Summer
Smaller, fast growing algae dominated in the cool waters of spring. As the water warms up, larger species of algae begin to play a more important role in the lake’s ecology. Some types of algae grow in mats or filaments, thus avoiding the pressure of grazing. Among these algae are the so-called “blue green algae.”

The Blue Green Algae
The blue green algae are not actually algae; they belong to a group of bacteria, called cyanobacteria, that can photosynthesize like plants. However, we have called them blue green algae for so long, the name just stuck. Blue greens are very primitive organisms, but can do some interesting things.

Certain blue green algae can take nitrogen gas out of the air and make it into a usable nutrient. This is VERY useful in lakes with abundant phosphorus. While other algae are struggling to survive due to a lack of nitrogen, the blue greens can make their own. Some land plants, such as clover and soybeans can also “fix” nitrogen, thanks to the bacteria that live in (i.e. infect) their roots.

Many types of blue green algae can control their position in the water column. They do this by filling a portion of their cells with gas to float. As the cells photosynthesize, they gain weight and sink again. In this way they can move vertically at a rate up to 10 feet per hour. That’s pretty fast for such a small organism, but it’s not fast enough to overcome even a small wind. As a result, the floating scum of blue green algae is most often seen on very still, sunny days. On windy days, they are mixed throughout the surface layer.

The blue green algae are not generally edible or desirable to zooplankton, and so are controlled by other factors. For example, the amount of blue green algae in a lake relative to other types of algae increases as phosphorus increases. So, if you have a lot of phosphorus in a lake, not only will you have more algae, you will have more blue green algae.