Microcystin at Little Rock Lake, MN- EH: Minnesota Department of Health

Microcystin at Little Rock Lake
July 2011


Little Rock Lake is a relatively large lake (approximately 1,270 acres) located in Benton County in central Minnesota. The lake is mostly shallow and is very nutrient-rich. Phosphorus runoff into the lake from agriculture is the main source of nutrients, which can trigger frequent algal blooms. A severe bloom occurred in July 2007. Because of the nutrient-rich conditions in the lake, as well as very high algal toxin levels measured in the summer of 2007, the lake was included on the 2008 Total Maximum Daily Load (TMDL) list for nutrient impairment. Because the conditions of the water in Little Rock Lake have not improved since 2007 the potential remains for another severe algal bloom. 

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What is microcystin?

Algal blooms are sometimes made up of cyanobacteria, commonly called blue-green algae.  Some cyanobacteria can produce toxins, called cyanotoxins.  Although many blooms contain non-toxic species of cyanobacteria, lab tests are needed to determine whether a bloom is toxic or nontoxic. The Minnesota Pollution Control Agency (MPCA) has identified microcystin as the primary cyanotoxin during blooms in Little Rock Lake.  During the blooms, microcystin levels have reached extremely high levels.

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Ways people are exposed to microcystin:

  • Swallowing of lake water while swimming, skin contact with lake water while swimming, and breathing in water spray while recreating on the lake.
  • Watering lawns and gardens with lake water – People can be exposed through eating produce from gardens, skin contact and breathing in water spray.
  • Fish consumption - Although there have been no confirmed reports of cyanotoxin-related health effects due to eating fish, it is uncertain whether enough toxins in fish can be consumed to pose a health risk.   
  • Private well water– It is unknown if microcystin will migrate into wells leading to exposure from ingestion, skin contact, and breathing water spray.  If a severe algal bloom occurs, the Minnesota Department of Health (MDH) will work with MPCA to test the shallowest wells located nearest the western shoreline for microcystin. 

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What are the possible health effects?

Individual health risk depends on many factors, including how much and how long a person is exposed, and their age, occupation, and health history. Exposure to cyanotoxins can cause short-term health effects that commonly include nausea, vomiting, fever, rashes, and eye and ear irritation. Low level exposures to microcystins over time can cause liver toxicity.  Residents concerned about health problems associated with microcystin exposure should consult their physician. Children are at a higher risk from cyanotoxins than adults because their exposure is likely to be greater when adjusted for lower body weight. They are also more likely to swallow water during swimming/wading in water. 

Offensive odors can be generated during algal blooms. Concentrations of hydrogen sulfide may get high enough in the air that people can experience temporary symptoms such as eye and throat irritation, headaches, nausea, and dizziness.

Are there community health standards for microcystin?

There are no federal standards for microcystin in drinking water or recreational waters.  Some states regularly monitor for cyanotoxins in surface water and have standards based on World Health Organization guidelines.  In Minnesota, there is no formal cyanotoxin monitoring program, or standards for microcystin, but people are encouraged to call the MPCA with concerns regarding a particular bloom or lake.

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Recommendations for what to do if you have a concern about lakes with algal blooms:

  • Avoid swallowing lake water, skin contact with lake water, and breathing in water spray from recreational activities. Contact with lake scum in or out of the water should be avoided. 
  • Fish caught from the lake should be eaten in moderation. To avoid the highest concentrations of toxins in fish, avoid eating the intestines and organs, especially the liver. 
  • If odors cause discomfort, leave the area if possible.
  • Pets should not be allowed to swim in or drink water from the lake.
  • Phosphorus contributions to Little Rock Lake need to be lowered to reduce the likelihood of a severe algal bloom. Strategies for reducing phosphorus in the lake are identified in the draft 2011 TMDL and should be implemented.

If experiencing health effects, contact a medical professional. In addition, you are encouraged to report human health effects to MDH’s Foodborne Illness Hotline at 1-877-366-3455.

For health questions, contact MDH’s Acute Disease Investigation and Control group at 651-201-5414.

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Printable Information Sheet

Microcystin at Little Rock Lake (PDF)

More information: 

There is still much to be learned about cyanotoxins and microcystin.  MDH will work to update the community as new information becomes available. 

Complete Health Consultation: Microcystin in Little Rock Lake (PDF)

For information about blue-green algal blooms, visit Blue-green Algae and Harmful Algal Blooms on the Minnesota Pollution Control Agency website.

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If you have questions or comments about this information, please contact us.

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Updated Thursday, January 28, 2016 at 06:39PM