Cyanobacteria (Blue-Green Algae)

What are Cyanobacteria?

Cyanobacteria, formerly known as blue-green algae, are photosynthetic microscopic organisms that are technically bacteria. They were originally called blue-green algae because dense growths often turn the water green, blue-green or brownish-green. These algae are found in all lakes and are a natural part of the lake ecosystem. Unfortunately, high nutrient concentrations can promote a population explosion of these organisms and result in algal blooms, especially during warm weather.

What are Cyanotoxins?

There have been reports from around the globe of cyanobacteria producing toxins in fresh waters. In 2014, cyanotoxin concentrations so high in Lake Erie, the water source for Toledo, Ohio, for the water utility to issue a ‘Do Not Drink/Do Not Boil’ warning that lasted three days.  These toxins can damage the liver or nervous system, produce gastrointestinal symptoms, or cause skin irritation. The World Health Organization established levels of concern for some algal toxins in 1998; the Environmental Protection Agency established similar guidelines for drinking water in 2015 and in early 2017 released draft guidelines for recreation. Although a few states have established a mechanism to issue advisories, many are awaiting final EPA guidelines before moving forward. Maine DEP has been measuring cyanotoxin concentrations since 2008 to evaluate the range of conditions in Maine lakes. Fortunately most Maine lakes do not produce cyanotoxins at concentrations that are of concern, however some lakes that experience annual blooms have been found to produce cyanotoxins.

What does a Bloom look like?

A bloom is any dense growth of algae that reduces transparency and discolors the water (neon green, pea green, blue green, reddish brown). Maine DEP tracks 'nuisance blooms' - blooms that reduce the transparency of the water to less than 2 meters, or about 6 ½ feet. Blooms not only turn the water murky, they can also cause: bad odors (musty or fishy smell), green or blue-green scums or streaks near-shore, and foam. Water may look like pea soup and/or look like someone dumped paint in the water. Algal material may accumulate on the water surface and create swirls when disturbed. Two other algal forms are often referred to as algal blooms. The first is the presence of bright green algae that has risen to the surface of the water and has been blown into shore; in this case the water nearby is quite clear. Although it is not safe to consume water with this algae present, it often dissipates within a day or two. The second is algae that looks like cotton candy attached to the bottom of the lake, sticks or rocks. This is known as metaphyton and although it is not pleasant to feel against one's body, it is in an entirely different group of algae than cyanobacteria and does not produce toxins.

Are all Blooms Toxic?

Not all blue-green algae or algal blooms are toxic. The reasons algae produce toxins at any given time are not well understood. Standard monitoring techniques cannot predict when a bloom has toxins in it. While heavy growths of blue-green algae often show detectable levels of toxins, only the most intense blooms like those above create a potential for significant toxin exposure for humans and animals. By far the most common and best known toxins are the various forms of microcystins.

How Toxic are these Compounds?

Toxic reactions depend on a number of factors, including the degree of exposure (how much water is swallowed, how long the water is in contact, etc.), the concentration of toxins in the water, and, the sensitivity of the individual. Toxins are not readily absorbed through the skin and it is not clear if health problems can arise from inhaling water droplets with toxins. Though small amounts can cause mild reactions in sensitive individuals, significant human illness has been only rarely reported. However, severe reactions and death of pets or livestock drinking contaminated water have been reported from many locations outside of Maine.

Are these Toxins found in Maine Lakes?

Since 2008, DEP has been measuring concentrations of microcystins in lakes that regularly support algal blooms and in lakes considered to free of blooms. By sampling both types of lakes, biologists are confident that we are establishing a dataset that will characterize all Maine lakes and provide insight on how toxin concentrations compare to EPA’s guidelines.  In addition, these data will form the basis for how the State of Maine’s Department of Health and Human Services, Center for Disease Control will create an advisory specific to our lakes.

Preliminary results indicate that most Maine lakes that do not produce algal blooms are safe to drink (after disinfection) and recreate in.  In contrast, lakes that regularly produce algal blooms are most likely to produce toxins, the highest concentrations of which are found in the scums that accumulate along the shoreline; these scums may be 100-1000 times the levels of concern issued by EPA. Swimming areas and deep waters often produce toxin concentrations that exceed EPA guidelines, but in the range of 1-10 times EPA’s guidelines.  Results also indicate that toxins are produced later in the bloom period, when cell numbers are most dense and cells are beginning to die. 

How can I Avoid Problems?

While most adults will avoid green discolored water, a hot day can lure children and pets into the water. The following list of recommendations will help you minimize risk to yourself, your family and pets.

  • Do not accidentally ingest or drink lake water during a bloom. Well maintained domestic water treatment systems may make lake water safe to drink by removing bacteria and parasites, but they are not guaranteed to remove algal toxins.
  • If you shower with lake water, keep showers brief because breathing toxins in shower mist could cause health issues.
  • Do not swim, water ski, or boat in areas where algae are visible (e.g., pea soup, floating mats, scum layers, etc.), where water is discolored, or where musty odors are present.
    Rules of thumb: if you are standing in water chest deep (4-5 feet ) and you can’t see your toes because the water is so green, you should get out; if you are looking into water that is 4-5 feet deep and can’t see the bottom of the lake because the water is so green, you should not to go in. 
  • Because algal scums along the shoreline have the highest concentrations of toxins, do not let children play in water that is discolored, where you see mats of algal material, foam, or where musty odors are present. Do not allow pets or livestock swim or drink water from these areas.
  • Rinse off with fresh water and soap if available, as soon as practical if exposed to water that has dense algae present. This will reduce skin exposure for humans and pets.

Can you tell me about a specific lake?

Lakes at risk of having an algal bloom. Any lake can have an algal bloom. Fortunately the majority of Maine lakes are not known to support blooms. Lakes that may be at risk of having an algal bloom are shown on this map and also listed in a table format. The frequency of annual blooms and the risk of future blooms is included.

Who do I call with questions?

If you want to report a bloom, contact the DEP Lakes Staff at 207-287-7688 or visit the DEP Report a Bloom page. For more information on blooms visit the DEP Algal Blooms in Maine Lakes page. For more information on cyanobacteria, visit the U.S. EPA cyanobacteria/cyanotoxin web page.

I have done a jar test on a suspected bloom, what do I do next?

Here are links to the Jar Test directions and the Jar Test report form. Please email your results to