In July, the City of Austin confirmed a harmful algal bloom, or HAB, on Lady Bird Lake. HABs are intense levels of algae growth with the potential to harm human health and aquatic ecosystems.

Austin's current bloom is caused by blue-green algae, which can produce neurotoxins and has reportedly killed five dogs that swam in the lake earlier this summer. Because of the reported deaths, the city closed Red Bud Isle and announced that all pets should stay out of Lady Bird Lake until further notice, a ban that is still in effect.

Blue-green algae are actually bacteria, and many types are common and normally present in water bodies, including those in Austin. These bacteria are photosynthetic, meaning they produce their own food using sunlight, as do plants (which explains their misnomer; algae are aquatic plants). The small bacteria can grow into colonies large enough to see.

But certain events cause these colonies to grow much larger and produce enough toxins to become dangerous, sort of like The Hulk.

“Cyanobacteria have always been around. They have a healthy relationship with other species in the water body,” says Eyal Harel, CEO of BlueGreen Water Technologies. “Like lions and zebras, there are healthy numbers of both. But a disruption in the ecosystem causes the bacteria numbers to explode.”

Blue-green algae thrives in hot temperatures, high nutrient levels, and low water flow — basically summer in Austin. Factors that may have contributed to our current bloom include flooding the previous fall, rising temperatures, and nutrient runoff, including from fertilizer use in the Colorado River watershed.

“Once the situation escalates, it continues spiraling out of control from cycle-to-cycle and year-to-year,” Harel says. “Even when a bloom subsides because winter is coming, it’s not going anywhere. The bacteria are not dying, just waiting for conditions next summer. You have to treat the water to bring it back to a healthy balance.”

The incidence of HABs is increasing both in the U.S. and worldwide, according to the U.S. Environmental Protection Agency, due primarily to human activities. Harel says he has seen increases both in numbers of HABs and their scale and severity.

Blue-green algae has caused taste or odor issues with Austin’s drinking water in the past, says Stephanie Lott, spokesperson for the city Watershed Protection Department, but she is not aware of previous incidents similar to the one currently in Lady Bird Lake. The department is currently evaluating the problem.

“I do think it is something we’ll see again,” Lott says. “But we don’t want to do something in knee-jerk reaction fashion that could make things worse.”

The blooms form large scummy mats or streaks on the water surface that tend to be green or blue-green in color, but also can look reddish-purple or brown. Sometimes it resembles foam.

To keep yourself and your pets safe, the department recommends not allowing pets to drink from or swim in Lady Bird Lake or parts of Barton Creek near the lake. People should not touch the algae and should minimize contact with the lake water, although it is probably safe to paddleboard or kayak (swimming is prohibited in Lady Bird Lake by city ordinance). Rinse yourself and your dog immediately after any contact with the water.

If symptoms occur, seek immediate medical attention. Symptoms in people include rash, irritation, swelling, or sores; gastrointestinal or respiratory symptoms; fever; headache; and ear or eye irritation. Those in pets include excessive drooling, vomiting, diarrhea, foaming at the mouth, dark urine, stumbling, loss of appetite and abdominal tenderness, muscle twitches and respiratory paralysis.