From prolonged drought to interbasin water transfer: evidence of increased cyanobacterial biomass and cyanotoxins in a tropical semiarid reservoir

The global expansion of interbasin water transfer (IBWT) projects has raised growing concerns regarding their environmental impacts on receiving aquatic ecosystems, particularly in water-limited semiarid regions. Using a long-term, high-frequency time series, we investigated cyanobacterial and cyanotoxin dynamics in a reservoir receiving water from the São Francisco River Integration Project (Brazil). Following the onset of water transfer, cyanobacterial biomass increased markedly, WHO Alert Level 2 events (biomass > 4 mg L⁻¹) became more frequent, species dominance shifted, and concentrations of microcystins, saxitoxins, and cylindrospermopsins rose. Eight cyanobacterial species were recorded exclusively after the transfer, including Dolichospermum circinale , a potentially toxic taxon that formed dense surface accumulations. These changes were associated with increased nutrient availability (particularly total phosphorus and nitrate), higher turbidity, and variations in reservoir water volume. The transfer began during a prolonged drought and overlapped with the subsequent resumption of seasonal rainfall, indicating that observed shifts reflect the combined effects of IBWT as an initial disturbance and hydrological recovery that amplified bloom development. Our findings highlight the importance of interpreting IBWT impacts within their hydroclimatic context and reinforce the need for adaptive management and continuous monitoring to mitigate toxicological risks in vulnerable semiarid systems.