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Blue-green Algae Quick Links

Frequently Asked Questions


What is blue-green algae?

  • Blue-green algae, or “cyanobacteria,” are natural to the environment’s food chain and are found all over the world. They are actually a type of bacteria but, like plants, they can use sunlight to grow. Many live with other types of algae and microscopic animals, collectively termed “plankton.”
  • Blue-green algae are a group of organisms that are among the oldest on the planet. They are ubiquitous - found in marine waters as well as freshwater and brackish habitats. Blue-green algae are also common in other states, including Georgia, Texas, Alabama and Michigan.
  • Some – not all – blue-green algae can produce toxins that can contribute to environmental problems and affect public health. Those blue-green algae that are known to produce toxins do not always do so. Little is known about exactly what environmental conditions trigger toxin production.
  • Blooms can float on the surface and be several inches thick or they can lie below the surface of the water. Blooms can disappear from view or move to different parts of a pond or lake.


What causes blue-green algae blooms?

  • Blue-green algae multiply quickly in water bodies with high nutrient levels such as phosphorous or nitrogen, and particularly when the water is warm and the weather is calm. That's why blue-green algae are most common in Florida in the summer and early fall, with its high temperatures and abundant sunlight, although they can occur at any time.
  • Waters that flow slowly with low turbulence – such as impounded rivers, dams, or water storages – are at particularly high risk of algal blooms.
  • Blue-green algae growth can become seemingly explosive when lots of nutrients are present, providing a rich feast that can then cause “blooms” that may look like floating carpets of blue-green algae turning the water green. Many other states, and countries for that matter, are experiencing problems with algal blooms.


Are the blue-green algae blooms a new problem?

  • Algal blooms are not a new phenomenon. Algal blooms were documented in Florida’s coastal waters as early as the 19th century. However, increased nutrient loading in our waterways has likely contributed to increased frequencies of blooms in our freshwater systems.
  • In 1998, recognizing the need to assess the status of toxic microalgae in Florida, the state legislature approved funds to the Florida Harmful Algal Bloom Task Force (FLHABTF) to address any potential concerns regarding microalgae, including blue-green algae. Collaborative studies funded by the FLHABTF were initiated between the St. Johns River Water Management District, the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute, Florida Department of Health (DOH), private laboratories, and Wright State University to investigate the distribution of toxic blue-green algaeand their toxins (cyanotoxins) in Florida waters.  For more information on this topic, please visit HAB Task Force Blue-Green Algae Studies.


How long do the blooms last?

  • Blooms can last days, weeks or months, depending on conditions. Scientists cannot predict when or where blooms will occur or how long they will last; the blooms run their course and dissipate naturally.

Can blue-green algae blooms be treated?

  • No. The Florida Fish and Wildlife Conservation Commission does not recommend treatment because it may release the toxins.

  • The Bureau of Invasive Plant Management does not favor, and does not issue permits for, the application of algaecides to treat blue-green algal blooms in surface waters of the state. Such treatments are generally not very effective and would need to be repeated frequently, potentially resulting in unacceptable levels of algaecide in surface water bodies.

  • Surface water bodies used as drinking water reservoirs may be treated on occasion by water treatment plants in some counties. These treatments are exempt from permit requirements related to the application of algaecides in surface waters of the state.


Are large blue-green algae blooms harmful to lake/river ecosystems and cause fish kills?

  • Blooms of blue-green algae that last more than a few months can be harmful to lake/river ecosystems and cause fish kills because of the decrease in oxygen levels and direct ingestion in the food chain:
    •  Persistent blooms block sunlight that feeds plants growing on the bottom of lakes, resulting in the loss of rooted aquatic vegetation, which is valuable fish habitat. Intense blooms can cause extreme fluctuations in daytime and nighttime water oxygen levels, due to the oxygen produced during the daytime photosynthesis and the oxygen consumed during the nighttime respiration. The nighttime respiration as well as the decay of the blue-green algae, can consume much of the oxygen in the water, causing fish to die.
    • Algae use sunlight to make food and are eaten by microscopic animals (zooplankton). Small fish eat the zooplankton, and larger fish and other large animals eat the small fish. However, blue-green algae are often difficult to eat or are of poor nutritional value for zooplankton. Consequently, zooplankton, particularly types that are the best fish food, often decline during blooms. This decline can impact the number of desirable game fish. 


What is the state doing about the blue-green algae blooms?

  • As part of our mission and responsibilities, the state – including Florida Department of Environmental Protection, Florida Fish and Wildlife Conservation Commission, Florida Department of Health, St. Johns River Water Management District, South Florida Water Management District-and the University of Florida is responsibly concerned, not alarmed, about the current situation as we continue to monitor, evaluate and provide information to other concerned local, county, state and federal agencies.
  • The state is looking to increase the frequency and expand some of its current routine algal bloom monitoring in the major water bodies to try and get a better understanding of the phenomenon. This information will be shared with our government partners and the public as it becomes available.
  • We’ve increased communication with:
    • Media
    • Local government
    • Elected officials
    • Partnering agencies
    • General public

The St. John’s River Water Management District regularly monitors algal growth in the St. John’s River and routinely collects water and algae samples, particularly during periods when conditions are right for algal proliferation.


Whom can I contact to report a blue-green algae bloom or to get more information about water quality?

  • There are several toll-free hotlines available for people to report fish kills or illness associated with blue-green algae.
  • Fish Kill Hotline (Florida Fish & Wildlife Conservation Commission) 800-636-0511
  • Human Illness (Florida Aquatic Toxin Hotline) 888-232-8635


Are there web sites for more information about blue-green algae?


Last updated: July 14, 2016

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