Blue-green algae and harmful algal blooms

Summertime in Minnesota: When in doubt, best keep out!

When temperatures climb and the summer sun beats down, conditions are ripe for Minnesota lakes to produce harmful algae blooms, some of which can be harmful to pets and humans.

Green-gray water with patches of scaly blue-gray and tan scum

What are blue-green algae?

Blue-green algae are not algae at all, but types of bacteria called cyanobacteria that are normally present in many lakes. This type of bacteria thrives in warm, nutrient-rich water. When conditions are right, the bacteria can grow quickly forming “blooms.”  

What do blue-green algal blooms look like? 

Lake with thick blue-green algal bloom Blue-green algal blooms are often described as looking like pea soup or spilled green paint. However, blooms aren’t always large and dense and can sometimes cover small portions of the lake with little visible algae present. Blooms can also produce a swampy odor when the cells break down. Here are some examples of algae blooms.

A couple of easy tests can tell you if the green stuff you're seeing in your body of water is likely to be blue-green algae:

What are harmful algal blooms?

When blue-green algal blooms produce cyanotoxins (toxins produced by cyanobacteria) that can make humans and animals sick, they are considered harmful. In general, algae are not harmful.    

When do harmful algal blooms occur?

Blue-green algae prefer warm, calm, sunny weather and water temperatures higher than 75 °F. Blooms usually occur during summer and early fall, but can occur other times of the year, if conditions are right. 

Where are harmful algal blooms found?

Harmful algae can be found everywhere in Minnesota, but thrive in warm, shallow, nutrient-rich lakes. They will often be found on the downwind side of a lake or in a secluded bay or shoreline.

What are the possible health effects?

Harmful algae on a shore in Alexandria

You can become sick if you swallow, have skin contact with, or breathe in airborne water droplets while swimming, boating, waterskiing, tubing, bathing, or showering in water that has harmful algae or if you drink water that contains algal toxins. If you become sick, you might experience vomiting, diarrhea, rash, eye irritation, cough, sore throat, and headache. Symptoms generally begin hours to two days after exposure.

What should I do if I see blue-green algae in my drinking water source?

Avoid using untreated lake or river water for drinking, cooking, and brushing teeth, particularly for infants and small children. Boiling water will not destroy toxins and could actually increase toxin levels. Simple treatment options are also not effective, because multiple treatment steps are typically required to remove algal toxins. 

Water that may be contaminated can be used for handwashing, bathing, washing dishes, or laundry, though it may irritate skin. Young children should be supervised when bathing to prevent them from swallowing water. After washing, skin and items that go into the mouths of infants and young children (i.e., teething rings, nipples, bottles, toys, and silverware) should be rinsed with uncontaminated water.

Can animals be affected?

Pets, especially dogs, are susceptible to harmful algae because they swallow more water while swimming and doing activities like retrieving a ball from the water. They are also less deterred by green, smelly water that may contain harmful algae. Animals can experience symptoms within minutes of exposure to the toxins. Symptoms they might experience include vomiting, diarrhea, weakness, difficulty breathing, and seizures. In the worst cases, animals have died. If your pet experiences these symptoms after exposure to algae, contact your veterinarian. 

What should I do if I see a bloom?

There is no way to tell if a blue-green algal bloom is toxic just by looking at it. Adults, children, and animals should avoid contact with water with blue-green algae.  Toxins can persist in the water after a bloom; watch for signs of recent blooms, such as green scum on the shoreline. When in doubt, stay out! If you or your pet go into water where there may be a bloom, wash off with fresh water immediately afterwards.

How can we get rid of harmful algae blooms?

We can't eliminate blue-green algae from a lake -- they are an inherent part of the overall algal community. What we really want to do is control their overall intensity and the frequency of the blooms. Since we can't control the water temperature, the best thing we can do is to reduce the amount of nutrients getting into the lake. This can best be accomplished by reducing the amount of phosphorus and nitrogen from man-made sources such as lawn fertilizer, and runoff from cities, cultivated fields, feedlots, and a myriad of other sources. Though a reduction of nuisance algal blooms will not be immediate, it is the best long-term solution to minimizing the frequency and intensity of algal blooms.


Photos of non-toxic plants and algae

Chara, a form of filamentous algae; often found in lakes with good water clarity.

Chara, a form of filamentous algae often found in lakes with good water clarity

Duckweed a non toxic aquatic plant

Duckweed, a non-toxic aquatic plant often mistaken for algae

Nuisance growth of a filamentous green algae

Filamentous green algae, a non-toxic form of algae that can create recreational nuisances