Part 3 – What are Blue-Green Algae?

Blue-greens are very primitive organisms that are not really algae. They photosynthesize like algae, but they are actually bacteria. Scientists refer to them as “cyanobacteria” to acknowledge that they are bacteria. “Cyan” means “blue”, which refers to the fact that these organisms often appear blue-green in color. However, they can also be red, purple, blue, brown, and yellow in various shades.

The blue-greens split into two major groups: the planktonics and the mat-formers. The planktonic blue-greens are microscopic and cause the typical pea-soup green color to water. When they rise to the surface of calm or still water, they form a surface scum that tends to block the light out for other algae and aquatic plants. By shading out their competitors, blue-greens can completely dominate a body of water.

The mat-forming blue-greens form dark green or black slimy mats. These mats start growth on the bottom but eventually float to the surface where they can be quite smelly and noxious looking.

Like green algae, cyanobacteria growth is determined by conditions like adequate nutrients (mostly phosphorus but also nitrogen), light levels, pH, temperature, etc. Usually, the amount of phosphorus available controls the amount of cyanobacteria found in a freshwater lake or water body.

A dense growth of algae or cyanobacteria is called a “bloom”. Most blooms are harmless, but when the blooming organisms contain toxins, other noxious chemicals, pathogens, or other impacts to recreation or economic activities, it is known as a harmful algal bloom (HAB). Some blooms are not visible.

This series about algae in our waters was written by Lake Specialist Reesa Evans of the Adams County Land & Water Conservation Department. She is also a lake manager certified by the North American Lake Management Society.