Warmer Weather Fueling Development of Green Algae in Lincoln Memorial Reflecting Pool

An unwelcome visitor is crashing the nation's 250th birthday on the National Mall. Read on for the details about the invasive green algae that has taken over the Lincoln Reflecting Pool, and what is causing it to bloom and spread.

Algae Blooms Overtake Lincoln Reflecting Pool

National Park Service (NPS) crews are working overtime to control a massive algae bloom that has invaded the newly remodeled Lincoln Reflecting Pool. The pool that sits between the Lincoln Memorial and the World War II Memorial on the National Mall was recently refilled after receiving a new paint job. According to the Trump administration, the pool was drained so that the surface could be painted "American flag blue."

However, the 2,030-by-167-foot pool now appears green thanks to the sludge that has overtaken the blue surface. University of Maryland Center for Environmental Science Prof. Patricia Glibert is an expert on algae and what fuels its growth. According to Glibert, algae growth is a natural process that is found all over the world, noting that "every body of water from the smallest pond to the biggest ocean has algae."

While a reflecting pool is different than a natural body of water, the same scientific principles apply to how the algae blooms form and spread. Glibert hypothesizes that the algae bloom got started when crews began to refill the newly painted pool in June. She said that, "As the pond was refilled, it probably had residual water in some of the infrastructure, and algae was probably residual in that water."

The timing of the pool's remodel is another factor that is supporting the algae. The pool was initially drained in April. It took weeks for the surface to be repainted. This means that the refilling process happened just as toasty temperatures were starting to dominate the weather in Washington, D.C. The nation's capital hit a sizzling high of 100 degrees on June 12 at the hands of an early-season heat wave.

Glibert believes that the soaring temperatures helped to contribute to the amount of algae. The new water also introduced enough nitrogen or phosphorus to act as fertilization for the algae.

The lingering heat has resulted in the faster spread of the algae. Algae coverage can double in the span of a day during times of sunshine and warmth. It is easy to see how the bloom has found the right ingredients to spread throughout the pool.

Lastly, the darker blue hue of the remodeled pool may also be playing a role in the algae's growth. This is because darker colors inherently help to warm the water.

President Donald Trump has been relatively quiet about the growing issue. A spokesperson for the Interior Department used the occasion to mock past administrations, saying that “Unlike under Obama and Biden, the National Park Service is actually maintaining the beautifully completed Reflecting Pool."

What is Next for the Famous Site of Reflection?

The Lincoln Reflecting Pool is one of the most recognizable sites at the National Mall. Crowds routinely gather around the pool during events in the heart of D.C. With the nation's big 250th birthday celebration right around the corner, the NPS is working hard to get the algae blooms under control.

Crews are using water pumps and skimmers in an effort to remove the algae. These tools are also being used to push the algae closer to an aeration area located in the middle of the pool.

Glibert warns that "killing the algae is the easy part." The challenge comes in getting rid of the algae using methods that do not lead to more issues. Every solution comes with a potential trade-off that could complicate the removal.

For example, the pool's filtration system needs to be able to remove the biomass completely. The last thing that you want is a slimy substance that decomposes and rots in the pool. Should this happen, the leftover nutrients are able to jumpstart a new bloom cycle.

Any residual algae can settle in the form of scum at the bottom of the pool. These remnants could feed another cycle of algae growth if it is not completely removed.

For now, crews are working to restore the pool by removing the algae without leaving behind the nutrients that could spark another bloom. The Interior Department said that the NPS was relying on “high-tech nanobubble ozone technology” to kill the algae and other contaminants. In addition, the NPS is using hydrogen peroxide to treat the pool, a treatment that is not harmful to marine life or the environment.

The clock is ticking as July 4 inches closer.