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Algal Blooms Consistently Produce Complex Mixtures of Cyanotoxins and Co-Occur with Taste-and-Odor Causing Compounds in 23 Midwestern Lakes

Frequently Asked Questions

USGS limnologist collecting a surface sample of algae from Binder Lake, IA
Jennifer Graham, USGS limnologist, collecting a surface accumulation dominated by Microcystis sp. from the surface of Binder Lake, Iowa. Photo Credit: Jennifer L. Graham, USGS.

1. What are cyanobacteria?

Cyanobacteria are a natural part of aquatic ecosystems and commonly occur at low abundances. They are true bacteria that function like algae and are commonly referred to as “blue-green algae”. Since cyanobacteria are photosynthetic organisms like green plants, they actually consume carbon dioxide, which is considered a green house gas, and produce oxygen. As primary producers, they are also an important part of the food web and are eaten by simple organisms and some fish. Some types of cyanobacteria have the potential to produce toxins called cyanotoxins, which can be harmful to humans and animals. They can also produce unpleasant earthy or musty taste and odor compounds, which are not toxic, but may indicate the presence of cyanotoxins in surface water.

2. What are cyanobacterial blooms and accumulations?

Blooms are specific cases where the cyanobacteria population grows so large that they are visible to the eye. Large cyanobacterial accumulations occur when blooms are concentrated, which usually happens near shore due to wind and wave action. Blooms most often occur in the summer, but can occur any time of the year.

3. Are harmful algal blooms (HABs) different from cyanobacterial blooms?

Freshwater and marine harmful algal blooms (HABs) can occur anytime water use is impaired due to excessive accumulations of algae. Several types of algae may cause HABs, including cyanobacteria. Cyanobacterial blooms (CHABs) may or may not be toxic, but refer to the overabundance of the cyanobacterial population.

4. What are cyanotoxins?

Cyanotoxins are natural toxins that are sometimes produced by cyanobacteria. Scientists do not currently understand when or why these compounds are produced. In mammals, including humans, cyanotoxins can cause allergic and/or respiratory issues, attack the liver and kidneys, or cause issues with the nervous system. Exposure routes to cyanotoxins can include inhalation, contact with skin and eyes, and consumption of contaminated water. Symptoms of mild cases of exposure may include no observable effects, skin irritation or rash, difficulty breathing, headaches, and/or nausea. As exposure increases symptoms may include inflammation of target organs, organ failure, paralysis, and death in the most severe cases.

Dock and a dispersed algal bloom. Crystal Lake, IA
A Microcystis sp. dominated bloom dispersed throughout the water column in Crystal Lake, Iowa (Hancock County). The total microcystin concentration (~ 40 µg/L) was approximately equal to that measured in Binder Lake, IA, where a surface accumulation of cyanobacterial was present. Photo Credit: Jennifer L. Graham, USGS.

5. How frequent do cyanotoxins poisonings occur?

There is currently no comprehensive assessment of cyanotoxin poisonings in the United States. However, anecdotal evidence of human and animal poisonings, and in some cases death, have been reported in over 50 countries including at least 36 U.S. states. Animal poisonings are more frequent than human poisonings because people generally avoid contact with dense cyanobacterial accumulations.

6. What are taste-and-odor compounds?

Taste-and-odor compounds, such as geosmin and MIB, are a group of chemicals naturally produced by cyanobacteria and other types of bacteria that typically impart undesirable taste and/or odor to surface water. Occasionally, these compounds cause aesthetic issues in finished drinking water.

7. Are taste-and-odor compounds toxic?

There is no known data indicating that the taste-and-odor compounds are associated with any human health effects.

8. If I smell or taste earthy, musty odors in the water in which I am swimming or boating, does that mean that I may be exposed to cyanotoxins?

Maybe. Cyanotoxins and taste-and-odor compounds have been found to frequently co-occur in cyanobacterial blooms. If you see algae and/or smell earthy, musty odors, it is best to use caution and avoid the area. If there is any doubt regarding your condition seek medical attention. It is also a good idea to notify local authorities regarding the potential for others to be exposed in the affected area such as the state health department, lake managers, or other relevant state agencies.

9. If I smell or taste earthy, musty odors in my drinking water, does that mean I am drinking cyanotoxins also?

No, it does not. Cyanotoxins and taste-and-odor compounds do not always co-occur in source water for drinking water treatment. In addition, it appears that cyanotoxins are more easily removed by most drinking water treatment processes commonly used in the United States compared with taste-and-odor compounds. However, there have been a few documented cases of cyanotoxins occurring in finished drinking water in the United States.

10. Are there other reasons to be concerned about cyanobacterial blooms besides cyanotoxins and taste-and-odor compounds?

Harmful algal blooms, including cyanobacterial blooms, indicate a general ecosystem imbalance. In other words, the ecosystem is not “healthy.” Frequently, large algal blooms lead to oxygen depletion and subsequent fish kills.

11. Why do cyanobacterial blooms occur?

Climate, geology, geography and other natural factors influence cyanobacterial growth and community composition. Land use seems to also have a substantial impact. Excess nutrients caused by human activities appear to be one of the largest contributors to cyanobacterial bloom occurrence. Studies are in progress to determine how all of these factors impact cyanobacterial populations and toxin production.

Algal bloom near the shore of Rock creek lake in Iowa
At Rock creek lake in Iowa, 13,000 micrograms per liter (µg/L) of microcystins was measured by enzyme linked immuno sorbent assay in this Microcystis sp. dominated accumulation. World Health Organization recreational guidelines for microcystin exposure for very high probable risk of acute health effects is anything greater than 2,000 µg/L.

12. Are toxic cyanobacterial blooms occurring more frequently?

We don't know yet. There is a perception that the occurrence of cyanobacterial blooms are occurring more frequently. It is possible that higher frequencies of blooms are correlated with human activities that result in increased nutrient loading to lakes and streams. Further study is currently planned/being conducted to assess whether these blooms are more prevalent.

13. Can we just eliminate cyanobacteria from a water body since they produce the toxins?

Our goal should not be to completely remove cyanobacteria from ecosystems, but rather to manage ecosystems to minimize the occurrence of cyanobacterial blooms. Cyanobacteria serve an important role in ecosystems as primary producers and play a role in converting carbon dioxide to oxygen.

Unfortunately, there is usually not an easy, permanent solution in the near term. Some in-lake treatments exist, but they can be extremely expensive, temporary, and may have limited effectiveness. Some treatments exacerbate the problem through unintended consequences. Long term solutions include implementation of best management practices in the watershed to minimize pollution such as excess nutrients and sediments.

14. What is the USGS doing to address cyanotoxins and taste-and-odor compounds?

The U.S. Geological Survey (USGS) approach to these issues has been three fold. First, the USGS is developing standardized methods to reliably measure cyanobacteria, toxins and taste-and-odor compounds in streams, lakes, and reservoirs. Second, the USGS is conducting studies that determine the potential human and ecological risk posed by cyanobacteria and their toxins. The third aspect of addressing this issue is to have two-way communication with the public to make sure real problems are being addressed in the areas where this problem is most prevalent.

More Information

Additional Information About Harmful Algal Blooms (HABs), Cyanobacterial Blooms, and Cyanotoxins

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