When the Lincoln Memorial Reflecting Pool turned green with algae just days after a US$15 million renovation, the U.S. government scrambled for chemicals and expensive technical solutions to fix the iconic landmark.

Trying to kill algae with chemicals is a common response when community ponds or other water features go green. But as a scientist who studies freshwater ecology, I can tell you there are better solutions that cost far less, last longer and carry less risk of harm to pets and wildlife.

Rather than battling against nature, these alternatives work with nature for long-term solutions.

What went wrong on the National Mall

The algal bloom that turned the Reflecting Pool a vibrant green shouldn’t have been a surprise.

The pool is big, more than a third of a mile long and around 165 feet wide. But it’s shallow, meaning it warms up quickly in the sun. When it was repainted “American flag blue” during the renovations in spring 2026, the new color darkened the pool, and darker colors absorb more heat.

On top of those conditions, the pool was refilled with water from the nutrient-rich tidal basin of the Potomac River. The combination of warm water and nutrients created prime conditions for algae to bloom, turning the water pea soup green.

As the national conversation over the Reflecting Pool shifts to political finger-pointing, an important environmental question deserves careful scrutiny: What is the best approach to maintain water quality in a case like this, whether for a national........

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