Do Phosphates Cause Algae Growth? A Full Explanation

Phosphates are one of the most potent triggers of algae growth in lakes and rivers, and in most freshwater systems they are the single nutrient that determines whether algae bloom or stay in check. This connection has been demonstrated repeatedly since the 1970s through large-scale experiments that essentially turned whole lakes into laboratories. The relationship is not always straightforward, though, because the form of phosphorus, the nutrient balance already present, and even what is buried in the lake bottom all shape how a water body responds to phosphorus loading.

The Experiments That Made It Clear

The most famous evidence comes from the Experimental Lakes Area in northwestern Ontario, Canada, where researchers spent decades fertilizing entire lakes with different nutrient combinations. In one landmark experiment, Lake 226 was physically divided in half with a curtain. One side received carbon and nitrogen, while the other side received carbon, nitrogen, and phosphorus. The side that got phosphorus developed a heavy algae bloom. The other half did not.¹ That visual contrast, captured in an aerial photograph that became iconic in environmental science, drove home the point that phosphorus was the critical ingredient.

The companion lake in those experiments, Lake 227, was fertilized with phosphate and nitrate for years and saw enormous increases in phytoplankton, even when dissolved carbon was low. When researchers later stopped adding phosphorus to other test lakes but kept adding nitrogen and carbon, the blooms disappeared almost immediately.² That rapid recovery was striking: it suggested that controlling phosphorus inputs could produce visible results in a relatively short time frame.

The experiment on Lake 227 eventually ran for 37 years, making it one of the longest ecological experiments ever conducted. During the final 16 years, the lake received phosphorus alone with no added nitrogen at all. Algae kept blooming anyway, because certain species of cyanobacteria were able to pull nitrogen from the atmosphere through a process called nitrogen fixation, supplying what they needed to keep growing as long as phosphorus was available.³ The conclusion was blunt: you cannot stop freshwater algae blooms by cutting nitrogen alone. Phosphorus has to be addressed.

Why Phosphorus Is Usually the Bottleneck

In most freshwater lakes, phosphorus is the nutrient in shortest supply relative to what algae need. Algae require both nitrogen and phosphorus (among other things) to grow, but freshwater environments tend to have more nitrogen available than phosphorus. So phosphorus acts as the bottleneck: add more of it, and the algae population expands to match.

This is not universally true. In temperate coastal seas, nitrogen tends to be the limiting nutrient rather than phosphorus. The difference comes down to water chemistry. In saltwater, high sulfate concentrations change how nutrients cycle through sediments, making phosphorus relatively more available and nitrogen relatively scarcer.⁴ This means that what drives algae in a lake and what drives algae in a coastal bay can be fundamentally different.

Even within freshwater systems, things can vary. In highly polluted lakes where both nitrogen and phosphorus are very abundant, the ratio between them starts to matter more than the absolute amount of either one. Research on Lake Taihu in China, a hyper-eutrophic lake with massive cyanobacterial problems, found that adding nitrogen alone stimulated algae growth, but adding only phosphorus did not, because the lake’s nitrogen-to-phosphorus ratio was already low enough that nitrogen was the more scarce resource.⁵ When both nutrients were added together, the resulting bloom was larger than from nitrogen alone. So for badly degraded lakes, the picture can be more complicated than “just reduce phosphorus.”

The ratio at which algae grow fastest depends on what species you are talking about and what concentrations are involved. For Microcystis aeruginosa, one of the most common bloom-forming cyanobacteria, experiments have shown that growth peaks at a nitrogen-to-phosphorus ratio around 16 when nitrogen is fixed at a certain level, but can shift to a ratio of 40 when phosphorus is the fixed variable.⁶ The practical implication is that nutrient management often needs to target both phosphorus and nitrogen, even in freshwater, depending on local conditions. The stoichiometric balance between nutrients also influences which algae species dominate, with imbalances in certain river basins favoring cyanobacteria over less problematic groups like diatoms.⁷

Not All Phosphorus Behaves the Same Way

When people talk about “phosphates” causing algae growth, they usually mean dissolved inorganic phosphate, specifically orthophosphate, the form that algae can absorb directly. But phosphorus in waterways exists in multiple forms, and the distinction matters for both monitoring and management.

Dissolved organic phosphorus is a second pool that many bloom-forming species can also tap into. Microcystis aeruginosa, for instance, can grow on both orthophosphate and dissolved organic phosphorus, even when overall phosphorus levels are low.⁸ However, the two forms do not do the same thing inside the cell. Research comparing them found that cells growing on organic phosphorus absorbed less phosphorus overall and shifted their internal metabolism toward protein synthesis and stress responses rather than pure growth. Inorganic phosphorus, by contrast, was used more directly for cell division.⁹ This means that measuring only total phosphorus in a lake can be misleading. The form of phosphorus present shapes both the ecological risk and which management strategies will actually work.

