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Blue-green Algae and Harmful Algal Blooms

Who should I call?

If you believe you or your pets are experiencing adverse health effects due to contact with, or ingestion of, lake water/algae, you should seek medical attention immediately.

If experiencing health effects, contact a medical professional. In addition, people are encouraged to report human health effects to the Minnesota Department of Health (MDH) Foodborne Illness Hotline at 1-877-366-3455. For health questions, citizens can contact MDH's Acute Disease Investigation and Control group at 651-201-5414 or visit the MDH's Blue-green Algal Bloom web page.

MPCA lake monitoring staff track reports of potential HAB for assessment monitoring. If a lake is assessed and found to exceed nutrient water quality standards, an investigative study designed to determine the sources and extent of the pollution problem is conducted. For more information on harmful algal blooms, please contact Steve Heiskary, MPCA, at 651-757-2419, or toll-free from Greater Minnesota at 1-800-657-3864.

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

Photo of toxic algaeWhen 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.

What: Harmful algal blooms (HAB) are blue-green (cyanobacterial) algal blooms containing toxins or other noxious chemicals, which can pose harmful health risks.

Why is this a concern? People or animals may develop skin irritation or upper respiratory problems from exposure to HAB, and in extreme cases, dogs and other animals have even died after drinking lake water containing these toxins.

Where: Severe blue-green algal blooms typically occur on lakes with poor water quality (high in nutrients), and look like green paint, pea soup, or a thick green cake (see photo gallery below for examples). HAB often result in extremely low water clarity (less than 1 foot). There is no visual way to predict the toxicity of an algal bloom

What should I do if I suspect a HAB on my lake? When these conditions are present, people should avoid contact with the water and they should prevent animals from swimming in or drinking the water. Scientists do not yet know what causes some blooms to produce toxins while others do not, so the safest course of action is to avoid contact with all blue-green blooms.

What does it look like? Blue-green algae can be hard to distinguish from other types of algae. While it's often described as looking like pea soup or spilled green paint, it can take other forms as well.  The photos below show some of the diversity of blue-green's appearances and also provides some examples of other types of algae or plants that may be mistaken for blue-green algae.

Slide Show of Blue-green Algae Photos

Photos of Non-toxic Plants and Algae

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

Photo of Duckwood and nontoxic algae

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

Nuisance growth of a filamentous green 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.

Algae Information


Algae (i.e., phytoplankton) are microscopic plants that are a natural part of the aquatic environment, and they play a critical role in aquatic ecosystems. Algal concentrations vary considerably through the year, but are most abundant during warm weather in water that is alkaline and rich in nutrients (primarily phosphorus and nitrogen, which promote the growth of aquatic plants). Excessive algal growth, or algal blooms, tend to be more common in mid- to late-summer.

Minnesota’s lakes and streams support numerous varieties of algae, and most are harmless; however, under certain conditions some algae species can be harmful. Blue-green algal blooms are referred to as “Harmful Algal Blooms” (HAB) because several forms produce toxins. However, not all blue-green algae blooms are toxic; in fact, the vast majority is not. But because blooms can become harmful so quickly and can vary so much in toxicity and frequency, all blue-green algae blooms are potentially dangerous. Exposure to HAB may cause skin irritation and upper respiratory problems.   In extreme cases, people and animals have gotten ill, and have even died, after drinking water containing these toxins.  

There is no visual way to predict the toxicity of a given algal bloom, or to visually determine if an algal bloom may be harmful. Laboratory tests do exist for measuring some types of blue-green algal toxins; however, many algal blooms are short-lived and dissipate in a single day by rainfall or heavy winds, so a bloom may have dispersed before we receive lab results. We also know very little about what triggers a bloom to produce toxins, so an algal bloom testing negative or low for toxins one day could become hazardous the next day. These factors make it important for us to take a prevention-minded approach to exposure to blue-green algal blooms.


Research has identified the conditions listed below that tend to occur along with a harmful algal bloom. If you observe these conditions on your lake or pond, it is best to avoid contact with the water and keep pets and children out of the water until the bloom dissipates. 

  • Very low transparency, Secchi often 1.5 foot or less;
  • Very high chlorophyll-a concentrations, generally greater than 30-50 ppb; and
  • Very high pH, generally 9.0 or greater

Interagency Work Group

After several dog deaths suspected to have been caused by contact with HAB were reported in the summer of 2004, the MPCA began meeting with representatives in the Minnesota Departments of Health and Natural Resources, and the Minnesota Veterinary Medicine Association.  The work group created the following poster for display. Copies  may be ordered by contacting Kelly O’Hara, 651-757-2622:

The group includes the Minnesota Departments of Health and Natural Resources, Pollution Control Agency, and the Minnesota Veterinary Medicine Association. For further information on the work group, please contact Steven Heiskary at MPCA, at 651-757-2419 or toll free from Greater Minnesota at 1-800-657-3864.

Research and Reports

While there have been long-standing concerns regarding blue-green algal toxins, significant research on the subject has been somewhat lacking until recent years. The MPCA has conducted two research studies in the past few years to advance our knowledge on the extent, magnitude and frequency of HAB in Minnesota; to identify factors associated with high algal toxicity; and to use the study findings to educate the public about HAB.

The MPCA has recently analyzed all Minnesota data for the blue-green algal toxin microcystin. The article describes the range in concentrations and where higher concentrations are likely to be found in Minnesota. This article reflects several years of study on this freshwater toxin.

  • PDF Document Summary of Microcystin Concentrations in Minnesota Lakes (wq-s1-66) . This is an Author's Accepted Manuscript of an article published in Lake And Reservoir Management online May 21, 2014.  Steve Heiskary, Matthew Lindon & Jesse Anderson (2014) Summary of microcystin concentrations in Minnesota lakes, Lake and Reservoir Management, 30:3, 268-272 (copyright Taylor & Francis) available online.

To obtain a copy of any of the following reports, please contact Kelly O’Hara, MPCA, at 651-757-2622, or toll-free from Greater Minnesota at 1-800-657-3864.

  • A Study of the Blue-green Algal Toxin Microcystin, Based on Select Lakes in McLeod and Blue Earth Counties
  • National Lake Assessment Project: Microcystin Concentrations in Minnesota Lakes
  • July 2008 Environmental Bulletin
  • In 2007, Little Rock Lake in Benton County, Minnesota experienced a severe algal bloom. The MDH website has several items pertaining to algal toxins and the Little Rock Lake event.
Last modified on September 04, 2015 10:16

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