MPCA Home > Water > Lakes > Blue-green Algae and Harmful Algal Blooms

Blue-green Algae and Harmful Algal Blooms

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. 

Interagency Work Group on Blue-Green Algae

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 Matt Lindon – (651) 757-2530:

• HAB Fact Sheet
• HAB Poster
• Harmful Algal Bloom Workshop and Webcast

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.

MPCA 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.

• A Study of the Blue-green Algal Toxin Microcystin, Based on Select Lakes in McLeod and Blue Earth Counties
• Presentation: A Study of Microcystin Levels in Eutrophic South Central Minnesota Lakes (as presented to the North American Lake Management Society, October 31, 2007)
• National Lake Assessment Project: Microcystin Concentrations in Minnesota Lakes
• July 2008 Environmental Bulletin
• Fall 2001 Minnesota Environment

Recognizing Potentially Harmful 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.

Photo Gallery of Severe Algal Blooms and Other Algae

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.  This photo gallery shows 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.

Blue-green algae:

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

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

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

Contact Information and Blue-green Algae Incident Tracking

MPCA lake monitoring staff does track reports of potential HAB for follow-up 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.  Ultimately, the goal is to reduce the load of nutrients flowing to the lake, which in turn diminishes occurrences of excessive algal blooms. 

For more information on harmful algal blooms, please contact: Matt Lindon, MPCA, at 651-757-2530, or toll-free from Greater Minnesota at 1-800-657-3864. To report a blue-green algae incident, please complete this form.