Cyanobacteria are known as blue-green algae because they are are aquatic and use sunlight to create food and support life.  However they are not algae.  Cyanobacteria are the oldest known fossils, more than 3.5 billion years old.  They usually are too small to be seen, but sometimes can form visible colonies (called algai blooms) in slow moving water that are rich in nutrients (e.g. fertilizer runoff or septic tank over flow). These blooms are often found in water where little mixing occurs, and can occur at any time, most often in late summer or early fall. Eutrophication and a thinning ozone layer can contribute.