Every summer, it seems like another lake, reservoir, or water feature turns bright green and makes the news.
The explanation is usually some combination of hot weather, sunshine, or stagnant water. Those conditions certainly help algae grow, but they aren't the reason blooms occur in the first place.
This summer, the algal bloom in the Lincoln Memorial Reflecting Pool sparked a lot of discussion about what caused it. Was it the residual heat from the recently painted bottom? Was it fertilizer in the newly refilled water? Was it the source of the water itself?
Those are reasonable questions, but they all point back to a more fundamental one: why were there nutrients available for algae to grow at all?
Whether we're talking about a drinking water reservoir, a recreational lake, an ornamental pond, or the Reflecting Pool, the same basic principles apply. Warm temperatures and sunshine create ideal growing conditions, but nutrients determine how dense a bloom can become.
If you want to understand algal blooms, start with phosphorus.
Why Phosphorus Matters
Like every living organism, algae need nutrients to build new cells — carbon, nitrogen, phosphorus, and a range of trace elements. In most freshwater systems, phosphorus is the nutrient that runs out first, which makes it the primary limiting nutrient for algal growth.
Once enough phosphorus becomes available, algae reproduce remarkably quickly. A bloom that seems to appear overnight has often been building beneath the surface for days as conditions turn increasingly favorable.
Heat and sunlight don't create blooms on their own. They simply let algae take advantage of nutrients that are already present.
Where Do Those Nutrients Come From?
When people think about nutrient pollution, agriculture is often the first thing that comes to mind — and for good reason. Fertilizer runoff is an important source of phosphorus in many watersheds.
But it's far from the only one. Stormwater, wastewater discharges, groundwater, decaying vegetation, animal waste, and even dust and pollen can all add nutrients over time. In shallow water bodies, phosphorus also accumulates in bottom sediments, creating a long-term reservoir that can keep feeding phosphorus back into the water column for years as conditions favor its release.
That's one reason bloom management is so challenging. Even after you reduce one source of nutrients, others often follow.
Two Ways to Manage Algae
From a treatment perspective, there are really two approaches to managing algal blooms: reactive and proactive.
The reactive approach kills or suppresses algae after they start growing. Algaecides such as copper sulfate or peroxide-based formulations are valuable tools for rapidly reducing biomass — particularly when harmful cyanobacteria produce toxins that create an immediate public health concern or cause treatment challenges at drinking water plants.
The proactive approach prevents the growth from taking hold at all. One of the most effective ways to do that is to remove the nutrients that let a bloom develop.
Both strategies matter, but they accomplish very different things. Eliminating algae doesn't solve the underlying problem if nutrients remain available. When algae cells die and break apart — a process called cell lysis — they release nutrients back into the water. It only takes a small number of surviving cells to restart a bloom, and those cells are constantly reintroduced by wildlife, sediments, wind, rain, and surrounding water. Given enough sunlight, warm temperatures, and available nutrients, they'll multiply again.
Removing the nutrients, on the other hand, keeps a dense bloom from becoming established in the first place.
How Aluminum-Based Coagulants Help
This is where aluminum-based coagulants become such an effective management tool.
Unlike algaecides, coagulants don't rely on killing algal cells. Their primary role in lake management is to reduce the bioavailable phosphorus that fuels future growth. At Usalco, this is the chemistry behind much of our lake, pond, and wastewater work — including UltraPHOSx™, engineered specifically for phosphorus removal, along with sodium aluminate and aluminum sulfate.
When an aluminum-based coagulant is added to water, it binds with dissolved phosphorus and forms particles that settle to the bottom. That pulls phosphorus out of the water column, where algae would otherwise use it as food, and it can create a protective layer over the sediments that helps keep phosphorus from releasing back into the water. Less bioavailable phosphorus means less fuel for future algal growth.
These same principles have been used in wastewater treatment for decades to reach extremely low phosphorus concentrations, and they're widely applied in lakes and reservoirs to reduce both dissolved phosphorus and sediment release. Rather than reacting to blooms after they appear, phosphorus removal addresses one of the underlying conditions that lets them form.
