Prymnesium parvum s.l. (sensu lato, in the broad sense) is a cryptic species complex of toxic, unicellular haptophyte algae responsible for recurrent fish kills and major ecological disruptions in freshwater and brackish systems worldwide. Although historically considered marine, P. parvum s.l. has increasingly expanded into inland freshwater environments. Even though the strains within this species complex are morphologically indistinguishable, recent genomic studies have revealed substantial cryptic diversity. Distinct genetic clades produce different prymnesin toxins (types A, B, or C), indicating that genetic divergence may be associated with functional and ecological variation. Previous studies have documented phenotypic differences among P. parvum strains, including variation in growth and toxicity. However, these differences have not been evaluated in a comprehensive, comparative framework across a broad spectrum of strains or between genetically divergent strains co-occurring within the same bloom. In this study, we investigate phenotypic variation among multiple P. parvum s.l. strains isolated from a single bloom, with a focus on differences in growth rates. By comparing growth performance across genetically distinct strains under controlled conditions, we assess whether phenotypic variation is structured primarily by clade identity rather than by strain-level variation. This approach provides insight into potential mechanisms that allow genetically distinct strains to coexist within the same bloom and how such differences may scale up to influence bloom dynamics, including bloom formation, persistence, and impact. By linking genotypes to measurable ecological traits, this work highlights the ecological relevance of cryptic diversity in harmful algal blooms and provides a framework for integrating genetic and phenotypic data to improve prediction, monitoring, and management of P. parvum s.l. blooms in inland waters.