Intense Cyanobacterial Blooms in Brealito: An Unprecedented Case in a High‐Altitude Lake of Argentina

Cyanobacterial blooms represent one of the major threats to continental aquatic ecosystems. Reports of blooms occurring in unexpected regions provide valuable information for understanding the true extent of the problem and its underlying causes. This study involved sampling in Brealito lake (Salta, Argentina), located at over 2500 m.a.s.l., and reports, for the first time, the phytoplankton composition of the site. Sampling campaigns were conducted in February 2022, February 2023 and March 2025. Chemical and physical parameters, as well as the micro‐ and nanophytoplankton structure, were analyzed. A total of 14 phytoplanktonic species were identified, with Aphanizomenon gracile and Planktolyngbya limnetica dominating and forming very intense blooms. This lake is difficult to access and remains relatively undisturbed, only used for recreation without a clear pollution factor. The blooms, dominated by Aphanizomenon gracile and Planktolyngbya limnetica , appear to be sustained by a combination of historical fish introduction, decline in water level and cyanobacterial UV‐protective adaptations, rather than by conventional eutrophication drivers. Reports of cyanobacterial blooms in high‐altitude lakes are scarce in South America, and this case represents the highest‐altitude bloom reported to date in Argentina.