A harmful algae bloom occurs when cyanobacteria, also called blue-green algae, produce toxins. Lady Bird Lake experienced a harmful algae bloom in 2019, which sickened and killed several dogs. Another one occurred in 2020.

Current Status

September 27, 2021 – The City of Austin has detected concerning levels of a toxin called cylindrospermopsin in Barton Creek at Sculpture Falls, near MoPac and Loop 360. The samples were taken on September 9 after a report of a human illness and results were received on Sept 22. This toxin is produced by certain types of cyanobacteria or blue-green algae. The levels detected were above EPA guidance values for recreational use. People and pets should not ingest or swim in the water. The City of Austin has posted signs at access points to the Barton Creek Greenbelt near Sculpture Falls. More info about Barton Creek. More info about Barton Creek...

On Lady Bird Lake, we found three different toxins in algae samples taken on September 13 from Cold Spring, between MoPac and Red Bud Isle, Auditorium Shores, South First Street bridge and the Festival boat ramp. Some sites had only dihydroanatoxin and some had both cylindrospermopsis and homoanatoxin-a. We found all three toxins in samples from Red Bud Isle. In general, we are seeing more mats of algae on the lake. They are often mixed in with other vegetation, especially an aquatic plant called Cabomba. On Red Bud, the algae is also growing on rocks. Boaters may want to be extra cautious on Lady Bird Lake and avoid contact with floating vegetation.

Photo of brightly colored algae growing in a patch of Cabomba

In this photo from Friday, September 17, 2021, Cyanobacteria is showing up as brown, yellow and blue blobs among Cabomba plants.

Earlier this year, we also detected dihydroanatoxin in algae in Lake Austin. LCRA has detected it in algae samples from other Highland Lakes this year.

Use our dashboard for algae testing results, water temperature and flow and nutrient levels in Lady Bird Lake and Lake Austin.


At this time, we are recommending that dog owners not allow their dogs to ingest or touch algae in any area lakes, creeks or water bodies. People should also avoid the algae. 

The risk may also be elevated at stock ponds or stormwater ponds. Although these ponds may be attractive, their water quality tends to be poor and is not suitable for recreation at any time for either people or pets.

Lady Bird Lake continues to meet State of Texas contact recreation standards, which are based on bacteria levels. 


There is always some level of risk in a natural water body. There is currently an increased risk in Austin's waterways.

  • Avoid stagnant areas or areas with algae.
  • People and pets should not touch or ingest algae.
  • Rinse your dog after contact with the water.
  • If your dog becomes sick after swimming, take your dog to a veterinarian immediately.

If you allow your dog to swim, you do so at your own risk. 

    Risk for People

    Cylindrospermopsin, which was detected in the water of Barton Creek at Sculpture Falls, may cause liver and kidney damage. According to the EPA, acute human and animal health effects of exposure to this toxin include:

    • Fever
    • Headache
    • Vomiting
    • Bloody diarrhea

    Seek medical treatment immediately if you believe you have been exposed. Please call 3-1-1 to report possible illness in people or pets from harmful algae.

      The 2019 harmful algae bloom appears to have only affected dogs. The toxins were contained in the algae and not released into the water. It is always recommended that people avoid stagnant areas of the lake and handling or eating algae. Remember that people are not allowed to swim in Lady Bird Lake (Ord. 640611-C). 

      Symptoms of Dihydroanatoxin Exposure

      Dogs who ingest algae with dihydroanatoxin could have a number of symptoms, including respiratory paralysis and death. Dogs can also be exposed to the toxin by licking algae from their fur. Look for these signs in your pet within minutes to hours of exposure:

      •     Excessive drooling, vomiting and diarrhea
      •     Foaming at the mouth
      •     Jaundice and hepatomegaly
      •     Blood in urine or dark urine
      •     Stumbling
      •     Loss of appetite
      •     Photosensitization in recovering animals
      •     Abdominal tenderness
      •     Progression of muscle twitches
      •     Respiratory paralysis

      Bull Creek

      The City was informed in early August that an individual became sick after playing in Bull Creek at Bull Creek Preserve. Watershed Protection tested algae and water samples and did not detect any toxins in the initial samples or the followup samples.

      Testing and Monitoring

      We will be monitoring three sites on Lake Austin and three sites on Lady Bird Lake, visiting them every other week throughout the summer. We will continue monitoring throughout the year, but may scale back the frequency as the year progresses. We will also begin monitoring one site on Lake Walter E. Long, visiting it at least three times during the summer and fall.

      Map showing six monitoring sites on Lake Austin and Lady Bird Lake

      Red dots indicate biweekly monitoring sites for harmful algae.

      Cause and Type of Algae

      Blue-green algae are one of the earliest forms of life and are common worldwide. Keep in mind:

      • There are many types of blue-green algae, but only some species can produce toxins. 
      • There are several different types of toxins possible. 
      • Even if a species is capable of producing toxins, that doesn't mean it will always do so. Generally, harmful algae need warm water, low flow and high levels of nutrients.  

      Blue-green algae can be single cells spread throughout the water. When they form mats that are big enough to see, they usually look like dark green, slimy blobs. Mats can be on the bottom or floating on the top of the water. Blue-green algae are often mixed in with other types of algae.  

      We are not aware of any human or pet health problems from harmful algae in Austin prior to 2019. Zebra mussels, flooding during the fall of 2018 and climate change are potential contributing factors to the harmful algae bloom in 2019. 

      Algae at Red Bud Isle on October 14.

      Algae at Red Bud Isle on October 14, 2019.

      Drinking Water

      Austin Water regularly tests algae levels on Lake Austin and Lake Travis near their intake pipes and has not seen levels of concern for drinking water. Currently, Austin Water does not use Lady Bird Lake as a source for drinking water.

      Other Bodies of Water

      Stock ponds and stormwater ponds have the highest risk for harmful algae. Most are privately owned. Although these ponds may be attractive, their water quality tends to be poor and is not suitable for recreation at any time for either people or pets. 

      We recommend avoiding water bodies that are warm and stagnant at any time.