What D.C.’s Algal Bloom Reveals About a Growing Water Threat

When bright green water appeared in the newly renovated Lincoln Memorial Reflecting Pool, it drew national attention and sparked political finger-pointing. The culprit: cyanobacteria—sometimes called blue-green algae—a type of bacteria that can produce toxins harmful to people, pets, and wildlife.

Environmental engineer Steven Chapra, a research professor and Louis Berger Chair in Civil and Environmental Engineering, Emeritus at Tufts University School of Engineering, was not surprised.

“The three things that make algal blooms grow fast are nutrients, sunlight, and warm water,” said Chapra. “The Reflecting Pool has a shallow basin with little water movement. It is fed by nutrient-rich water from the Potomac River through the Tidal Basin, receives full sun, and now has a dark-blue bottom that absorbs heat. If you wanted to grow algae, that’s a perfect recipe.”

Beyond creating an eyesore at one of the nation’s most recognizable landmarks, the event points to a broader concern, Chapra warned.

Chapra led a 2017 study published in Environmental Science & Technology that projected climate change could raise the number of harmful algal blooms in the contiguous United States between two and more than five times their then-current level. The study predicted the largest increase in the Northeast. 

Chapra spoke with Tufts Now about what happened in the Reflecting Pool, why harmful algal blooms are a growing concern, and what they may mean for freshwater ecosystems.

What is happening in the Reflecting Pool?

The pool has always had some green algae growing on its bottom. What’s different now is that a large algal bloom has reached the surface. The primary reason is that it is filled with nutrient-rich water containing enough phosphorus and nitrogen to support rapid algal growth. The dark blue bottom likely compounds the problem because it absorbs more heat during the day and releases it slowly at night, keeping the water warmer around the clock. Since both cyanobacteria and green algae reproduce more rapidly in warm water, these conditions create an ideal environment for blooms. 

Can the issue be fixed?

I don’t think this would be a difficult or expensive problem for a good environmental engineer to solve. Recirculating and filtering the water could lower the nutrients to levels that would limit algal growth. This could be done using good engineering design principles without creating water jets strong enough to disrupt the pool’s reflective surface. Repainting the bottom a lighter color also would make blooms less likely.