Published on: 2.12.2019

Information produced by Finnish Environment Institute

How do you recognize blue-green algae?

You can see little specks in water. Could it be blue-green algae, or some other algae species? Or something else altogether? How can you tell blue-green algae from pollen, for example?

Blue-green algae are actually bacteria, also known as cyanobacteria. Unlike many other bacteria, they can perform photosynthesis just like plants.

In reality, blue-green algae do not ‘flower’, and the phenomenon known as algal bloom refers to a mass occurrence of blue-green algae. In calm weather, the tiny algae cells rise to the surface and form clearly visible, greenish or yellowish mats. Algae pushed to shore by waves can form thick gloop.

Mass occurrences of blue-green algae are caused by high levels of nutrients in the water, especially phosphorus, and they also need warmth. This is why the blue-green algae blooms usually occur in July or August, whereas pollen is seen in water in spring and early summer.

To tell blue-green algae from other types, you can use a stick: if you can pull up the algae from water with the stick, it is not blue-green algae. Another method is collecting some water into a glass and letting it stand for an hour. Buoyed by their gas vacuoles, blue-green algae rise to the surface.

Image: © Heikki Ketola, Vastavalo