How likely am I to drink water contaminated with cyanobacteria and its toxins?

All in all, this is very unlikely.

Relatively few incidents of human poisoning have been reported. This is because people usually won't drink water contaminated with cyanobacteria because of the scum and the accompanying smell (fresh blooms smell like newly mown grass; older blooms smell like rotting garbage).

What purpose do blue-green algae serve in the environment? Algae, including blue-green algae, are very important to the food chain. They are known as “primary producers ” – a name that is given to living organisms that can convert sunlight and inorganic chemicals into usable energy for other living organisms. Most algae are microscopic and serve as the main supply of “high energy” food for larger organisms like zooplankton which in turn are eaten by small fish, larger fish, mammals, raptors, and even people.

Blue-Green Algae (Cyanobacteria), also known as “pond scum,” is made up of cells that range in color from olive-green to red. These cells form in shallow, warm, slow-moving or still water and can house poisons called cyanobacterial. A mass of cyanobacteria in a body of water is called a bloom. When this mass rises to the surface of the water, it is known as surface scum or a surface water bloom.

Does the presence of blue-green algae always mean the water is contaminated? The short answer is “No” and “Yes.” Many blue-green algae do not produce toxins and while algae blooms may be unsightly, it does not always mean the water is contaminated with toxic substances. Simply seeing a bloom will not tell you whether or not algal toxins may be found in the water. The only way to be sure if the toxins are present is to have water samples analyzed in a laboratory using sophisticated equipment. These tests currently cost about $600 for each water sample. On the other hand, these blooms often reach a nuisance state forming ugly, smelly scum layers that look like peasoup or a green milk-shake if they have a sufficient supply of nutrients like phosphorus and nitrogen. While both phosphorus and nitrogen are natural and important elements for all life forms, human activity has clearly allowed more than necessary to enter our streams and lakes.

It is possible for people to unknowingly drink water containing cyanobacterial toxins released from blooms that have died naturally. Because all blue-green algae blooms are potentially toxic, it's always best to stay away from contaminated water until it has been tested and declared safe. Even after the bloom is gone, you should wait until health authorities declare the water safe before swimming in it. It can take more than three weeks for the toxins to clear from the water after being treated.If your water comes from a source that is prone to blue-green algal contamination (dugouts, for example), you should monitor the water for bloom formation. If you detect a bloom in your water supply, contact your local health authorities for advice.

Even though it is unlikely you could ever drink water contaminated with cyanobacteria, how will you know if you have accidentally come into contact with cyanobacterial toxins? If you ingest water, fish or blue-green algal products containing elevated levels of toxins, you may experience:

• Headaches
• Fever
• Diarrhea
• Abdominal pain
• Nausea
• Vomiting

If you swim in contaminated water, you may get itchy and irritated eyes and skin, as well as other hay feverlike allergic reactions. If you suspect you might have come into contact with blue-green algae, rinse any scum off your body and consult your physician right away