of freshwater cyanobacteria blooms are increasing in response to anthropogenic nutrient loading, posing significant risks to water quality and human health. Understanding how nitrogen (N) and phosphorus (P) stoichiometry and lake trophic status influence phytoplankton community composition can inform management strategies that reduce or prevent toxic cyanobacteria blooms. Large-scale ecosystem studies have indicated that nutrient-rich lakes with low N:P ratios are at risk for toxic cyanobacteria blooms, due to some genera’s ability to fix atmospheric N. In contrast, culture studies have indicated that lakes with higher N:P ratios may be at greater risk, with toxin concentrations increasing with lake total N. In this study, we examined how extreme N:P imbalances in hypereutrophic lakes influence phytoplankton community dynamics over a growing season. Using microscopy, we analyzed phytoplankton biomass and composition in 12 in-lake mesocosms that were open to the sediment and atmosphere. We manipulated the N concentrations relative to P to simulate eutrophic conditions representing four different molar ratios: 2.2 (low N), 16 (medium-low N), 55 (medium-high N), 110 (high N). The high N and medium-high N treatments had significantly more phytoplankton biomass and supported greater cyanobacteria abundance, including toxin-producing genera Cylindrospermopsis, Limnothrix, and Aphanizomenon, particularly in July and August. In contrast, low and medium-low N treatments experienced brief, small blooms of Microcystis and Cylindrospermopsis, however they were mainly dominated by chlorophytes, specifically Monoraphidium, Chlamydomonas, and Staurastrum. The unfertilized controls were dominated by diatoms and dinoflagellates, Cyclotella and Glenodinium, respectively. These results highlight the strong nutrient dependence not only of phytoplankton biomass, but also phytoplankton community composition and indicate that hypereutrophic lake ecosystems with high N:P stoichiometry are more susceptible to toxic cyanobacteria blooms.