ABSTRACT The increasing demand for freshwater, in combination with climate change and pollution, compromises the quality and quantity of water resources. These pressures have intensified cyanobacterial blooms, including toxic events that threaten human health and aquatic ecosystems. This study complements the objectives of the Water Framework Directive by addressing the current lack of systematic data on toxic cyanobacteria and cyanotoxins. Therefore, the occurrence of toxic cyanobacteria in Mediterranean reservoirs was revisited, and the findings were complemented by case studies of two Portuguese reservoirs. Phytoplankton community composition and toxin concentrations were analysed, and cyanobacterial strains were screened for toxin‐producing genes. The data gathered from literature revealed widespread cyanobacterial blooms across Spain, Portugal and Italy, with cell densities frequently exceeding the WHO medium‐risk threshold (20,000 cells/mL) and occasionally reaching the very high‐risk level (1000,000 cells/mL). Microcystins were the most frequently detected toxins, often occurring above the WHO guideline (1 μg/L), whereas saxitoxin, anatoxin and cylindrospermopsin were detected less frequently. In Portugal, the Aguieira reservoir exhibited recurrent blooms and high phytoplankton biomass under eutrophic conditions, while the Rabagão reservoir showed no relevant cyanobacterial proliferation. Molecular analyses confirmed the presence of the mcyA gene in Microcystis aeruginosa isolated from Aguieira, indicating its potential for microcystin production, whereas Aphanizomenon flos‐aquae lacked detectable toxin biosynthesis genes. Overall, these findings highlight the ecological variability of cyanobacteria in Mediterranean reservoirs and emphasize the importance of integrating toxin monitoring and molecular tools into water quality assessments to strengthen management strategies and safeguard public health.
Pinto et al. (Thu,) studied this question.