In addition to Iran, inflation, and the courts, President Donald Trump has been fighting a losing war against another opponent these past weeks: pond scum.

As part of his broader effort to “beautify” Washington, DC, ahead of the nation’s rapidly approaching 250th birthday, President Trump wanted to clean up the Lincoln Memorial Reflecting Pool and turn it “American Flag blue.” The 6.5-million gallon pool was drained, lined with a darker blue coating, and refilled — at a cost of more than $16 million.

That effort so far seems to have backfired.

Shortly after the pool was filled earlier this month, algae started spreading in the water, turning the whole thing slime green. The blue liner started peeling off. And even after federal workers dumped jugs of hydrogen peroxide into it, the algae bloom nevertheless persisted.

Can the president clear the pool of algae by July 4? Several scientists I spoke with were skeptical that this microorganism could be exterminated. And if the water does soon appear cleaner, they say it’s only a matter of time before algae returns.

Algae, less affectionately known as pond scum, is a plant-like organism that gets its energy from sunlight. Some varieties are large, such as kelp or seaweed, whereas others are microscopic, comprising just a single cell. And it’s these microscopic ones that bloomed in the reflecting pool. Reporting suggests they were a common variety of non-toxic green algae called Desmodesmus or Scenedesmus.

The thing to know about algae is that they can multiply incredibly fast. A single algae cell can divide two to three times per day, which grows a population exponentially, said Jim Elser, an ecologist at the University of Montana who studies algae. That’s how a waterbody can go from clear to murky in 24 hours, he said.

But for that to happen, the right conditions need to be in place.

Green algae need a few key ingredients to thrive: sunlight; carbon dioxide; warmth; and, crucially, nutrients. And it’s hard to imagine a better place for them than the Reflecting Pool in the summer, at a time when the planet is warming, no less.

For one, the pool has an enormous surface area — roughly the size of 6 NFL football fields — and it’s shallow. That means there’s a lot of space for sun-drenched water. Plus, it’s hot outside, with temperatures in the 80s and 90s.

But what’s even more important for a full-on algae bloom is nutrients — namely, phosphorus and nitrogen, the same elements that we use to fertilize our crops. And the water used to refill the Reflecting Pool is, apparently, full of them.

Water in the pool often comes from the Tidal Basin, a humanmade reservoir connected to the Potomac River. But when that supply is too murky, government officials can fill the pool with DC Water, the city water supply. That’s what they did after the renovation, according to a spokesperson from the Department of the Interior. But even city water can contain algae-fueling nutrients, according to Ashley Bair, a senior research developer at the water treatment company Usalco. Indeed, DC adds a form of phosphorus to its water supply to prevent lead and other heavy metals from leaching out of old pipes and into the tap. So, even a pool fed by city water could fuel an algal bloom if not properly treated, Bair said.

Another helpful (for the algae) source of nutrients is the pool’s location: in the middle of a city, near parks frequented by geese and ducks. Their droppings and those of other wildlife can contribute more food to fuel the bloom, according to Ana “Mindy” Morales, an algae researcher at the University of Vermont. When it rains, water can run off fertilized grass and pick up feces, spilling phosphorus into the pool, which sits lower than the surrounding ground, Morales told me.

While the Reflecting Pool is a dream home for algae, it’s not, in fact, impossible to clean it up.

The Reflecting Pool is equipped with a system that filters and treats water coming in, in part to curtail algae. But it’s imperfect, not always on, and does not appear to filter out nutrients that fed the recent bloom. The Trump administration has tried to kill the algae — with hydrogen peroxide and a technology called nanobubbles, which can damage their cells — and then vacuum it up. But, Elser said, this doesn’t address the root of the problem: the nutrients.

To avoid another bloom and keep the pool clean, water should be thoroughly treated before and after it comes in. They would need to continue to remove nutrients from the water even after it enters the pool, he said, such as by using aluminum compounds that bind to phosphate and can then be filtered out.

“They’re treating the symptom, not the cause,” Elser told me. “The cause is nutrients.”

Bair, of the water treatment company, said she’s confident that she could have cleared up the algae. She’d take out the nitrogen and phosphorus as the water came in; kill the algae with chemicals, such as hydrogen peroxide, as the administration used; and clean them up. Then, regularly monitor the concentration of nutrients over time. That’s possible to do by July 4, she said.

Usually, however, chemical treatments are only temporary solutions, said Alan Wilson, an expert in algal blooms at Auburn University. He offered up another idea: adding in other organisms that eat algae, such as zooplankton. These are essentially freshwater sea monkeys. “If you can promote them, they’ll do their job,” he said. That would be a slower solution, he said. (I also asked researchers about adding snails, which love algae, though they tend to only go after stuff on the bottom and sides and, thus, won’t fix a bloom where the algae is floating throughout the water.)

The Reflecting Pool still has a green-ish hue, though the color appears to be fading, according to my colleague Cameron Peters, who visited it on June 23. He saw a family of ducks standing by the pool’s edge.

And some people are fine with a more natural look to the water, anyway.

“My feeling is that water with algae in it reflects the Washington Monument just as well with water without algae,” Elser said. “You will still get a reflection from a green pool.”