Despite the name “Harmful Algal Blooms” or “Blue-Green Algae”, these cyanobacteria blooms are not really algae. They are bacteria, a more ancient type of organism. Among the first life forms on earth, cyanobacteria emerged some 3.5 billion years ago, at a time when the earth’s atmosphere contained virtually no oxygen. As the first photosynthetic organisms and the ancestors of modern plants, cyanobacteria produced oxygen as a waste product that accumulated in the atmosphere. Over the course of some 2.5 billion years, cyanobacteria made it possible for life forms that depend on oxygen, including eventually humans, to evolve. Cyanobacteria are a diverse group of organisms with a range of morphologies and adaptations. Some genera can fix nitrogen, others have gas vacuoles that enable them to move up and down in the water column to seek out favorable conditions. Many species of cyanobacteria produce chemical compounds whose purposes are not fully understood. A few of these natural compounds, cyanotoxins, are toxic to humans and other animals. Their presence is what makes a cyanobacteria bloom harmful.

Under the right conditions, freshwater cyanobacteria experience explosive growth, dividing rapidly and increasing their population until they become a visible bloom. Three common groups of bloom-forming cyanobacteria in Cayuga Lake are Microcystis, Dolichospermum, and Aphanizomenon. While these groups have distinctive features that can be recognized under a microscope, the blooms they form are indistinguishable to the naked eye. Additionally, there is no visual indicator of toxins that may be present in a bloom. Cyanobacteria blooms may look like parallel streaks or green clumps on the water, or they may have a spilled paint or pea soup appearance (see below).