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Lake Atitlán is a deep, clear lake in the heart of Guatemala's volcanic region. Last fall and this fall, it has been overwhelmed by blooms of Lyngba hieronymusii, a species of microscopic cyanobacteria, for the first time in its history. The blooms are prompted by erosion of phosphorus-rich volcanic soil into the lake as well as increasing pollution from nearby communities. Biologists are now testing the blooms for toxicity to better gauge the impact of this little-understood cyanobacterial species, which has rarely bloomed in other lake environments.