Mass developments of cyanobacteria, so-called blue-green algae blooms, repeatedly threaten the quality of water bodies and drinking water resources worldwide. Cyanobacteria are considered to be heat-loving, and massive algal blooms are reported mainly in summer, when monitoring is particularly close. Now, an international research network involving IGB has shown that cyanobacterial blooms can also occur at colder temperatures - even under ice. If the algal blooms go unnoticed, there are risks to drinking water production.