If you're a frequent visitor to a lake or river, at some point you may have encountered something weird happening in the water. It might be a green film on the surface of the water or even a fish kill. What causes the green film, and why are the fish dying?
It's likely the cause comes from some type of algae bloom in the water. Many types of algae occur naturally in all surface water. Some can be a simple nuisance, and some can even be toxic.
The harmful algal bloom is a sudden, massive growth of microscopic and macroscopic organisms that develop in surface water. They can occur in warm freshwaters, marine waters or brackish, salty waters. They are frequently associated with bodies of water that contain plenty of nutrients, often caused by lawn fertilizer runoff.
A harmful algae bloom occurs when an alga capable of producing toxins grows rapidly. Sometimes, those toxins can cause harm to aquatic organisms, animals, or people.
Harmful alga 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.
Here's what you need to know about a variety of harmful algae blooms that can be found within the Brazos River Basin.
Blue-green algae (cyanobacteria)
Sometimes blue-green algae is referred to as "pond scum" and isn't even a true algae because they have no membrane-bound nucleus and have no chloroplasts. Very few organisms consume cyanobacteria . There are more than 2,600 species of blue-green algae and most do not produce toxins.
There are two types of blue-green algae known to occur in Texas that can produce toxins: Anabaena sp. and Microcystis sp.
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.
Other environmental concerns that can be caused by blue-green algae include discolored water, reduced light penetration, and dissolved oxygen depletion.
When some blue-green algae end their lifecycle, they can emit an oily substance called geosmin, which has a distinctive earthy taste that humans can detect, even in small concentrations.
The woody, musty, earthy taste that shows up from time to time in our drinking water is normally due to the die-off of a portion of the algae population. While the taste can be annoying, it does not pose a health hazard.
There have not been any recorded toxic blue-green alga blooms in the Brazos River Basin in more than 25 years. Jenna Olsen, Brazos River Authority’s environmental programs manager says, “Please note though, that if you’re on a water body, you likely will be able to tell there is an algal bloom by looking; however, you won’t be able to tell if it is toxic unless it is professionally analyzed. If you see water with a “pea-soup” color and consistency, just use common sense and avoid it.”
You can report any blue-green algae to the Harmful Algal Bloom (HAB) tracking system by visiting https://arcg.is/14COnK. You can learn more about blue-green algae here.
Golden Algae (Prymnesium parvum)
Golden alga is unique from other alga found in the basin in that occasional blooms can be toxic to fish, mussels, and clams.
Though golden algae can occur in all types of water bodies, harmful events typically occur in brackish or salty waters. It is the only species of algae in the Brazos River basin to have caused documented toxic events.
The toxins produced by golden alga are not known to be harmful to humans, livestock, or wildlife. It characteristically appears as brownish or tea-colored water.
While golden alga is frequently present in the waters of the Brazos River Basin in low concentrations, it is most likely to bloom, causing fish kills, usually during cold weather events. A golden alga fish kill may last for days, weeks, or months, and may affect whole water bodies or isolated portions of water bodies.
In 2003 and 2005, the freshwater organism nearly devastated some fish populations in Possum Kingdom Lake, Lake Granbury, and Lake Whitney in the Brazos basin and many other Texas lakes outside the basin. The most recent reports of fish kills happened in 2022 and 2023.
Indicators of toxic golden algal blooms include:
- Water that appears to be tea-colored.
- Impacts on small fish occur first, with larger fish impacted as the duration of the toxic event continues, expands in area, or increases in intensity. All species of fish can be affected.
- Fish may be gulping for air at the water's surface because the toxin attacks their gills and the gills no longer function appropriately.
- Fish swimming in irregular patterns or manners often appear to be intoxicated.
- Hemorrhaging is visible in the fish. It is first noticeable in the gills, head, and base of the fins, then moves into the tissue, giving the fish a pink to red appearance.
Prompt notification is key to successfully determining the cause of a fish kill. When there is a report of dead fish in the Brazos River Basin's reservoirs or rivers, the BRA's environmental team will assist the Texas Parks and Wildlife Department's Kills and Spills Team in documenting the event and investigating the cause.
You can report golden algae blooms by providing information on what you've seen through our HAB tracking system. You can find the reporting system here: https://arcg.is/14COnK. You can learn more about harmful golden alga blooms here.
To see all golden alga bloom reports from December 2018 to March 2024 click here.
Red Tide Algae
Red Tide alga blooms occur mostly in the Gulf of America, where the mouth of a river meets the Gulf, mixing it with salt water.
Fish and shellfish that are contaminated with red tide toxins can cause illness in humans and pets. Swimming in red tide-infested water may irritate the skin, and breathing airborne toxins can lead to respiratory problems.
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.
Prevention
To prevent exposure to harmful algal blooms, stay out of discolored, scummy, or smelly water and keep pets away. Don't swim in water where dead fish are found.
Runoff is a contributing factor to the growth of some algae blooms. The best way to prevent the formation of the organisms is with watershed management. Strategies that limit the use of nutrients that feed algae in areas surrounding lakes, rivers, and streams.
Lake-side homeowners can be mindful of the chemicals they add to their lawns through fertilizer and weed control. Heavy rains cause excess chemicals to be washed directly into the lake and those chemicals can provide nutrients to the algae growing in the lake.
Use only the recommended amount of fertilizers on your farm, yard, and garden. This will reduce the amount of nutrients running off into nearby water bodies. Nutrients in the water can help harmful algal blooms grow.
Maintain your septic system to keep wastewater from getting into nearby bodies of water. Wastewater is full of nutrients that can feed harmful algal blooms.