How Algae Stockpile Phosphorus

Algae do not just use phosphorus at a steady rate; they can gorge on it when it is available and store the surplus for later. When algae that have been starved of phosphorus suddenly encounter a fresh supply, they absorb it at roughly ten times their normal rate in a rapid burst lasting an hour or two. During that binge, their internal phosphorus content can climb to around 5% of their dry weight, stored as chains of polyphosphate molecules inside specialized compartments.¹⁰ Cell division slows during this hoarding phase, then ramps up afterward as the stored phosphorus is spent.

This ability is known by two names depending on the trigger. When algae overload on phosphorus after a period of starvation, it is called the overplus response. A related phenomenon, called luxury uptake, occurs when algae accumulate extra phosphorus even without prior starvation, often triggered by some other nutrient running out.¹¹ Both mechanisms mean that a single pulse of phosphorus into a water body, from a rainstorm washing fertilizer off a field, for example, can feed algal growth for longer than you might expect from the brief spike in water-column phosphorus levels.

Where the Phosphorus Comes From

Agriculture is the dominant source in most watersheds. Animal manure and synthetic fertilizers applied to cropland contain large amounts of phosphorus, and rain can wash a significant fraction of it into nearby streams. In runoff experiments, dissolved reactive phosphorus from surface-applied manure and fertilizer accounted for about 64% of the total phosphorus leaving the field, compared to only 9% from unamended soils.¹² The losses are strongly tied to how the fertilizer is applied. Injecting swine manure into the soil rather than spreading it on the surface reduced dissolved reactive phosphorus in runoff by about 93% and total phosphorus loads by roughly 94%.¹³

Urban and industrial wastewater is the other major pathway. Wastewater treatment plants have become much better at removing phosphorus over the past several decades, and bans on phosphate-containing laundry detergents, enacted in many countries starting in the 1970s and 1980s, removed a significant source. But treated effluent still contributes phosphorus, particularly in densely populated areas where treatment does not include a dedicated phosphorus-removal step.

The Trouble Buried in the Sediment

Even after external phosphorus inputs are reduced, lakes can keep experiencing blooms because of phosphorus that accumulated in their sediments over years of pollution. Sediments act as a bank: phosphorus binds to iron and other minerals in the lake bottom, and under certain conditions, it gets released back into the water column. This internal loading can sustain high phosphorus levels long after the external source is eliminated.¹⁴

The key trigger is dissolved oxygen. When the water above the sediment contains oxygen, iron minerals hold phosphorus tightly. But when that water goes anoxic, typically during summer when warm temperatures increase microbial oxygen consumption and thermal stratification cuts off deep water from atmospheric resupply, iron minerals release their phosphorus. Research in Lake Erie’s central basin found that phosphorus flux from sediments under normal conditions was slow, but within 24 hours of the water going fully anoxic, the release rate jumped dramatically, reaching levels ten to a hundred times higher.¹⁵ A study of a drinking water reservoir confirmed the same pattern: warm temperatures and low oxygen promoted phosphorus release from sediments, while cooler and well-oxygenated conditions kept it locked in place.¹⁶

This creates a vicious cycle. Algae blooms deplete oxygen as they decay, which triggers sediment phosphorus release, which feeds more algae growth. Breaking the cycle requires getting phosphorus concentrations low enough that the oxygen-depleting blooms stop forming in the first place.

Legacy Phosphorus and the Slow Road to Recovery

The accumulated phosphorus from decades of agricultural overapplication and industrial discharge has a name among scientists: legacy phosphorus. Soils in intensively farmed areas have been loaded with phosphorus well beyond what crops can use. That surplus slowly leaches into groundwater and erodes into surface water for years or decades after fertilizer application stops or is reduced. This means that water quality improvements from better land management can take frustratingly long to materialize.¹⁷

The timescales involved vary widely depending on the watershed. In some cases, decades of legacy phosphorus continue to flow from soils and modified drainage systems into receiving waters, undermining restoration efforts even when current practices have improved.¹⁸ This lag effect is one of the most common reasons that phosphorus-reduction programs appear to fail in their early years, discouraging the public and policymakers who expect visible improvement soon after regulations take effect.

Lakes themselves can exhibit hysteresis, meaning they do not recover along the same path they degraded. Lake Veluwe in the Netherlands provides a striking example. The lake lost its clear, plant-dominated state in the late 1960s when total phosphorus rose above roughly 0.20 milligrams per liter. But the clear-water state did not return until phosphorus dropped below about 0.10 milligrams per liter, half the concentration that triggered the collapse.¹⁹ In other words, undoing the damage required much cleaner water than would have been needed to prevent it in the first place. Multiple shifts between different ecosystem states occurred over 30 years of recovery.

