Blue-green algae, correctly cyanobacteria, are among the oldest forms of life on earth. They are very common both in water and on land. Under very special conditions, they can spread en masse in lakes and can form mats and scum on the lake surface. One speaks then of a bloom. Because there are those among the several thousand species that can produce toxic substances, many questions arise. In response, Eawag has opened an FAQ page, which can be found here.