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Blue-green algae blooms appeared earlier and on more lakes this summer than usual. Learn more about these blooms and the health risks they can pose.

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For information on Lakes in Wisconsin, contact:
Wisconsin DNR Lakes
Division of Water
Bureau of Water Quality
Blue-Green Algae Contacts

Blue-Green Algae

Contact and General Information

If you think you are experiencing symptoms related to exposure to blue-green algae (e.g., stomach cramps, diarrhea, vomiting, headache, fever, muscle weakness, difficulty breathing), contact your doctor or the Poison Information Hotline (800-222-1222) right away.

If your pet displays symptoms such as seizures, vomiting, or diarrhea after contact with surface water, contact your veterinarian right away.

Report a Case with potential health effects caused by blue-green algae, visit the Department of Health Services [exit DNR]. or contact the Bureau of Environmental and Occupational Health at 608-266-1120.

For more information about contacting your local health department, check the Department of Health Services Web site [exit DNR].

If you are (or your local community is) interested in collecting samples for analysis, please contact the Wisconsin State Laboratory of Hygiene at (800)442-4618. The Wisconsin Department of Natural Resources is not currently conducting any routine monitoring for blue-green algae or blue-green algal toxins.

What are blue-green algae?

Blue-green algae, also known as Cyanobacteria, are a group of photosynthetic bacteria that many people refer to as "pond scum." Blue-green algae are most often blue-green in color, but can also be blue, green, reddish-purple, or brown. Blue-green algae generally grow in lakes, ponds, and slow-moving streams when the water is warm and enriched with nutrients like phosphorus or nitrogen.

When environmental conditions are just right, blue-green algae can grow very quickly in number. Most species are buoyant and will float to the surface, where they form scum layers or floating mats. When this happens, we call this a "blue-green algae bloom." In Wisconsin, blue-green algae blooms generally occur between mid-June and late September, although in rare instances, blooms have been observed in winter, even under the ice.

Many different species of blue-green algae occur in Wisconsin waters, but the most commonly detected include Anabaena sp., Aphanizomenon sp., Microcystis sp., and Planktothrix sp. It is not always the same species that blooms in a given waterbody, and the dominant species present can change over the course of the season.

How do blue-green algae differ from true algae?

Blue-green algae, like true algae, make up a portion of the phytoplankton in many water bodies. However, blue-green algae are generally not eaten by other aquatic organisms, and thus are not an important part of the food chain. True algae (e.g., green algae) are very important to the food chain. They are known as "primary producers", a name given to living organisms that can convert sunlight and inorganic chemicals into usable energy for other living organisms. Most algae are microscopic and serve as the main supply of "high energy" food for larger organisms like zooplankton, which in turn are eaten by small fish. Small fish are then eaten by larger fish, and both small and large fish are eaten by mammals, raptors, and people. Green algae, shown here [PDF], can also bloom at nuisance levels and may be mistaken for blue-green algae.

What are the concerns associated with blue-green algae?

Concerns associated with blue-green algae include discolored water, reduced light penetration, taste and odor problems, dissolved oxygen depletions during die-off, and toxin production. Discolored water is an aesthetic issue, but when blue-green algae reach bloom densities, they can actually reduce light penetration, which can adversely affect other aquatic organisms both directly (e.g., other phytoplankton and aquatic plants) and indirectly (e.g., zooplankton and fish that depend on phytoplankton and plants). Blue-green algae blooms can be quite smelly, and though it is recommended that people never drink raw water, blue-green algae have been known to affect the taste of drinking water that comes from surface waters experiencing a bloom. Here in Wisconsin, most of the state relies on groundwater, rather than surface water, for drinking water. When a blue-green algae bloom dies off, the blue-green algae cells sink and are broken down by microbes. This breakdown process requires oxygen and can create a biological oxygen demand. Increases in biological oxygen demand result in decreases in oxygen concentration in the water, and this can adversely affect fish and other aquatic life, and can even result in fish kills.

Blue-green algal toxins are naturally produced chemical compounds that sometimes are produced inside the cells of certain species of blue-green algae. These chemicals are not produced all of the time and there is no easy way to tell when blue-green algae are producing them and when they are not. When the cells are broken open, the toxins may be released. Sometimes this occurs when the cells die off naturally and they break open as they sink and decay in a lake or pond. Cells may also be broken open when the water is treated with chemicals meant to kill algae, and when cells are swallowed and mixed with digestive acids in the stomachs of people or animals. The only way to be sure if the toxins are present is to have water samples analyzed in a laboratory using sophisticated equipment.

Are blue-green algae blooms a new problem?

No. Fossil evidence suggests that blue-green algae have been around for millions of years. Scientists have recorded blue-green algae blooms dating back to the 12th century and they have documented the toxic effects to livestock for more than 100 years. However, it is possible that the frequency and duration of blooms are increasing in some Wisconsin waters as a result of increased nutrient concentrations. Nutrients, particularly phosphorus and nitrogen, can be carried into water bodies as a result of many human activities, including agriculture, discharge of untreated sewage, and use of phosphorus-based fertilizers and detergents.

What is Cylindro and how is it different?

