Cyanobacteria (Blue-Green Algae)

Lake Ecology Introduction: Cyanobacteria

Download a pdf of our cyanobacteria FAQs.

What is cyanobacteria (or blue-green algae)?

These microscopic organisms are not actually algae but belong to an ancient group of bacteria called cyanobacteria that have been on earth for at least 2.1 billion years.

Scientists have described 2,700 species of cyanobacteria, though the total number is probably thousands more. It is no surprise that cyanobacteria have evolved to survive in almost every aquatic environment, though they all share some similarities.

Cyanobacteria may live as single cells or colonies that create filaments, spheres or irregular globs. The distinctive blue-green color comes from a pigment used to capture sunlight called ‘phycocyanin,’ although cyanobacteria can be many colors (see “I see something green in the water. Is it a cyanobacteria bloom?” below).

Blue green algae on Yahara River

Blue-green algae bloom on the Yahara River near the Tenney locks, June 2017

Why is cyanobacteria a problem?

When blue-green algae grows too quickly it can deplete oxygen in the water, cause fish die-offs, close beaches and even release toxins.

1. Cyanobacteria can deplete oxygen

Cyanobacteria may grow so fast (often called a “bloom”), that they turn entire shorelines green or rise to the surface in huge, stinking mats. Blooms of cyanobacteria can become so thick that they block sunlight at the surface of the lake. Without sunlight, photosynthesis stops, which means no oxygen is released into the water. Dead bacteria sinks and decays, using up more oxygen in the process. Severe blooms can completely deplete oxygen and cause dramatic fish kills.

2. Cyanobacteria may release toxins

True algae can grow problematically in our nutrient-rich lakes, but cyanobacteria blooms are of special concern because some species can produce potent toxins. It is impossible to tell whether a bloom is producing toxins without specialized tests, so it is best to stay out of the water when you see a bloom.

June 2017 blue-green algae bloom

Fish, insects and even a few ducklings died in a large blue-green algae bloom in June 2017. Photo by Tyler Tunney via Center for Limnology.

How do blooms form?

On the Yahara Lakes, cyanobacteria blooms can appear and disappear in a matter of hours, or persist for days. Often, the worst blooms will form on really warm, still days. Blooms can be pushed across the lake by winds or concentrated in bays by currents. Tracking blooms is a key objective of lakeforecast.org and our network of volunteers across all five lakes.

You can read more about the large cyanobacteria bloom that formed in June of 2017 on the UW-Madison Center for Limnology blog. 

I see something green in the lake! Is it a cyanobacteria bloom?

It could be!

First, take a closer look at the water, and make sure you are not looking at uprooted plants. Algae and bacteria lack differentiated tissues, like leaves, stems, and roots.

Next, note the color. Cyanobacteria and normal green algae may both appear bright green (or brown when decaying), but only cyanobacteria can showcase brilliant hues of blue and white. This is caused by cells rupturing and releasing pigments as they are damaged by the sun. You can also look for a paint-like sheen or pea-soup appearance, especially if the cyanobacterial bloom is still young and growing.

Another way to tell the difference between cyanobacteria and green algae is the “stick test”. If you reach a stick into the bloom and come up with a clump hanging off the end, you are likely looking at a green algae bloom.

Cyanobacteria grow as single cells or colonies, and most species will not cling together enough to hang off the end of a stick or paddle. Many species of green algae, however, grow in long interconnected filaments and can look like green hair or spaghetti.

How can I enjoy the lakes and stay safe from cyanobacteria?

Fortunately, it is easy to avoid potentially toxic blooms of blue-green algae, because the most dangerous blooms also make the water looks unappealing. The risk from a bloom is proportional to the number of cells in the water. So, if the water looks clear, your risk is very low.

If you see green specks floating in the water column but you can still see the bottom in two to three feet of water, cyanobacteria may be present, but not in bloom concentrations. At this point, sensitive individuals may want to find another place to swim.

Large surface scums should always be avoided, as ruptured cells may release their toxins all at once. In general, you should avoid swimming in any water that appears murky.

Know your risk level – keep kids and pets safe

Individuals with allergies or sensitive immune systems may react more strongly to low concentrations of toxins. Children and pets are at greater risk because they tend to ingest more water when swimming and playing. Dogs can also ingest toxins when they try to clean themselves after swimming in a bloom. Always provide your pets with clean drinking water, so they are less likely to drink lake water.

Find a safe, clear beach

Even during the worst blooms, other public beaches may remain clear and safe to swim. To find them, check the conditions at your local beaches using lakeforecast.org, and choose a beach with clear water.

What are cyanobacteria doing in our lakes?

Cyanobacteria have probably been present in Madison’s lakes since they formed, but large blooms have only become a problem in the last hundred years due to human impacts. We have made substantial changes to the land surrounding the lakes and the quality of the water flowing into them, which have tended to make things better for cyanobacteria.

Several factors give blue-green algae an advantage.

  1. Cyanobacteria like it hot.
    In general, cyanobacteria grow optimally under slightly higher temperatures than true algae. The risk of a bloom increases greatly after several days of high temperatures, meaning that we will see more blooms as climate change drives average summer temperatures higher.
  2. Cyanobacteria love to eat plant food.
    Human activities have increased the amount of nitrogen and phosphorus entering our lakes. Lake weeds, algae, and cyanobacteria all need these elements for growth, but cyanobacteria have a special advantage here as well. Amazingly, cyanobacteria can actually take up nitrogen from the atmosphere. Not many bacteria can do this, so cyanobacteria have a competitive advantage when phosphorus is available in the water but nitrogen is scarce.
  3. Cyanobacteria can float up and down in water.
    Cyanobacteria need both energy to power their cells and nutrients to build special molecules. In deep lakes like Mendota and Monona, cyanobacteria can sink down to take up nutrients and float up to gather energy, allowing them to grow faster than competing algae and other microorganisms. They do this by filling tiny chambers with gas, much like a submarine.

How do we stop the blooms? What can I do?

The Yahara lakes will probably never be totally free of blooms, but Clean Lakes Alliance and other community partners are working to reduce the amount of phosphorus entering the lakes. Over time, with less of this food available, blooms will become smaller and less frequent. Find out what you can do to help.