Cyanobacteria are also known as blue-green algae. They're a type of bacteria that occur naturally in fresh and marine bodies of water. There are many types of cyanobacteria, and some can produce harmful toxins called cyanotoxins.
Cyanobacteria can grow quickly in freshwater lakes and rivers under favourable conditions that can include:
slow moving waters
higher water temperatures
an increase of nutrients in the water, such as nitrogen and phosphorus
These conditions can sometimes lead to large cyanobacteria blooms. When a bloom containing toxin-producing cyanobacteria develops, toxins can reach hazardous levels.
Marine waters generally don't have problems with cyanobacteria blooms.