Unexpected cyanobacterial communities in highly heterogeneous toxic blooms from a Mediterranean protected area

The negative effects associated with cyanobacterial blooms are of particular concern in protected ecosystems, as these areas are ecologically significant and attract a high number of visitors. This study aims to explore the cyanobacterial communities and associated toxicity in three reservoirs located within a Mediterranean National Park with a compromised situation at basin-level. Our results demonstrate the occurrence of dense toxic blooms containing microcystins (reaching values close to 280 μg L^-1) and low levels of anatoxin-a and saxitoxins (up to 0.02 μg L^-1). Comprehensive metabarcoding analyses based on cyanobacterial 16S rRNA and cyanotoxin-biosynthesis genes (mcyE, anaF and sxtA) unveiled three highly heterogeneous communities, despite the spatial proximity of reservoirs. Additionally, our results suggested the influence of water conductivity on the blooms composition. Among the diverse bloom-forming taxa found, Microcystis sp. and Planktothrix sp. were revealed as the microcystins-producer candidates, and Cuspidothrix issatschenkoi and Dolichospermum/Aphanizomenom sp. as the potential producers of anatoxin-a and saxitoxins, respectively. A polyphasic characterization confirmed the first report of the tropical-related species Planktothrix spiroides in Europe, showing elevated level of dominance. As a whole, we present the scenario of an ecologically important protected area facing significant challenges in the proper management of cyanobacterial blooms.