Harmful Algal Blooms

Many types of algae occur naturally in all surface water in Texas. Algae is a base organism in the aquatic food pyramid. Alga are photosynthetic, meaning they harness energy from sunlight and turn it into chemical energy. Any number of the different types of Algae may exist in one waterbody at the same time.

While most algae are not problematic, some types have the potential to produce toxins that are harmful to aquatic organisms and, in some cases, also harmful to humans, pets, livestock and wildlife. Toxic algae events are called “blooms.”

Harmful Algal Blooms

The word “bloom” may remind you of springtime and flowers; but, harmful algae blooms do not produce flowers. They can discolor the water and produce scum or foam, but during this process, they grow out of control, producing harmful toxins.

An algal bloom is a sudden, massive growth of microscopic and macroscopic organisms that develop in surface water. The rapid growth is often associated with water bodies with abundant nutrients. Blooms can occur in warm freshwaters, marine waters or brackish waters and are often associated with water bodies with ample nutrients.

A harmful algal bloom occurs when an alga capable of producing toxins that can cause harm to aquatic organisms, animals or people grows rapidly.

Harmful algal blooms are becoming more frequent with climate change, according to the Centers for Disease Control and Prevention. Water bodies with an ongoing bloom may look blue, green, brown, yellow, orange, or red.

Golden Algae

Golden Algae

Golden algae, or Prymnesium parvum, is one type of microscopic alga that is frequently present in the waters of the Brazos basin in low concentrations.

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Golden Algae

In the Brazos River basin, there have been numerous documented golden algae blooms, a few that nearly decimated the fish populations at Possum Kingdom Lake, Lake Granbury and Lake Whitney in 2003 and 2005. There have also been blooms that were localized, causing smaller, isolated fish kills.

While golden alga is frequently present in the waters of the Brazos basin in low concentrations, it is most likely to bloom, becoming toxic, and cause fish kills in cold weather. A golden alga fish kill may last for days, weeks or months, and may affect whole water bodies or isolated portions of water bodies. To learn more about golden algae, go here.

Red Tide Algae

Red tide is an algal bloom that occurs mostly in the Gulf of Mexico and associated estuaries, where freshwater from rivers flows into the Gulf, mixing with salt water. While rare in Texas, red tide events have occurred in Galveston Bay near the Brazos basin estuary. Red tide blooms occur mostly in late summer or early fall and cause the water to appear to be red to brown in color. Toxic red tide events not only result in fish kills but are also harmful to humans, pets, livestock and wildlife.



Cyanobacteria are commonly called “blue-green algae” or “pond scum”, but are actually not an algae at all.

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Blue-green Algae

Most blue-green algae, also called cyanobacteria, are not known to produce toxins; but, there are two species, Anabaena and Microcystis, both known to occur in Texas, that can produce toxins.

Unlike golden alga, these two blue-green algae species not only cause fish kills when toxic blooms occur, but their toxins can be harmful to humans, pets, livestock and wildlife. Blue-green algae blooms are believed to have caused the death of several dogs in the Austin area in recent years. To learn more about blue-green algae, go here

Effect on Humans

The human illnesses caused by harmful algal blooms, though rare, can be debilitating or even fatal, according to the National Oceanic and Atmospheric Administration. To date, there have been no known, documented algal blooms in the Brazos River basin that were considered harmful to humans, pets or wildlife in the past 25 years.

How do these blooms cause harm?

According to the Centers for Disease Control and Prevention, a harmful algal bloom can cause harm by:

  • Producing toxins that can poison humans, fish, seabirds, aquatic animals, livestock, wildlife, and household pets that are near the water, drink the water, or swim in the water.
  • Causing illness when a person or animal eats fish or shellfish contaminated with algal toxins.
  • Becoming dense enough to keep sunlight from reaching the lower depths of the water.
  • Removing the oxygen from the water as it decomposes, starving fish and plants of oxygen, and damaging the local ecology.

Protecting yourself from harmful algae blooms

If you’re on a water body, you will most likely be able to see an algal bloom, but you will not be able to tell which type of algae it is unless you look at it under a microscope.

Some of the signs of an algal bloom are:

  • a strong odor
  • discolored water
  • algal mats or scum floating on the surface
  • dead fish or other dead animals

Experts advise that if you see water with a “pea-soup” color and consistency, avoid it. Do not boat or swim near a suspected algae bloom.

The Brazos River Authority environmental services team tracks harmful algae blooms in the Brazos River basin.

If you’re out on a BRA reservoir and notice water that appears green and thick or has an abnormal odor, don’t swim or let your pet go into the area. Do not catch and consume fish that have been caught in the area of an algae bloom.

If you feel you may have spotted an algae bloom, please contact the BRA at (888) 922-6272 with the location.

For more information on harmful algal blooms, go to:

Click here to learn more about the species of interest and here to learn about invasive species monitored by the BRA.