After Treatment, Potentially Harmful Algae Bloom Takes Over Lake Thoreau

Reston Association is encouraging residents to avoid contact with Lake Thoreau after a major algae bloom has taken over parts of the lake.

Lab testing is underway to determine if the algae bloom is harmful. Residents should avoid contact with the water until algae concentrations return to “acceptable levels,” according to a statement released by RA last night (Wednesday).

It’s unclear if the bloom was directly caused by RA’s recent treatment of the lake for Hydrilla, an invasive plant that had taken over roughly 30 percent of the lake. Typically, algae blooms thrive when there are more nutrients available for algae growth.

Some RA members criticized the association for attempting to treat the lake late in the summer season.

“I’m at a loss how the RA dumped a bunch of chemicals into a healthy lake without thinking through the consequences of the outcome,” one RA member wrote on Facebook.

Others called the issue a “man-made” problem.

“The algae bloom is due to the irresponsible decision to treat the entire lake at one time for hydrilla growth very late into the season when temperatures were at an all time high! This is not a natural occurrence but a man-made problem,” an RA member wrote.

Jeannine Santoro said she’s at a loss for how “RA dumped a bunch of chemicals into a healthy lake without thinking through the consequences of the outcome.”

Here’s more from RA told Reston Now on whether the Hydrilla treatment caused the bloom:

Algae blooms can be caused as a result of multiple factors. This includes water temperature, air temperature, amount of nitrogen and phosphorous present in the lake, amount of rain, and runoff from the Watershed that can carry fertilizers. The main sources of nutrients are runoff from the watershed and phosphorous released from the anerobic zone of the lake. Anerobic decomposition releases phosphorous. While the hydrilla may be contributing, it is not the causal factor.

RA acknowledged that summer is not the best time period to treat the lake. But the association wanted to see if the grass carp would impact the Hydrilla plant before using herbicide management methods. The dying hydrilla is expected to sink to the bottom of the lake and decay in the next few weeks.

Harmful algae can cause skin rashes and gastrointestinal illnesses. Anyone concerned about the effects of exposure to a bloom should contact the Virginia Harmful Algal Bloom hotline at 1-888-238-6154.

In previous years, RA stocked more grass carp — a freshwater fish species — to help control the plant. But after the fish proved ineffective, RA hired a contractor to treat the Hydrilla, which has floated to the surface after the July 29 treatment.

RA believes the blue-green algae bloom happened as Hydrilla plant began to die, creating conditions primed for the bloom to thrive.

“The blue-green algae bloom in Lake Thoreau has the potential, if concentrations are high enough, to provide microsystins, which can be harmful to both humans and pets,” RA wrote in a statement.

The decomposing Hydrilla on the surface of the water will sink to the lake bottom and decay within the next few weeks. For this reason, the association is not removing the decomposing hydrilla.

RA currently has no plans to treat the bloom until more appropriate conditions — cooler air and water temperature — occur. Treating the bloom as the Hyrdilla plant dies could compromise the dissolved oxygen levels at the lake and put aquatic life in danger.

In the future, RA hopes to explore better ways to manage aquatic plants on the lake.

One option includes treating the plants easy in the season when they begin to come up. This would require three low-dosage treatments — a decision that must “must be made way before the plants are a problem,” RA said.

RA did not treat the water earlier this year because the grass carp were stocked in 2018.

Photos courtesy Jeannine Santoro and staff