After blue-green algae samples collected in some Highland Lakes tested positive for toxins that can be fatal to dogs, the city of Austin is raising its risk level on Lady Bird Lake and Lake Austin from “low risk” to “increased risk.”

The harmful algae killed five dogs in Austin in 2019, and in the summer of 2020 samples taken from Lady Bird Lake also tested positive for the toxins. While there have been no positive samples yet in 2021, staff from the Austin Watershed Protection Department said they have seen accumulations of algae on Lady Bird Lake and that they are arranging to take samples. Samples from Lake Austin have already been sent to The University of Texas for testing.

The Lower Colorado River Authority took samples in March that showed the toxins released by the blue-green algae were present in multiple areas throughout the chain of Highland Lakes, including areas around Lake Travis, Inks Lake and Lake Marble Falls. The blue-green algae is most common in warmer times of the year. In a news release, Brent Bellinger, an environmental scientist with the watershed protection department, said the specific times when the algae produces toxins are still under investigation.

“With the exception of the winter storm, we had a fairly warm winter this year, which probably contributed to the toxicity concerns we have seen so far in the Highland Lakes,” Bellinger said in the release. “We will continue to track algae quantity and toxicity through the spring as we get ready for the summer bloom season.”

Dog owners in Austin are cautioned to let their dogs swim in Lady Bird Lake or Lake Austin at their own risk. Swimming in Lady Bird Lake has been banned for humans since 1964.