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What is a Harmful Algal Bloom?

discolored water

A harmful algal bloom (HAB) is the proliferation of a toxic or nuisance algal species that negatively affects natural resources or humans. Scientists prefer the phrase “harmful algal bloom” to “red tide” because blooms are not always red and are not related to the tides. 

To further define a harmful algal bloom, let’s look at the phrase more closely.

  • Harmful algal blooms damage the environment because they replace vital food sources, clog fish gills, prevent sunlight from reaching seagrass and contribute to low oxygen “dead-zones” when they degrade. Some HAB species produce potent toxins that can persist in the water and enter the food chain. These toxins can be harmful to humans and animals.
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  • Algae are plantlike organisms. In aquatic systems, most algae are microscopic and cannot be seen by the naked eye, though some such as "sea lettuce" are macroscopic and look like plants. Most microscopic algae are beneficial to natural systems and humans; they produce about half of the oxygen we breathe and serve as the base of the food web in fresh- and saltwater.
  • Blooms occur when algae reproduce or accumulate far beyond their normal levels for specific geographic areas. Blooms are influenced by chemical, physical and biological factors.  

There are several main groups of algae that form HABs: flagellates, diatoms and blue-green algae. Flagellates are single-cell organisms that move about or swim with whiplike appendages called flagella. They can cover up to 20 meters in a day. Diatoms live in glass boxes made of silica, either individually or in chains. Blue-green algae (also called cyanobacteria) are bacteria with plant pigments that can occur individually, or in colonies.

Learn more about the Gulf of Mexico’s prominent HAB species, their effects and their management by reading A Primer on Gulf of Mexico Harmful Algal Blooms.