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Spoiling the Fun: Harmful Algal Blooms and Microcystin

About Harmful Algal Blooms


Harmful algal blooms (HABs) are overgrowths of blue-green algae (also known as cyanobacteria) in water. HABs can produce dangerous toxins that present significant threats to our health, our environment, and our economy. 


HABs occur when algae that are normally present in water grow excessively due to increased nutrients (nitrogen and phosphorus). They most often form in slow-moving, warm water, and can accumulate along shorelines.  

According to the U.S. EPA, "animal manure, excess fertilizer applied to crops and fields, and soil erosion make agriculture one of the largest sources of nitrogen and phosphorus pollution in the country."

Other sources of nutrient pollution include stormwater runoff, wastewater discharges, domestic fertilizer use, and wildlife or pet waste.

Health Impacts 

Health Impacts Icon

HABs can produce toxins that cause illness in people and animals. The most common group of toxins are microcystins. According to the Centers for Disease Control, direct exposure to microcystin can cause serious health problems, including:  

  • Rashes 
  • Diarrhea 
  • Vomiting 
  • Respiratory problems 
  • Liver or kidney damage 
  • Neurological effects 

How People & Pets are Exposed

According to the Iowa Department of Public Health, humans can be sickened by microcystin if they have direct contact with blue-green algae. Contact can happen by intentionally or accidentally swallowing water, by direct skin contact when swimming or wading, or by inhaling contaminated airborne water droplets, such as during boating or waterskiing.

Pets and other animals that drink from the water’s edge, where scum accumulates, can be exposed to deadly levels of microcystins. Dogs and other animals that swim in water with a recent HAB are especially at risk when licking themselves after leaving the water.

Environmental Impacts

According to the EPA, toxins sometime present in HABs can kill fish and other animals. These toxins are consumed by small fish/shellfish, and then "move up the food chain" to other larger animals like turtles, birds, other fish, etc.

HABs can also form thick layers of scum that can harm aquatic life by blocking out sunlight and even clogging fish gills. 

Economic Impacts

Economic Impacts

HABs resulting from nutrient pollution can increase a variety of costs, ranging from drinking water treatment to medical costs associated with HAB exposure. HABs can also decrease spending on recreational lake tourism and the associated economic benefits