killing algae – The Lincoln Memorial Reflecting Pool—rebuilt as part of Donald Trump’s Washington, D.C., beautification plan for the nation’s 250th anniversary—turned from “American flag blue” to green algae soon after water returned in early June. Crews have tried hydrogen p
For a brief moment, the Lincoln Memorial Reflecting Pool looked like the vision behind Donald Trump’s Washington, D.C., beautification plan: its bottom freshly coated in what he called “American flag blue,” and a patriotic showpiece ready for the country’s 250th anniversary.
Then, early in June, water began to flow back into the pool. Soon after, the blue gave way to green. Algae spread across the shallow basin, and crews moved quickly—trying to knock it back with hydrogen peroxide and other treatments. Efforts are ongoing.
The color shift didn’t stay local. The green pool became an online spectacle, and scientists started weighing in on what the water might be telling them. In an edited transcript. Ashley Bair. a senior research developer at Usalco—an industry maker of coagulants and other water-treatment chemicals—offered a direct read of what’s likely happening and what may be missing.
Bair said she wasn’t surprised by the images. “Honestly, my pool turns green all the time, so it didn’t shock me.” Still, she added that while people might be able to predict this kind of outcome, “there’s no guarantee that it was going to happen.”
Why would algae bloom in a place like the Reflecting Pool?. Bair’s answer is chemical and seasonal. Algae can include eukaryotic algae such as green algae, cyanobacteria—often called “bacterial algae”—or a mix of both. Either way, they need nutrients and the conditions to photosynthesize: light, nutrients, carbon dioxide from the atmosphere, and warmth. “At this time of year, though, there’s plenty of light, and there’s plenty of heat,” she said.
The obvious question is whether the newly painted bottom—meant to be vivid and symbolic—played a role by warming the water. Bair doesn’t buy it as the main trigger. “People are associating the painting of the bottom of the pool as causing more heat in the water,” she said. “And maybe it increased the temperature slightly. but there’s plenty of heat in Washington. D.C. right now to fuel an algal bloom.”.
She points to a historical echo: in 2012, when the pool was refilled, the same green outcome happened—even though it “hadn’t been painted at the time.” That’s why, in her view, the limiting factors for an algal bloom during this part of the year are nutrients rather than heat from a coating.
Those nutrients, Bair said, come down to phosphate and nitrogen. She described cyanobacteria as more likely to be limited by phosphates because “a lot of them are capable of fixing atmospheric nitrogen.”
The pool’s water source matters too. Bair said she understands that the National Park Service converted the pool to fill from the Tidal Basin in 2009. The reason. she said. was cost: it would be cheaper than using the city water supply. particularly when completely refilling the pool at “6.5 million gallons.” Water from the Tidal Basin comes from the Potomac River and can carry natural runoff contaminants.
High phosphate levels, she said, can come from pollution point sources or from agricultural runoff, which she called “a big issue.” The contradiction is stark: if phosphate is the fuel, how would the pool already contain enough of it to sustain a bloom?
In Bair’s view, the key step would be removing phosphate through treatment—specifically, coagulation. “I would have recommended that they do some sort of coagulation treatment to remove the phosphate,” she said. Coagulants, she explained, are metal-based chemicals—either aluminum coagulants or iron coagulants—that form complexes with organics and phosphates. Those complexes precipitate so the phosphates can settle to the bottom, where they can be vacuumed out. In some setups. the water is treated before it enters the pool; then “the water without phosphates” flows over a weir while sludge stays behind at the bottom.
Hydrogen peroxide, which crews have used, is a different kind of intervention. Bair said it can work, but the way it was applied may have doomed it. “They didn’t use enough of it,” she said. And the Reflecting Pool is designed to be still. “It’s stagnant. It’s reflecting,” she said, so peroxide needs to reach algae throughout the basin. “Peroxide doesn’t have time to diffuse into the middle before it’s done. So they only poured it around the edges…. They should have covered it evenly.”.
She emphasized that peroxide-based products can be effective algicides, but they often come as stabilized formulations because peroxide doesn’t last very long. The advantage is what it breaks down into: oxygen and water, which she said are nontoxic. Still, effectiveness can be temporary.
Bair warned about a more troubling effect: when peroxide kills algae. it lyses cells and releases the nutrients the algae were using. “Once you use it, you’re lysing those cells,” she said. “So you’re breaking down the cells. which means phosphates are released back into the water.” In her framing. that can create a cycle where the bloom is killed and then fed again—what she called “bloomageddon.” If peroxide is used without a follow-up phosphate-removal or copper treatment as a static algicide. she said she would expect rebound growth “within a week or two. more likely a week.”.
For many people, the green pool is a photo problem. For Bair, it is a safety problem.
She pointed to surface scums seen in pictures as potentially indicative of cyanobacteria. organisms she said “have the potential to produce cyanotoxins.” Those toxins. she added. “have the potential to be fatal if ingested.” Her message carried a note of frustration at how quickly misinformation had spread. “There was just a lot of misinformation going around,” she said. She wanted the public to understand the risk without turning it into mockery. “It’s important that we understand that this is a serious thing. It’s not just something to laugh about.”.
Even so, she kept her focus on action: “I just wanted to share my thoughts on ‘Hey, this is what I think is happening.’” Her bottom line was simple—if the pool is going to be dealt with, the underlying drivers have to be addressed, not only the visible green.
The Reflecting Pool remains green while crews try to control it. and the debate over what works—peroxide application. nutrient removal. and whether phosphate is being properly treated—hangs over every new round of coverage. The stakes are no longer just aesthetic. If the bloom contains cyanobacteria. the consequences extend well beyond the spectacle of a painted basin turning into a science experiment in public view.
Lincoln Memorial Reflecting Pool algae bloom hydrogen peroxide cyanobacteria phosphate nitrogen coagulants Usalco Tidal Basin Potomac River environmental science water treatment