Lake Petenwell Algae Bloom

People looking at lake property frequently ask about algae.  For good reason as algae has existed for hundreds of millions of years and is prevalent in most lakes.  Many species of algae are found in Wisconsin lakes and rivers. Algae act like other plants and convert sunlight into energy, forming the base of a lake’s food chain. Algae are eaten by zooplankton, which are in turn eaten by small fish, then larger fish, and eventually the larger fish are eaten by birds, shore animals, and people. True algae are a vital part of lake systems.

What are algal blooms and why do they occur?

When environmental conditions are right, the algal population can grow quickly and a bloom can occur. A bloom is a sudden increase in algae cells in a certain area of water. Little wind, warm water, sunlight, and plentiful nutrients –especially phosphorus- all increase the chance that a bloom will occur. Warm weather patterns and large rain events that wash agricultural and residential fertilizers (which contain phosphorus) into the water can also jump start a bloom. In Wisconsin, blooms typically occur during the warm weather months between mid June and mid September. Lakes and rivers in Wisconsin can become cloudy with rapidly reproducing algae.

What are blue-green algae?

True algae are a vital part of lake systems, however, blue-green algae are not true algae. Blue-green algae are photosynthetic bacteria known as cyanobacteria which can cause illness and death in humans and animals. While blue-green algae can convert sunlight into energy, they are not an important a part of the food chain because most organisms prefer not to eat them.

Blue-green algae will follow sunlight and nutrients by floating to the surface where they can form thick scum layers or mats and the surface may look bubbly or frothy. Algal scums can be pushed to different locations by wind or tide. When blue-green algae are present, the algal scum can be a variety of colors such as fluorescent blue, green, white, red or brown. Blooms can have more than one color present and may look like thick paint floating on the water.  Algal blooms can give off a foul odor, which is particularly offensive in the warm summer months.

Blue-green algae are a natural part of lake ecosystems and algal blooms have occurred for many centuries. While scientists are learning more and more about blue-green algae, researchers are only beginning to understand the health risk blue-green algae pose to humans and animals.

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.

Are all blue-green algae dangerous?

There are many species of blue-green algae found in Wisconsin lakes and rivers, but only certain species can produce the algal toxins that cause illness. Not all algal blooms produce toxin; a blue-green algae bloom may not be producing toxins or producing enough toxin to cause a health concern. Larger algal blooms have the potential for higher concentrations of toxin. Unfortunately, there is no immediate way to know if an algal bloom is dangerous or not. It takes at least several days for a laboratory to analyze a water sample to see if toxins are present. During that time, a harmless algal bloom may become more toxic, a toxic algal bloom may become harmless, or the bloom could blow away or dissipate all together. Algal blooms can appear and disappear within hours, which makes it difficult to decide when to close a beach. By the time water quality analysis is done, water conditions may have changed from the time a sample was collected. This is why it is so important to follow a common sense approach. When you see an algal bloom, avoid the bloom, enjoy a clean part of the lake, and remember that you should not allow your pets in water where an algal bloom is present!

Can water containing blue-green algae blooms be used for recreational activities?

Because local health officials cannot easily determine when blue-green algal toxins are being produced, anyone considering recreation on or in the water should use common sense. Simply put, if a scum-layer or floating mat is present, do not recreate in or on that water. The chance for health effects is greater if you or your children participate in water-related activities such as swimming, wading, water or jet-skiing, or wind surfing in areas containing algae. Try to find areas where a blue-green algae bloom is not present.

Can I eat fish from water containing blue-green algae?

The World Health Organization advises that people who choose to eat fish taken from water where a blue-green algae bloom is present eat such fish in moderation and avoid eating the guts of the fish, where accumulation of toxins may be greatest. Also, take care to not cut into organs when filleting the fish and rinse the fillets with clean water to remove any liquids from the guts or organs before freezing or cooking.

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.

Important Note About Hygiene

All natural surface waters contain bacteria, algae, viruses, and other pathogens that if consumed may pose health risks to humans, pets, and other domestic animals (e.g., cattle, swine). No one should ever ingest raw water.


The Problem: Phosphorus has long been recognized as the controlling factor in plant and algae growth in Wisconsin lakes and streams. Small increases in phosphorus can fuel substantial increases in aquatic plant and algae growth, which in turn can reduce recreational use, property values, and public health. See Reducing Phosphorus to Clean Up Lakes and Rivers for more information about phosphorus as a pollutant.

Sources of Excess Phosphorus:  Phosphorus entering our lakes and streams comes from “point sources” – piped wastes such as municipal and industrial wastewater treatment plants that release liquid effluent to lakes and rivers or spread sludge on fields; and from natural sources, including past phosphorus loads that build up in lake bottom sediments.

Phosphorus also comes from “nonpoint” or “runoff” pollution. Such pollution occurs when heavy rains and melting snow wash over farm fields and feedlots and carry fertilizer, manure and soil into lakes and streams, or carry phosphorus-containing contaminants from urban streets and parking lots.

New phosphorus rules that set standards for the amount of phosphorus allowed in different categories of lakes and streams took effect in December 2010, as did related rules allowing for those limits to be incorporated into municipal and industrial wastewater permits. A month later, new performance standards took effect for farmers requiring them to curb phosphorus potentially coming off their fields.

A more thorough review of the phosphorus impact on lakes, what is being done to reduce the phosphorus input, a chart of phosphorus input into Lake Petenwell since 1999, and steps you can take to minimize your phosphorus output can be viewed HERE.