Though President Donald J. Trump promised “American flag blue” waters, the Lincoln Memorial Reflecting Pool in Washington, DC, quickly turned bright green with algae after it was refilled on June 10 following renovations that cost more than $14 million.
Algae have bloomed in the pool many times since it was built in 1922. In a comment to CNN, the US Interior Department attributed this latest occurrence to “residual algae” in the pool’s supply lines and offered assurances that the algae would soon be eliminated with the help of hydrogen peroxide and an ozone nanobubble system.
But scientists say the conditions that produce algal blooms are hardly mysterious. While warm weather and the new, darker paint job inside the pool are considerable factors, the chemical ingredients also had to be there for the algae to thrive. Without an official identification from the National Park Service, scientists are still debating what is growing in the Reflecting Pool.
“Algae need nitrogen, phosphorus, and light to grow,” says James Jensen, a University of Buffalo environmental engineer with expertise in water chemistry. “Many waters in North America have enough nitrogen and phosphorus so that algae grow readily in the summer, especially in pretty stagnant waters like the Reflecting Pool.”
What nutrients are in the water?
Since the last major renovation in 2012—which was also accompanied by an algal bloom—the water in the pool has come from the nearby Tidal Basin. But managers are supposed to switch to the city water supply when algae levels in the basin become too high, according to Wired. The Interior Department did not respond to inquiries about which source was used during the recent refill.
Many public water systems use orthophosphates (such as PO43–) as corrosion inhibitors to keep lead and other metal contaminants from leaching out of old pipes and into drinking water. Orthophosphates build up an insoluble coating inside pipes and fixtures, acting as a physical barrier to metal ions and as a buffer to keep the water pH between 7.4 and 8.0 to prevent the pipes from corroding.
Orthophosphates can also serve as a nutrient for algae and cyanobacteria. In freshwater environments, phosphorus is often the nutrient that limits biological growth. When additional phosphorus becomes available, algae can multiply rapidly given appropriate light, warm temperatures, and relatively stagnant conditions, like those in the Reflecting Pool in summer. The phenomenon is known as “eutrophication, the enrichment of water bodies with nutrients, that lead to accelerated algal growth,” says Hans Paerl, an aquatic ecologist at the University of North Carolina at Chapel Hill.
Still, Jensen cautions against drawing conclusions without water-quality measurements. “The balance of phosphorus and nitrogen in the water is a crucial factor for algal bloom.” Without knowing actual concentrations of phosphorus and nitrogen, he says, it’s difficult to assess the role of the water source and what treatments are being applied.
Green algae or cyanobacteria?
Based on colors seen in photographs taken shortly after the pool was refilled, Paerl suspects the bloom was initially formed by cyanobacteria, also known as blue-green algae. Some cyanobacteria can obtain nitrogen from dissolved atmospheric nitrogen gas, allowing them to thrive even when biologically available nitrogen is scarce. “Cyanobacteria are more phosphorus limited,” says Dail Laughinghouse, a phycologist, or algae researcher, at the University of Florida.
After obtaining a water sample from the Lincoln Memorial Reflecting Pool on June 17, phycologist Euan Reavie analyzed it under a microscope at 400× magnification. Based on the images, he identified the bloom to be primarily Scenedesmus, or green algae. Credit:
Euan Reavie
Green algae, by contrast, generally rely on dissolved nitrogen compounds, such as ammonium and nitrate, that are already present in water. But open pools have plenty of opportunities for exposure to additional nitrogen, such as from atmospheric deposition or fecal matter from birds, according to Laughinghouse. “It’s all a beautiful soup of nutrients for algae to grow.”
The species distinction matters because some cyanobacteria produce toxins that can threaten people, pets, and wildlife. “It probably needs to be monitored,” Paerl says. “We should be cautious and aware of the potential environmental health problems.”
Green algae, meanwhile, are generally considered more of a nuisance than a health threat. They can discolor water, clog filters, and create unpleasant odors, but they are not known for producing the potent neurotoxins and liver toxins associated with some cyanobacterial blooms.
Although official confirmation of the species has not been made, citizens and scientists alike have eagerly sampled the water. After looking at a few drops with a microscope, Euan Reavie, a phycologist based in Minnesota, says he believes it to be a “community of non-toxic green algae, very likely fed by abundant nutrients from the water source. The stuff isn’t dangerous, but it might be stinky.”
How to address the issue
To control the bloom, federal officials have deployed hydrogen peroxide and an ozone nanobubble system. Both treatments rely on oxidation: hydrogen peroxide damages the algal cells wherever it’s poured, whereas the nanobubbles spread through the water to deliver ozone, which is a powerful oxidizing agent.
Laughinghouse says it’s important to know what species is present in the pool, because hydrogen peroxide can be particularly effective against cyanobacteria but not so much against many green algae, which possess enzymes that help break down peroxide: “If you have a mixed bloom with cyanobacteria and green algae, the latter will mitigate the effect of peroxides, so you need a higher concentration of peroxides, because the two algae are kind of working in tandem.”
Neither treatment, however, removes the nutrients that allow the bloom to form in the first place. “Once the nutrients get into the system, they don’t really leave very easily, particularly phosphorus,” Paerl says. “Nitrogen can be denitrified from nitrate to nitrogen gas, and leave the system that way, but that takes time.”
For that reason, water managers often view algae control as an ongoing maintenance challenge rather than a one-time fix. Removing nutrients, increasing circulation, and repeatedly treating blooms are all common parts of managing ponds, reservoirs, and ornamental water features.
Although Laughinghouse suspects the Reflecting Pool bloom is likely a nuisance rather than a public-health emergency, he hopes the conversation generated by a green pool on the National Mall helps draw attention to more-serious occurrences elsewhere, as harmful algal blooms have previously contaminated drinking-water supplies, poisoned pets and livestock, and closed beaches and fisheries across the US.
Additional reporting by Brianna Barbu.