What Are the Main Causes of Algae Growth?

Algae growth is driven primarily by an oversupply of nutrients, especially phosphorus, combined with warm water temperatures, adequate sunlight, and slow-moving or stagnant conditions. That short list covers the textbook explanation, but the real picture is messier and more interesting. Internal recycling from lake sediments, rising carbon dioxide levels, invasive species, and even road salt all play documented roles in feeding algae blooms, and climate change is reshuffling the deck in ways that make traditional nutrient-focused management harder than it used to be.

Phosphorus Is Usually the Limiting Factor

In most freshwater lakes and reservoirs, phosphorus is the nutrient that determines whether algae grow slowly or explosively. When phosphorus concentrations rise, the conditions for rapid bloom formation follow. Nitrogen matters too, and in some coastal or estuarine systems it can be the primary bottleneck, but decades of research have consistently pointed to phosphorus as the nutrient most responsible for freshwater eutrophication. The sources of phosphorus are familiar: agricultural runoff carrying fertilizer and manure, wastewater treatment plant discharges, urban stormwater, and erosion. Even modest increases in the amount of phosphorus reaching a lake can tip conditions toward bloom territory.

What makes phosphorus management so frustrating is that reducing external inputs does not always produce quick results. Lakes have long memories. Phosphorus that accumulated in bottom sediments over decades of pollution can be released back into the water column, sustaining algae growth even after the original pollution sources have been cleaned up. This internal loading is often the reason managers see a disappointing lag between cutting nutrient inputs and seeing clearer water.

How Sediments Feed Algae from Below

When the water at the bottom of a lake loses its dissolved oxygen, a chemical switch flips. Under oxygen-rich conditions, phosphorus stays bound to iron particles in the sediment and is mostly locked away. When oxygen drops to zero, that bond breaks, and phosphorus floods into the overlying water at dramatically higher rates. In Lake Erie’s central basin, researchers measured phosphorus release from sediments under anoxic conditions at roughly 5 to 31 milligrams per square meter per day, compared to less than half a milligram under oxygenated conditions. The transition happened within 24 hours of oxygen reaching zero.¹

This process is not limited to the deep parts of lakes. Research in Finnish lakes found that shallow areas contributed about half of the total phosphorus flux caused by sediment anoxia, and that phosphorus concentrations in the near-bottom water of shallow zones were a key driver of overall water quality.² Longer-term studies show that the release of iron-bound phosphorus from sediments increases as water temperatures climb during summer, meaning the problem intensifies exactly when algae are primed to grow fastest.³ In practical terms, a lake’s sediment can act as a slow-release fertilizer for years after external nutrient sources have been curtailed.

Temperature and Thermal Stratification

Warmer water accelerates algae metabolism directly: cells photosynthesize faster, divide sooner, and reach bloom-level densities in a shorter window. Simulations of freshwater reservoirs show that rising air temperatures increase the photosynthetic and respiratory rates of algae, shorten the time to peak bloom, and make blooms both earlier and more severe.⁴ But temperature does more than just speed up growth. It also restructures the water column in ways that favor the most problematic algae.

In summer, many lakes and reservoirs develop thermal stratification: a warm upper layer sits on top of a cold, dense lower layer, and the two barely mix. This layering reduces the supply of oxygen to the bottom, encouraging the sediment phosphorus release described above, while creating a warm, stable surface environment that certain algae exploit. Projections for reservoirs in the northeastern United States suggest that the number of days per year with extreme surface temperatures and strong thermal stratification will rise significantly, creating conditions that favor high-density cyanobacteria blooms.⁵ The combination of warm surface water and a strongly stratified water column is particularly dangerous because it allows buoyant cyanobacteria to dominate the sunlit zone while suppressing competitors that need mixing to stay suspended.

Light Availability

Algae are photosynthetic organisms, so light is a non-negotiable requirement. But different types of algae have very different relationships with it. Diatoms can grow at extremely low light levels and tolerate high light without damage. Dinoflagellates and cyanobacteria tend to do best at lower light intensities and can be harmed by too much exposure.⁶ This means water clarity is not simply “more light equals more algae.” Instead, changes in clarity shift which species dominate.

