What does this week's flooding mean for Austin's toxic algae issue?

While Monday's rains might have been a welcome reprieve amid a lengthy drought, the waterfall and flooding could spell some bad news for toxic blue-green algae blooms in city waterways.

AUSTIN (KXAN) — While Monday's rains might have been a welcome reprieve amid a lengthy drought, the waterfall and flooding could spell bad news for toxic blue-green algae blooms in city waterways.

Cyanobacteria, also referred to as blue-green algae, produces toxins that can be lethal for dogs when ingested. The toxins first emerged in 2019 and have been linked to several dog deaths in the years since.

Dr. Brent Bellinger, a senior environmental scientist with the city's Watershed Protection Department, said one working hypothesis is that the October 2018 extreme floods in Austin might have contributed to the emergence of toxic algae due to ecosystem changes.

After heavy rains and flooding, new clays, sand, silt and other muck flowing into a waterway can bring nutrients that fuel cyanobacteria. Bellinger and his team will be taking sediment samples next month to get a better understanding of the possible impacts of the rainstorm.

Currently, the Watershed Protection Department is in the second year of its toxic algae treatment pilot, using a resource called Phoslock that binds phosphorus — a key ingredient feeding toxic algae blooms. Once Phoslock binds phosphorus, it cuts off that nutrient as a food source for toxic algae.

"We know we got hammered pretty good, and Lady Bird Lake got hit hard," Bellinger said. "So we'll discuss what this means in terms of the frequency of our treatments, the timing of our treatments."

Residents might notice the muddy waters now filling Lady Bird Lake, courtesy of all the new clays and silt. They might also see the disappearance of the floating plants growing on the lake.

Known as fanwort, city officials said the plant is critical to cleaning pollutants out of waterways and offering a habitat for native fish and other aquatic life.

With Monday's rainfalls washing many of the plants away, it might make it easier for kayakers and paddleboarders out on the lake in the coming weeks — but it comes at a cost.

"The floating stuff is an eyesore, but you really want to have those aquatic plants present as they are competing for nutrients, they're competing for sunlight, and again, they're creating shade, and they can help stifle the growth of noxious algae," Bellinger said. "You really want to have the aquatic plants being present to compete with the cyanobacteria."

While this weekend's drier forecast and warm temperatures might entice residents to head out onto the lake, Bellinger said this week's rain and flooding has made local waterways a petri dish for bacteria.

"The first week afterwards, because of that influx of fresh organic matter, bacteria, E. coli, other issues in the water can be present now until everything kind of settles back down," he said. "The potential for swimmer's itch, for gastrointestinal issues, that type of thing is really elevated after large rainfall events."