Lake Thoreau Algae Bloom Expected To Disappear In 2-3 Weeks

RESTON, VA — Lake Thoreau's algae bloom should disappear within two to three weeks, according to the man in charge of maintaining Reston's waterways and the company monitoring water quality.

"Aquatic Environment Consultants was out doing their water quality monitoring on Monday," said Larry Butler, chief operating officer of Reston Association, who has been managing Reston's lakes for over 20 years. "Our watershed staff also did a little bit of sampling."

Butler told RA CEO Hank Lynch in a new video posted to YouTube that if they treated the algae bloom right now and it died back quickly, there could be a severe oxygen depletion in the water that could kill the fish in the lake.

"You have to be nimble in your response to those things," Butler said.

In the past, RA has used Grass carp as a way to control the algae, but Butler said in the future they may consider using an aluminum sulphate treatment, which precipitates phosphorus out of the water.

The current algae bloom in Lake Thoreau is nothing new. Blooms occur almost every year in the community's four lakes, Butler said. Lake Thoreau even had a small bloom last year.

"When conditions are right, regardless of the lake, if you've got the right amount of nutrients — phosphorus and nitrogen, primarily — and ample sunlight and heat, which certainly we had that in July with 23 or so days over 90 degrees, it creates a situation for an algae bloom," he said.

The blue-green algae currently has an advantage over all the other algae in Lake Thoreau, because it fixes atmospheric nitrogen, Butler said. Provided there's enough phosphorus in the water from roadway runoff and fertilizers, the blue-green algae will bloom.

Lake Thoreau's algae bloom followed on the heals of contractor Aquatic Environment Consultants using an aquatic herbicide on July 29 to treat the lake for Hydrilla.

An aggressive, invasive species, the Hydrilla could've been brought into Reston's waterways in the digestive system of a waterfowl or on the propeller or bottom of a boat that had been in the Potomac River.

"Once it gets in, it establishes really quickly," Butler said. "It could grow inches per day. If it breaks off, those fragments could reroot. It has a competitive advantage over the native plants that we want on the lake."

Aquatic Environment Consultants did not treat all of the Hydrilla in Reston's four lakes. The goal, according to Butler, it to maintain a balance. Ideally, the aim is keep plants on about 20-30 percent of a lake's bottom.

RA will be hosting an environmental health virtual meeting via Zoom on Monday, Aug. 31, at 6:30 p.m., to answer address community concerns and answer questions about the algae bloom.