This dynamic plays out vividly with benthic algae, the filamentous mats that grow on rocks and lake bottoms. In Lake Huron’s Saginaw Bay, light limitation was the factor that set the maximum depth at which filamentous algae could grow. After invasive mussels filtered the water and increased clarity, the depth limit for algae growth extended from roughly 3.3 meters to about 5 meters, greatly expanding the area covered by algae mats.⁷ Similarly, monitoring in eastern Lake Erie found that benthic Cladophora biomass was strongly controlled by light, and that shading from river plumes actually reduced algae growth despite those plumes carrying phosphorus.⁸ The counterintuitive result: nutrient-rich but murky water sometimes grows less benthic algae than cleaner water with modest nutrient levels.

Water Movement and Residence Time

Algae need time to multiply. If water moves through a lake or reservoir quickly, it flushes cells downstream before they can build up to bloom densities. Research on a Japanese dam reservoir found that it took a hydraulic residence time of at least two weeks for phytoplankton to increase substantially; shorter retention times kept algae populations in check regardless of nutrient levels.⁹

Flow velocity also matters at a cellular level. Experiments with Euglena gracilis, a common bloom-forming species in reservoirs, showed that flow velocities above about 0.1 meters per second inhibited growth, while the organism thrived under slow or completely static conditions.¹⁰ Stagnant ponds, sheltered embayments, and reservoirs during dry spells all create the slow-water conditions that favor bloom development. This is one reason why droughts, which reduce inflows and extend residence times, often precede bad bloom years.

How Climate Extremes Compound the Problem

The conventional model of algae blooms treats nutrients as the main lever: reduce phosphorus, reduce blooms. Recent work suggests that compound climate extremes are undermining that logic. When a pulse of heavy rain washes phosphorus into a lake, certain bloom-forming algae store it internally as polyphosphate reserves. If a heatwave follows that rainfall event, the algae draw on those reserves to fuel rapid growth, expand vertically through the water column, and sustain blooms for longer than nutrient-only models would predict. This sequence has been shown to drive bloom expansion even in lakes with low background nutrient levels.¹¹

The implication is unsettling for management: as extreme rainfall events and heatwaves become more frequent and more likely to cluster together, the conditions that trigger blooms may arise in lakes previously considered safe. Nutrient reduction remains essential, but it may not be sufficient on its own when the climate itself is loading the dice.

Carbon Dioxide as a Growth Accelerator

Algae need carbon to build biomass, and most of them get it from dissolved carbon dioxide. Rising atmospheric CO₂ levels increase the amount of dissolved CO₂ available in surface waters, and experimental evidence shows that many algae species grow faster when CO₂ concentrations are elevated. Lab studies on several freshwater microalgae strains found that growth rates increased at CO₂ concentrations well above ambient levels, with some species reaching their highest biomass production rates at concentrations far above what current atmospheric conditions provide.¹²

In marine environments, a meta-analysis of harmful algae species from temperate regions found that growth rates increased consistently with elevated CO₂, and that this advantage was specific to harmful bloom-forming species rather than a general boost to all phytoplankton.¹³ If harmful species benefit more than their competitors, rising CO₂ could shift the composition of algal communities toward more problematic blooms over time.

Why Cyanobacteria Keep Winning

Not all algae are equal threats, and the group that dominates headlines is cyanobacteria, sometimes called blue-green algae. These organisms have a suite of competitive advantages that the conditions described above play directly into. One of the most important is buoyancy regulation through gas vesicles, tiny gas-filled compartments inside the cell that allow cyanobacteria to float or sink to whatever depth gives them the best access to light and nutrients.¹⁴

This buoyancy trick is especially powerful in stratified water. During calm periods, buoyant cyanobacteria float into the sunlit upper layer and monopolize photosynthesis. Calculations for the Baltic Sea species Aphanizomenon flos-aquae showed that floating up after a mixing event nearly tripled net photosynthesis compared to staying dispersed, and that averaged over normal cycles of calm and wind, gas vesicle buoyancy roughly doubled the organism’s photosynthetic output.¹⁵ Warm, stratified water reinforces this advantage by making calm periods longer and the water column more resistant to mixing.¹⁶

Cyanobacteria also produce toxins that deter many grazers, and they tolerate warmer temperatures better than most of their eukaryotic competitors. The net result is that as lakes warm and stratify more strongly, the community structure of phytoplankton shifts toward cyanobacteria dominance, which is exactly the trend observed across freshwater systems worldwide.⁴

Invasive Mussels and the Paradox of Clearer Water

One of the more counterintuitive stories in algae ecology involves zebra mussels, an invasive species that has spread through lakes across North America and Europe. These mussels are powerful filter feeders that strip algae from the water column, which sounds like it should reduce blooms. In nutrient-rich lakes, that is roughly what happens. But in low-nutrient lakes, the story flips. Zebra mussels selectively reject cyanobacteria like Microcystis aeruginosa while consuming their competitors, effectively clearing the field for the organisms most likely to produce toxins.

