The nuisance raphidophyte Gonyostomum semen (Ehrenberg) Diesing blooms in lakes and is known to produce a mucilage which can cause human skin irritation. Parameters such as water temperature, iron and high dissolved organic matter loads are shown to be important drivers in temperate regions. However, the causes of blooms in warmer latitudes are less well understood. Over a 6 mo study period, we used field monitoring and a nutrient addition experiment within a water reservoir to examine the role of nutrients in promoting G. semen growth. Early in the study, an inflow event delivered nutrients which increased dissolved inorganic nitrogen (DIN) concentrations 2-fold and filterable reactive phosphorus (FRP = phosphate) 4-fold. This event shifted the phytoplankton community from a mixed community to one dominated by G. semen. Two months after the inflow event, the effect of nutrients in promoting G. semen was confirmed with a nutrient addition experiment. Total biovolumes of this species across the study were strongly predicted by FRP and nitrate + nitrite concentrations. G. semen biovolumes also decreased as ratios of total nitrogen (TN):total phosphorus (TP) and DIN:FRP increased, highlighting the importance of P inputs. The stable isotope tracer 15N-nitrate was also used to trace N through the G. semen-dominated phytoplankton community. The tracer rapidly cycled through the G. semen-dominated phytoplankton community in 1-2 d, settled and remineralized, providing an ongoing source of DIN for maintaining blooms. Overall, the results highlight the importance of FRP, and to a lesser extent nitrate, in promoting blooms of this nuisance species.