Just days after the Trump administration completed millions of dollars in renovations to the Lincoln Memorial's reflecting pool to turn it blue like the American flag, residents and online users noticed that it had turned phosphorescent green.
Here's why:
The calm, calm waters of Reflecting Pool make it an ideal nursery for algae growth. Algae need nitrogen and phosphorus to grow, and the reflecting pool is fed primarily by the Potomac River, which gets large 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 from Washington, although that event probably occurred too long ago to contribute to the current algal bloom.
Untreated wastewater is high in nitrogen and phosphorus. When nutrient levels are high, the feeding algae can reproduce quickly.
The Department of the Interior said that when the algae first appeared, it was “residual” from the supply lines to the pool.
Experts also speculate that the darker blue color may be helping the reflecting pool absorb more heat. Higher temperatures promote algae growth by allowing their metabolism to speed up.
Summer temperatures in DC don't help. This week, temperatures reach 95 degrees in the city, prompting a heat alert.
This combination likely explains the overgrowth, which turns the water surface a dull green and prevents viewers from seeing the concrete pond's new blue hue.
Algae are important and beneficial organisms when the ecosystem is in balance. They are the base of the aquatic food chain, feeding on herbivores of all shapes and sizes, including shrimp and juvenile fish, which in turn feed organisms higher up the food chain. Single-celled organisms use the power of the sun to produce energy through photosynthesis, similar to the houseplants on your balcony.
In an effort to combat algae in the reflecting pool, National Park Service employees were seen pouring gallons of hydrogen peroxide, a chemical commonly used in pool maintenance.
The Department of the Interior is also employing “high-tech nanobubble ozone technology” to destroy algae cells.
Ozone (yes, the same irritant found in smog) is a gas made up of three oxygen molecules, and the small size of the bubbles allows for the greatest transfer of gas to water, where it can damage algae cells, similar to how it irritates our lungs.
However, this only treats the symptoms. Ozone nanobubbles are generally effective as a temporary solution to algal blooms. Longer-term solutions would have to address what makes the reflecting pool so ideal for algae, such as its depth, darker color, and its contribution of nitrogen and phosphorus.
In California, ozone nanobubbles have also been used in a project to improve water quality in the Tijuana River. The 120-mile river that runs near the border between northern Mexico and southern California was the site of a pilot study in 2025. The U.S. section of the International Boundary and Water Commission reported that the nanobubbles reduced “odors and bacteria,” but the project ended prematurely after a flood washed some of the instrumentation into the river.
