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Harmful Algae Blooms

Harmful algae blooms (HABs) are overgrowths of algae in water. Some produce dangerous toxins in fresh or marine water, but even nontoxic blooms hurt the environment and local economies. Harmful algae blooms generally need sunlight, slow-moving water and nutrients (nitrogen and phosphorus). Because HABs can produce toxins that can affect public health, it is important to learn about how to spot them and avoid waters where they are present.

Harmful Algal Blooms (HABs) in Southwest Florida presentation
Red tide event on the coast

What is that green slime?

Some of our fresh waterways have a green slimy appearance. This can be from excessive blue-green algae growth, also known as cyanobacteria— a type of HAB.  Both regular green algae and blue-green algae/cyanobacteria can produce dense mats that can impede activities like swimming and fishing, and cause odor problems and oxygen depletion; however, cyanobacteria unlike green algae can produce toxins that affect aquatic life and human health. Cyanobacteria has been a major problem this year in local waterways such as the Caloosahatchee River and Cape Coral canals.

What is “Red Tide”?

Red tide” is another type of HAB formed from the excessive growth of Karenia brevis, a type of algae present in marine environments.  It also can produce a toxin, which has been the case with the wide-spread red tide event we have had the past few months in Southwest Florida. As a result of the severe red tide, oxygen-deprived “dead zones”  can form.

Cartoon of an algae bloom monster growing from a person over fertilizing

Image courtesy of P. Schmidt, Charlotte Sun.

Why do animals get sick or die?

HABs lower the amount of  oxygen in water, which inhibits the breathing of fish and other organisms who breath underwater. Red tide toxins attack the nervous systems of the animals exposed to it—which can inhibit their swimming or surfacing to breathe (as manatees and sea turtles do). The toxins also go up the food chain, causing birds who eat poisoned fish to get sick or die.


What can I do?

Some of the things you can immediately do to help in addition to volunteering or donating are:

  • Refrain from fertilizing during rainy season. Fertilizers containing nutrients often runoff yards into waterways before they can be absorbed with the frequent hard rainfalls we have this time of year. Plant Florida-friendly plants that do not require fertilizers.

  • Get a professional to inspect your septic system, maintaining/repairing if need be. Failing septics are often leaching nutrients into waterways.

  • Pick up your pet’s waste and dispose of it in a container so it does not wash nutrients or bacteria into nearby swales, canals or stormwater drains.

  • Support policies that better detect and control nutrient pollution, and projects that will clean it up (septic to sewer conversion projects, more water quality monitoring, etc.)

To report a bloom or find out more about whether harmful algae blooms are in your area, go to:

FL DEP Algae Bloom Monitoring and Response Website

 FL FWC Red Tide Weekly Status Reports

    FL DOH Healthy Beaches Website 


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