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. Beyond creating an eyesore at one of the nation’s most recognizable landmarks, the event points to a broader concern, Chapra warned.
“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.”
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 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.
Where do algal blooms like this come from?
Harmful algal blooms occur when cyanobacteria—which are everywhere—get the right combination of warm temperatures, sunlight, and nutrients. These single-celled organisms are among the oldest forms of life on Earth. You know the old saying that cockroaches would survive a nuclear war? Cyanobacteria are the “cockroaches of the water.” They have survived dramatic changes in our planet’s climate over billions of years.
How could climate change make the problem worse?
Though always present in lakes and reservoirs, climate change gives cyanobacteria several competitive advantages.
Cyanobacteria thrive in warm water. As temperatures rise, they continue to grow even as many other algae struggle. Because they are a poor food source for zooplankton, they face fewer natural checks on their populations. They can also float to the surface, where they capture more sunlight while shading competing organisms below.
Climate change is also extending their growing season. Ice covers lakes for less of the year, and warmer temperatures arrive earlier in the spring and last longer into the fall. In some regions, blooms that once did not develop until September now begin as early as April and persist into October.
Why are these blooms a problem?
Not all cyanobacteria are toxic, but many can release toxins that damage the liver or nervous system. People and animals can be exposed by drinking contaminated water or, in some cases, through contact. Dogs are especially vulnerable because they may ingest water while playing in affected lakes and ponds.
Beyond health risks, blooms create surface scums that discourage swimming, boating, and fishing. They also hurt aquatic food webs because most zooplankton and small fish do not eat cyanobacteria.
Why should people who don’t spend time around lakes care about algal blooms?
Most people think of harmful algal blooms as a lake or recreation problem. Increasingly, they are becoming a drinking-water problem as well. In developed countries, we tend to assume that our drinking water will always be safe and usable. But harmful algal blooms threaten those supplies. When cyanobacteria produce toxins, water utilities may need more costly treatment or may even have to temporarily stop using affected water sources.
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This isn’t a hypothetical concern. In 2014, nearly 500,000 residents of Toledo, Ohio, were told not to drink their tap water after toxins from a harmful algal bloom in Lake Erie contaminated the local water supply. Researchers have estimated that lakes and reservoirs providing drinking water to tens of millions of Americans may periodically contain algal toxins.
The bloom in the Reflecting Pool isn’t important simply because it is unsightly. It is important because it illustrates the same environmental processes that increasingly threaten lakes and reservoirs used for drinking water throughout the United States.
How can people help reduce the risk of harmful algal blooms?
Climate change is making harmful algal blooms more likely, but individuals can help reduce the conditions that allow them to thrive. One of the most important steps is limiting the nutrients—especially nitrogen and phosphorus—that wash into lakes, rivers, and reservoirs. Avoid fertilizing before heavy rainstorms (or, better yet, don’t overfertilize because fertilizer that isn’t taken up by plants may eventually wash into nearby waterways), clean up after your dog, and add native plants and rain gardens to absorb stormwater.
It’s also important to pay attention to water conditions. If water is bright green, covered with surface scum, or has a foul odor, avoid swimming, keep children and pets out of the water, and report the bloom to local environmental or public health officials.
Finally, let your representatives know that reducing nutrient pollution is important to you. As they say, “the squeaky wheel gets the grease.”
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