Cylindrospermopsis raciborskii, also referred to as "Cylindro," is a blue-green algal species that is not native to Wisconsin. Recent reviews of archived samples by DNR scientists have shown that Cylindro has been present in some southern Wisconsin lakes dating back to the early 1980s. It is likely that migratory waterfowl brought this algae to Wisconsin and other Midwestern states. In lakes where Cylindro has been detected, blooms typically occur any time between late July and late September. Cylindro is different from many other blue-green algae in that it does not typically float to the surface to form scums. Thus, it can be difficult to see a bloom of this species. Cylindro is capable of producing more than one toxin, including cylindrospermopsin, which can affect the liver. However, to date, cylindrospermopsin has not been detected in any Wisconsin waters.

Why do blooms sometimes appear overnight?

Even if you can't see blue-green algae floating on the surface of the water, that doesn't mean they aren't there. Blue-green algae can be suspended at various depths in the water, and their location depends on a number of factors. The most important of these are light and nutrients (phosphorus and nitrogen). Many species of blue-green algae have evolved to be able to control their buoyancy as the availability of light and nutrients change with the time of day and local weather conditions. At night, when there is no light, cells are unable to adjust their buoyancy and often float to the surface, forming a surface scum. So this scum can literally appear overnight and may linger until wind and waves scatter the cells throughout the water body.

Should I get treat a blue-green algae bloom with a chemical to get rid of it?

No. Treatment of a surface water that is experiencing a blue-green algae bloom with an herbicide or algaecide may kill the blue-green algae, but any toxin(s) contained in the cells will be released at once, resulting in a slug of toxin(s) in the water. So while the bloom may no longer be visible, toxin(s) may be present for some period of time following treatment. It is best to stay out of a water experiencing a bloom and wait for the bloom to dissipate on its own.

What can be done to reduce the frequency and intensity of blue-green algae blooms?

There are no quick or easy remedies for the control of blue-green algae once they appear in a lake or pond. Reducing the amount of nutrients that wash into our lakes and ponds will eventually reduce the frequency and intensity of blue-green algae blooms, but it may take a long time and a lot of community involvement to effectively change the nutrient concentrations in a water body. This is because there may still be large amounts of nutrients in the sediment at the bottom that may continue to serve as food for the blue-green algae.

Regulatory agencies like the Wisconsin Departments of Natural Resources and Agriculture, Trade, and Consumer Protection are working with communities around the state to reduce stormwater runoff, and to encourage agricultural practices that reduce soil erosion while maintaining high crop yields. Locally, landowners and interested citizens can help minimize the problems associated with algal blooms by working together with partners in their watershed to reduce the amount of nutrients that reach nearby lakes, streams, and ponds. You can help reduce nutrient concentrations by promoting the following practices in your community:

  • Use lawn fertilizers only where truly needed
  • Prevent yard debris (e.g., leaves, grass clippings, etc.) from washing into storm drains
  • Support local ordinances that require silt curtains for residential and commercial construction sites
  • Plant and maintain vegetative buffer strips along shorelines of lakes, ponds and streams. Note: Native plants are much more effective at filtering runoff than the typical grass species found on residential lawns.

Has the Wisconsin Department of Natural Resources conducted any monitoring for blue-green algae?

The WI DNR conducted a two-year study to investigate the frequency, severity, and duration of blue-green algae blooms, including information on which species of blue-green algae are present over the course of the summer. We also looked for the presence and concentrations of specific toxins: anatoxin-a (a neurotoxin), microcystin-LR (a hepatotoxin), and cylindrospermopsin (a cytotoxin). Samples were (for the most part) collected from five lakes in each of five regions, five times over the course of each summer (2004 and 2005). Samples were also collected from eight ponds in the south central region of the State. It is important to note that we chose to sample sites where blue-green algae blooms had occurred in the past or where they could potentially occur, based on nutrient concentrations. Therefore, this was not a random sample of lakes and ponds meant to represent all lakes and ponds in Wisconsin.

Also, unlike the beach monitoring study for bacteria , this study was not designed to provide real-time information on the presence of blue-green algae or blue-green algal toxins, and only a limited number of surface waters could be monitored in each region of the state. However, when DNR received information from the State Laboratory of Hygiene on the presence of high counts of blue-green algae or on the presence of blue-green algal toxins, this information was shared with the local public health agency. Only the Department of Health and Family Services or the local public health agency has the authority to close a beach.

The total number of samples collected in the statewide monitoring study was 187 in 2004, and 194 in 2005. Blue-green algae were present in 74% of all samples collected in both 2004 and 2005 (again, samples were collected from sites where we believed the potential for blooms was high). Blooms occurred in all regions of the state, with the biggest "hot spots" in the west central and south central regions. Species of blue-green algae most commonly detected included Anabaena sp., Aphanizomenon sp., Microcystis sp., and Planktothrix sp. Alerts were sent out to local public health agencies when concentrations of blue-green algae likely exceeded the World Health Organization (WHO) guideline of 100,000 cells/mL. This concentration represents a "moderate risk to human health." The total number of alerts sent out was 33 in 2004 and 42 in 2005.

A subset of the total number of samples collected was analyzed for toxins at the end of each summer (45 samples in 2004, and 34 samples in 2005). Microcystin-LR (a hepatotoxin) was the toxin detected most frequently and in the highest concentrations. This toxin was detected in the northern, south central, and west central regions of the state. The toxin anatoxin-a (a neurotoxin) was detected in samples collected in the northern and south central regions, and its presence was associated with a dog death in 2004. The toxin cylindrospermopsin was never detected.

Last Revised: Monday, May 05, 2014