The reports of cyanobacterial blooms and their impact on ecosystems and human health have increased in the last two decades, becoming of emerging concern for the World Health Organization (WHO). Most of these blooms are non-specific, although a few species showed more dominance, frequently, toxin-producing strains of Microcystis. Likewise, in Cuba, some cyanobacterial water blooms have been published, mainly focused on species composition and abundance, but studies do not approach both morphological and molecular analyses. Herein, we performed the characterization of an unprecedented mixed bloom in the Abreus reservoir using morphological features and molecular biomarkers. We detected high concentrations of some species of cyanobacteria, in addition to some groups of phytoplankton. Our results revealed Microcystis sp. as dominant species, followed by Sphaerospermopsis torques-reginae, being confirmed through molecular biomarker analyses, the presence of the 16S rRNA gene and Microcystis-specific 16S rRNA gene. Besides, the screening on cyanotoxin genes, revealed the gene mycE, which is involved in the biosynthesis of microcystins. This study constitutes one the few records of non-specific Harmful Algal Blooms (HABs) in Cuba, based on both morphological and molecular level. Although this study shows unpublished data, we consider this work as an extended version of the article “Valle-Pombrol A., et al., Planktonic cyanobacteria from the Abreus Reservoir, Cienfuegos, Cuba. Pan-American Journal of Aquatic Sciences (2021), 16(1): 20-29; http://panamjas.org/pdf_artigos/PANAMJAS_16(1)_20-29.pdf”. Indeed, our findings reinforce the importance of monitoring program of cyanobacteria in reservoir waters used for agricultural activities, animals, and human consumption.