Just days after the Trump administration completed millions of dollars in renovations on the Lincoln Memorial Reflecting Pool to make it American flag blue, residents and online users noted it had turned a phosphorescent shade of green.
Here is why:
The calm, still waters of the Reflecting Pool make it an ideal nursery for algae growth. Algae need nitrogen and phosphorous to grow, and the Reflecting Pool is primarily fed by the Potomac River, which gets heavy doses of those nutrients from nearby urban and agricultural lands.
The Potomac also absorbed one of the largest sewage spills in U.S. history earlier this year when a pipe burst five miles upstream of Washington D.C., although that event probably happened too long ago to contribute to the algal bloom today.
Other than untreated sewage being generally undesirable in waterways, it is also high in nitrogen and phosphorus. When nutrient levels are high, feasting algae can quickly reproduce.
The Department of the Interior said when the algae first appeared that it was “residual,” from the supply lines to the pool.
Experts speculate that the darker blue color also may be helping the Reflecting Pool to absorb more heat. The higher temperatures promote algae growth by allowing their metabolisms to shift into overdrive.
Summer temperatures in D.C. aren’t helping. This week temperatures are as high as 95°F in the city, prompting a heat alert.
The combination probably explains the excessive growth, turning the water surface opaque green and making sure no one gets to see the new blue hue of the concrete basin.
Algae are important and beneficial organisms, when the ecosystem is in balance. They’re the base of the aquatic food chain, fed on by herbivores of all shapes and sizes, including shrimp and juvenile fish, which in turn feed organisms higher up the food chain. The single-celled organisms use the power of the sun to produce energy through photosynthesis, similar to houseplants on your balcony.
In an effort combat the algae in the Reflecting Pool, employees of the National Park service were seen pouring in gallons of hydrogen peroxide, a chemical commonly used in pool maintenance.
The U.S. Dept of Interior also is employing a “high-tech nanobubble ozone technology” to destroy the cells of the algae.
Ozone — yes, the same irritant that is in smog — is a gas composed of three oxygen molecules, and the small size of the bubbles allow the most gas transfer into the water where it can damage algal cells, similar to how it irritates our lungs.
However, this only treats the symptoms. Generally, ozone nanobubbling is effective as a temporary solution for algae blooms. Longer term fixes would have to address what makes the Reflecting Pool so ideal for algae, such as its depth, darker color and inflow of nitrogen and phosphorus.
In California, ozone nanobubbles also have been used in a project to improve water quality in the Tijuana River. The 120-mile river that runs near the border in northern Mexico and Southern California was the site a pilot study in 2025. The United States Section of the International Boundary and Water Commission reported that the nanobubbling reduced “odors and bacteria,” but the project ended in a “premature conclusion” after a flood swept some of the instrumentation into the river.
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