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Cyanobacteria Blooms

Updated: Aug 31, 2023

Sometimes lakes in Wyoming can be closed to recreation due to what some people call Toxic Algal Blooms. Although these toxic blooms are actually caused by cyanobacteria, not algae, we thought you might want to know more about them.

Cyanobacteria are one of the most important groups of bacteria on earth. Having been around for at least 3.5 billion years, not only are they the oldest known fossils, they are also thought to be the first organisms to have produced oxygen on earth. Cyanobacteria are photosynthetic, which means they make their own energy from sunlight and release oxygen into the atmosphere. Although microscopic and usually unicellular, these bacteria can form colonies large enough to be seen by the naked eye. They are often called ‘blue-green algae’ because of their color but have no relation to algae. Cyanobacteria can be found in aquatic and terrestrial environments as well as in symbiotic relationships with plants, fungi, and animals.

Christian Fischer, Wikimedia Commons

Although cyanobacteria are found naturally in aquatic environments, an overgrowth of cyanobacteria, also known as a ‘cyanobacteria bloom’, can occur when an excess of nutrients from fertilizer run-off or septic overflows end up in bodies of warm, stagnant water. While not all cyanobacteria blooms are harmful some can be very toxic to humans, animals, and the environment. Blooms can show up as blue-green, red, or brown scum floating on the surface of the water. They can occur at any time of the year but are most likely to happen during the summer or early fall.

A cyanobacteria bloom is harmful to the environment when it becomes too dense, uses up all the oxygen in the water, produces toxins, and releases harmful gases. Humans are most likely to be exposed to cyanobacterial toxins through ingestion, inhalation, and skin/eye contact during recreation.

It is recommended that humans and pets have no contact with the water when blooms are present because some cyanotoxins can move through the air. The Wyoming Department of Environmental Quality (WDEQ) and Wyoming Department of Health (WDH) work to keep an updated map to inform the public of health risks where cyanobacterial blooms occur, noting that some affected waterbodies may not be discovered or reported yet. Bloom advisories, toxin advisory, and waterbodies under investigation are all categories listed on these maps:

The Wyoming Department of Health also has a resource page about harmful algal bloom (HAB) associated illness, signs and symptoms, treatments, and prevention measures. You can access this page by visiting:

Education is the key to preventing harmful cyanobacteria blooms. Homeowners and landowners should use only the recommended amounts of fertilizers on lawns, gardens, and fields to limit excess run-off. Homeowners should keep septic systems properly maintained. Those who own land at the edges of stream, rivers, and lakes can maintain a natural vegetation buffer at the edge of waterbodies to filter incoming water.


Infectious Disease Epidemiology Unit. (n.d.). Harmful Algal Blooms (Cyanobacteria). Wyoming Department of Health.

UC Museum of Paleontology. (n.d.). Introduction to the Cyanobacteria. U.C. Berkeley.

Wikipedia. (n.d.). Cyanobacteria. Wikipedia.

Wyoming DEQ. (n.d.). Wyoming Department of Environmental Quality. Harmful Cyanobacterial Blooms. Wyoming Department of Environmental Quality. (n.d.). Harmful Cyanobacterial Bloom (HCB) Advisories in Wyoming Waters. ESRI.

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