The excessive growth of harmful cyanobacteria, including Dolichospermum (formerly known as Anabaena), in freshwater bodies has become a pressing global concern. However, detailed information about the role of Dolichospermum in shaping bloom dynamics and producing cyanotoxins is limited. In this study, a bloom event dominated by Dolichospermum spp. at Detroit Lake (Oregon, USA) was examined from 2019 to 2021. In 2019, early summer cyanobacterial community succession reached up to 8.7 % of total phytoplankton abundance. Dolichospermum was the major microcystin (MC)-producing genus, with peak MC levels of 7.34 μg L⁻¹. The presence of MCs was strongly correlated with the abundance of Dolichospermum (r = 0.84, p < 0.05) and MC synthetase gene, mcyE-Ana (r = 0.63, p < 0.05). Metabolic analyses further showed that the presence of nif/pst genes linked to nitrogen and phosphorus metabolism was dominated by Dolichospermum from the bloom onset until September. In addition, the abundance of Dolichospermum was significantly correlated with the abundance of nitrogen-fixing nif-Ana gene (r = 0.62, p < 0.05). As the lake experienced a longer N and P scarcity period (May to September), the N₂-fixing Dolichospermum was able to dominate over other non-fixing cyanobacteria present, including Microcystis and Planktothrix. Overall, our results facilitate a better understanding of the organism and will help working toward managing/predicting future blooms.