Small clumps of potentially harmful algae are floating on Lake Anne. The Reston Association is warning residents to avoid contact with the water, especially dog owners.

The association issued an advisory Tuesday, Sept. 15, after residents reported spotting algae on the lake's surface. Warning signs have been posted at lake access points around the popular Reston waterway.

Boating and fishing are still allowed.

But the association said residents and visitors should avoid contact with algae and lake water wherever the clumps are visible. Dog owners face the highest stakes: the association identified ingestion of algae by dogs as the primary concern.

Blue-green algae, also called cyanobacteria, can produce toxins dangerous to animals and people. According to the CDC, cyanotoxins have killed dogs, cattle, birds and fish. In dogs, the most common symptoms are gastrointestinal, including vomiting or foaming at the mouth. Signs of illness can appear within minutes to hours.

There is no antidote for blue-green algae poisoning, and it can be fatal, according to Cornell University's Riney Canine Health Center. If a pet may have been exposed, Cornell advises rinsing the animal's fur with fresh water, calling a pet poison control hotline and getting to a veterinary hospital immediately.

This is the second algae alert at Lake Anne in 2026. The Reston Association's watershed team found blue-green algae during routine monitoring on July 21, and that earlier event led to a temporary suspension of stand-up paddleboard rentals. The Sept. 15 advisory does not mention any SUP rental suspension.

Lake Anne is one of Reston's four artificial lakes, which together cover 125 acres and draw residents for boating, fishing and lakeside recreation, according to the association's watershed page. Algae blooms can develop when warm water and nutrients combine, the association said. Most algae species are harmless, but some produce toxins.

The association said it will continue monitoring conditions and provide updates. No timeline was given for lifting the advisory.

Residents who spot algae should report it to the Reston Association's Watershed Department with the location, time, date and photos. Anyone who suspects health effects from bloom exposure can call the Virginia Harmful Algal Bloom Hotline at 888-238-6154.