When Blooms Turn Dangerous

Not all algae blooms are just an eyesore. When phosphorus enrichment favors cyanobacteria, which are the group most likely to dominate in nutrient-rich freshwater, the consequences extend to public health. Cyanobacterial blooms can produce a range of toxic compounds called cyanotoxins, and when the bloom eventually dies and decomposes, it depletes dissolved oxygen, sometimes killing fish and other aquatic life.²⁰ Contact with or ingestion of water during a toxic bloom can cause skin rashes, gastrointestinal illness, liver damage, and neurological symptoms in people and animals. Dogs are especially vulnerable because they tend to drink directly from affected water and may ingest scum that concentrates toxins.

Climate change is making this worse. Warmer water temperatures favor cyanobacteria over other phytoplankton, and in high-latitude lakes that were once too cold and too nutrient-poor for major blooms, the combination of warming and even modest phosphorus enrichment has been shown to shift communities toward cyanobacteria while reducing overall phytoplankton diversity.²¹ Lakes that were safe a few decades ago are increasingly at risk, and the threshold of phosphorus loading needed to trigger a harmful bloom is effectively dropping as temperatures rise.

How Phosphorus Is Removed From Lake Water

Given how stubbornly phosphorus persists in water and sediments, a range of active remediation approaches have been developed. Several commercially available materials work by binding dissolved phosphorus and locking it into insoluble forms that sink to the bottom. One widely used product, lanthanum-modified bentonite (sold under the brand name Phoslock), strips phosphate from the water column and forms a stable mineral. Aluminum-based treatments achieve similar results. Testing of three such materials showed that their effectiveness depends heavily on temperature and pH. Higher pH, for example, significantly reduced the binding capacity of aluminum salts, dropping by more than 95% when pH climbed from 6 to 10.²² This means that a treatment strategy that works in one season or one lake may not transfer to another without adjustment.

Combining approaches appears to help. In tests on eutrophic lake water, pairing a polyaluminum chloride flocculant with Phoslock at reduced doses of each achieved removal efficiencies for phosphate, chlorophyll a, and turbidity above 88%, outperforming either material used alone by a substantial margin.²³ These in-lake treatments are generally seen as stopgaps rather than permanent fixes; without also reducing external phosphorus inputs, the lake will eventually re-load.

The Economic Weight of Algae Blooms

Phosphorus-driven algae blooms are not just an ecological problem. They carry significant economic costs, particularly for lakefront property. A detailed analysis across six Ohio counties between 2009 and 2015 found that homes near lakes affected by toxic algae blooms lost between 11% and 17% of their value, with properties directly adjacent to the water losing more than 22%. For Grand Lake St. Marys, which experienced severe cyanobacterial blooms during that period, the total loss in property value for nearby homes exceeded $51 million, roughly double the $26 million the state of Ohio spent on cleanup.²⁴ Losses of that scale create a strong financial case for prevention, especially considering that the cost of phosphorus control at the source, through better agricultural practices and wastewater treatment upgrades, can be recouped many times over in protected property values alone, to say nothing of avoided health costs and preserved recreation.

The Paradox of Phosphate in Drinking Water

One of the stranger twists in the phosphorus story is that water utilities deliberately add orthophosphate to drinking water. They do this for a good reason: orthophosphate reacts with lead and copper inside old pipes, forming a protective mineral coating that reduces the amount of those metals leaching into tap water.²⁵ It is one of the most effective and widely used strategies for managing lead exposure in cities with aging infrastructure.

The catch is that this treated water eventually ends up in wastewater, which means the phosphorus added for corrosion control contributes to the total phosphorus load leaving treatment plants and entering rivers and lakes. The amounts are small compared to agricultural runoff in most regions, but in urban watersheds with limited farmland, drinking water treatment can be a meaningful part of the phosphorus budget. It is a genuine tradeoff: protecting people from lead exposure today versus adding to the phosphorus burden in downstream waterways. Most regulators have concluded that the public health benefit of corrosion control outweighs the downstream cost, but it is a reminder that phosphorus management involves competing priorities, not just a simple directive to eliminate all inputs.

Bacteria That Unlock Phosphorus for Algae

Algae do not always acquire phosphorus on their own. Certain bacteria living alongside them can convert insoluble or organic phosphorus into forms that algae can absorb. A recently described marine bacterium was found to actively solubilize phosphorus and convert organic nitrogen into ammonia, both of which benefited the algal species it was growing alongside. Co-culture experiments showed that the bacterium significantly boosted algal growth, especially when either nitrogen or phosphorus was scarce.²⁶ These microbial partnerships add another layer of complexity to the phosphorus-algae relationship. Even when dissolved phosphorus levels in the water column look reassuringly low, bacterial activity at the microscopic scale can be recycling and releasing phosphorus right where algae need it, helping sustain blooms that measured nutrient concentrations alone would not predict.