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Abstract: The toxigenic marine flagellate Prymnesium parvum was first recorded in Lake Texoma, OK-TX, USA, an impoundment of the Red and Washita Rivers, following a massive fish kill in January 2004. Results of a 4.5-year monitoring program, spanning five bloom periods, revealed that Prymnesium abundances in the lake were temporally and spatially variable—densities were higher in winter, near-shore, and in Red River-associated sampling sites; the largest blooms were in Lebanon Pool, a large backwater basin often disconnected from the main reservoir. Prymnesium blooms appeared to have been fueled by high nutrient concentrations, and winter-spring densities were positively correlated with chlorophyll a, conductivity, total phosphorus, total nitrogen, and microzooplankton biomass, and negatively correlated with molar total nitrogen:total phosphorus and cladoceran and total crustacean zooplankton biomass. Comparison of Prymnesium densities with hydrological data suggested that Prymnesium blooms in Lebanon Pool were highest when the pool was disconnected from the main reservoir; no bloom occurred in the winter of 2004–2005, the only year since the 2003–2004 invasion in which Lebanon Pool and Lake Texoma were connected during the winter months. [ABSTRACT FROM AUTHOR]