Algal blooms in the Lincoln Memorial Reflecting Pool in Washington, D.C., have long been a visible public nuisance. When the pool turned green again on June 15, less than two weeks after President Donald Trump’s US$14 million renovation, it sparked outrage, a rush to use chemical treatments and even claims of sabotage by “radical left lunatics.”
To limnologists like us — scientists who study inland bodies of water such as lakes, rivers and reservoirs — this algae bloom isn’t especially surprising.
The Reflecting Pool is a reminder of a lesson we have learned repeatedly from lakes around the world: algal blooms are usually symptoms of environmental conditions, not the underlying problem itself.
When algae proliferate, public attention naturally focuses on the bright green smelly water covering the surface. But the real story lies beneath. Blooms develop when nutrients, warm temperatures, abundant sunlight and calm water combine to create ideal growing conditions.
This helps explain why blooms are becoming more common in freshwater ecosystems and why they can be so difficult to eliminate.
What is eutrophication?
Like crops growing in a field, algae need nutrients to grow. Although they require more than 20 essential nutrients, in freshwater ecosystems their growth is usually limited by the amount of phosphorus and, in some cases, nitrogen that is available.
(AP Photo/Rahmat Gul)
When excess nutrients enter lakes and ponds through agricultural runoff, urban stormwater, wastewater or decaying organic matter, algae respond quickly. This process is known as eutrophication.
Decades of research have shown that adding nutrients almost invariably leads to more algae. Think of phosphorus and nitrogen as the fuel that feeds the fire. Without them, blooms struggle to develop. With them, algae can multiply rapidly.
But nutrients alone are not enough.
Perfect recipe for a bloom
(Paul Frost)
Algae also need favourable environmental conditions. Warm water speeds up their growth, which is one reason blooms are becoming more common as lakes warm under climate change.
Longer summers, shorter periods of ice cover and more frequent heatwaves all extend the window for algae to flourish.
Sunlight is equally important. Shallow water bodies allow light to penetrate much of the water column, giving algae ample energy for photosynthesis. Calm water provides enough time for algae to accumulate into the dense blooms we see from shore.
Put these ingredients together — nutrients, warmth, sunlight and relatively still water — and blooms become much more likely, if not almost guaranteed.
The Reflecting Pool checks nearly every box. It is shallow, exposed to full sunlight and designed as a decorative landscape feature rather than a naturally functioning lake. As one limnologist colleague put it: If we were trying to design a system to grow algae, it would look remarkably similar to the Reflecting Pool.
Quick fixes rarely last
If scientists understand what causes algal blooms, why is it so difficult to get rid of them?
The answer is that most management strategies target the bloom itself rather than the conditions that allow it to grow.
(AP Photo/Mark Schiefelbein)
Scientists and water managers have developed many ways to suppress or remove algae. These include mechanical removal, algaecides, hydrogen peroxide treatments, aeration systems and newer technologies, such as the nanobubbles used in the Reflecting Pool, that can temporarily improve water quality.
Many of these treatments are highly effective in small, controlled water bodies such as swimming pools, but become much more challenging to apply in larger systems like lakes. Others are unsuitable because they may pose risks to people or harm other aquatic organisms.
Ultimately, however, these treatments only address symptoms rather than the ecological conditions that cause algae to bloom in the first place. Without removing the fuel that feeds the fire — the nutrients algae need to grow — the algae will happily return over and over.
A global problem
The Reflecting Pool has attracted international attention because it sits between two of the world’s most famous monuments. But the ecological processes responsible for its green water are remarkably ordinary.
Across Canada and around the world, communities are confronting similar problems. Harmful algal blooms threaten drinking water supplies, close beaches, reduce fisheries and damage aquatic ecosystems.
In places like Lake Erie, recurring cyanobacterial blooms have become an annual reminder of the consequences of excess nutrient pollution.
(AP Photo/Paul Sancya)
The lesson from the Reflecting Pool therefore extends well beyond a single landmark in Washington.
Algae themselves are not the enemy. They form the foundation of healthy aquatic food webs and produce much of the oxygen in freshwater ecosystems. Blooms become a problem only when environmental conditions allow algae to grow out of balance.
Rather than viewing algal blooms as maintenance failures that require another round of treatment, we should see them for what they are: visible ecological warnings. Like the “check engine” light in a car, they tell us something deeper is happening beneath the surface.