What the Reflecting Pool Really Tells Us
One question I was asked repeatedly after the Reflecting Pool bloom was whether the pool should have been refilled with water from the Tidal Basin or with treated drinking water.
From a nutrient perspective, the answer matters less than most people expect. Both sources can contain phosphorus.
Water from the Tidal Basin in Washington, D.C., which is commonly used to refill the Reflecting Pool, receives nutrients from the Potomac watershed. Treated drinking water, meanwhile, commonly contains orthophosphate for corrosion control. Orthophosphate is an important and safe drinking water additive that helps prevent lead from leaching out of pipes, and it's reduced to environmentally safe levels by wastewater plants before discharge. But it's still a form of phosphorus that algae can readily use if it enters a system where they're able to grow.
The Reflecting Pool simply offered nearly ideal conditions for algae this summer. It's shallow, gets abundant sunlight, has very little water movement by design, and doesn't maintain a disinfectant residual the way most backyard pools do. If nutrients are present, algae will take advantage of them — and because the entire pool was refilled at once following renovations, plenty of nutrients were available to fuel a bloom.
Looking Ahead
Algal blooms will never disappear entirely. Natural waters are dynamic systems, and nutrients are constantly entering from both human and natural sources.
But bloom severity isn't inevitable. By reducing bioavailable phosphorus, we reduce the fuel available for algal growth. That's why phosphorus management has become one of the cornerstones of lake restoration, wastewater treatment, and drinking water source protection.
Summer will always bring sunshine and warm water. Whether those conditions produce a nuisance bloom depends, in large part, on the nutrients we've allowed to accumulate — and on how proactively we manage them.
If you're working out how to get ahead of phosphorus in a lake, pond, or wastewater system, that's exactly the kind of problem our team solves every day.
Ashley Bair, Ph.D., is a Senior Research Developer at Usalco, where she leads research and innovation efforts focused on water treatment chemistry, phosphorus removal, coagulant optimization, and digital decision-support tools for water quality management. Usalco is the essential catalyst for clean water, serving lake and pond management, municipal drinking water, wastewater, and industrial customers across North America.
What you need to know.
Does hot weather cause algal blooms?
Not by itself. Heat and sunlight create ideal conditions for algae to grow, but nutrients — especially phosphorus — determine whether a bloom develops and how dense it becomes. Warm water only becomes a problem when the fuel is already there.
What is the main cause of algal blooms?
In most freshwater systems, phosphorus is the limiting nutrient, so it's the primary driver. It enters the water from fertilizer runoff, stormwater, wastewater, decaying vegetation, animal waste, and even bottom sediments that release stored phosphorus back into the water column over time.
Nitrogen can also be limiting for many types of algae, but many species of blue-green algae (cyanobacteria) can fix atmospheric nitrogen, giving them opportunity in even nitrogen-depleted environments. Nitrogen is also very challenging to remove or sequester deliberately. For these reasons, phosphorus is the best nutrient to manage.
Is treated drinking water safe to use in a pond or fountain?
Yes — but it isn't nutrient-free. Treated drinking water commonly contains orthophosphate for corrosion control, which is a safe and important additive that helps keep lead from leaching out of pipes and fixtures. It's still a form of phosphorus that algae can use, so any shallow, sunlit, slow-moving water body can bloom regardless of its water source.
What's the difference between algaecides and aluminum-based coagulants?
Algaecides are reactive — they kill or suppress algae after growth begins, which is valuable when toxin-producing cyanobacteria create an immediate health concern. Aluminum-based coagulants are proactive: instead of killing cells, they bind dissolved phosphorus and remove it from the water, reducing the fuel available for future blooms. Both have a role, but only phosphorus removal addresses the underlying cause.
How do aluminum-based coagulants remove phosphorus?
When added to water, an aluminum-based coagulant binds with dissolved phosphorus and forms particles that settle to the bottom. That pulls phosphorus out of the water column and can form a protective layer over sediments that limits phosphorus from releasing back into the water. Usalco products like UltraPHOSx, sodium aluminate, and aluminum sulfate are used for exactly this purpose in lakes, ponds, and wastewater systems.