Surveys comparing lakes with and without established zebra mussel populations found that invaded lakes had roughly 3.6 times higher Microcystis biomass and 3.3 times higher concentrations of the cyanobacterial toxin microcystin, despite having lower overall phosphorus levels.¹⁷ The positive influence of zebra mussels on Microcystis dominance was statistically significant in lakes with total phosphorus below 25 micrograms per liter but not in lakes above that threshold, suggesting that the mussel’s selective grazing matters most where nutrients are scarce and competition is tighter.¹⁸

A more recent analysis estimated that zebra mussel establishment leads to an overall 1.4-fold net increase in microcystin levels, the combined result of higher cyanobacterial abundance, a shift toward more toxic cyanobacteria species, and a partial offset from reduced total phosphorus.¹⁹ This finding complicates the traditional management assumption that cutting nutrient inputs will control harmful blooms. In mussel-invaded lakes, the biological community itself can push conditions toward toxic cyanobacteria dominance, even in relatively clean water.

Grazing Dynamics Are Not Always Straightforward

Zooplankton grazing is often described as a natural check on algae growth, and in many cases it is. But the relationship can be more complex than a simple predator-prey balance. Experiments with Daphnia, one of the most common freshwater grazers, and the colonial green alga Sphaerocystis showed that most algal cells survived gut passage intact. During the trip through the Daphnia’s gut, surviving algal cells absorbed phosphorus from digested remains and from the grazer’s own metabolic waste. The cells that emerged were primed with nutrients and divided faster, so the enhanced growth after gut passage compensated for the minor population losses caused by grazing itself.²⁰ In other words, grazing can sometimes recycle nutrients so efficiently that it actually stimulates the algae population it is supposed to control.

Ocean Upwelling and Iron

The causes of algae growth in the open ocean differ in some important ways from freshwater systems. While phosphorus and nitrogen still matter, vast stretches of the ocean have enough of both but still produce surprisingly little algae. In these high-nutrient, low-chlorophyll regions, the limiting factor is iron. Iron is essential for photosynthesis and nitrogen fixation, and its supply to the open ocean comes largely from wind-blown dust originating in deserts. The amount of iron deposited from the atmosphere strongly influences primary production and community structure in these regions.²¹

Coastal ocean systems have a different set of drivers. Upwelling zones, where deep, nutrient-rich water rises to the surface along continental margins, are among the most productive marine environments on Earth and are particularly susceptible to harmful algal blooms precisely because they deliver such large quantities of nutrients to the sunlit layer.²² These are blooms driven not by human pollution but by the natural circulation of the ocean, though warming-driven changes in wind patterns and stratification can intensify them.

Freshwater Salinization

A growing body of evidence links increasing salt concentrations in freshwater to shifts in algae communities. Road salt, irrigation return flows, and mining discharge are raising chloride levels in lakes, streams, and stormwater ponds across temperate regions. The biological response is not uniform: salt tends to harm green algae and many zooplankton while leaving cyanobacteria relatively unscathed, which tilts the competitive balance.

In urban stormwater ponds, chloride concentration emerged as a leading predictor of cyanobacterial dominance, with cyanobacteria compensating for the decline of other algal groups as salt levels rose.²³ Mesocosm experiments confirm the mechanism: added salt at environmentally relevant concentrations reduced the abundance of cladocerans and copepods (important algae grazers), which led to an increase in phytoplankton.²⁴ The pattern held at the physiological level too: differences in growth responses to salt were consistent with field observations documenting a shift from green algae toward cyanobacteria in salinizing freshwater systems.²⁵ Salt is not causing algae blooms on its own, but in nutrient-rich waters, it may be an underappreciated amplifier of harmful cyanobacteria.

Benthic Algae and the Factors That Set Them Apart

Most public attention focuses on floating planktonic blooms, but algae that grow attached to rocks, sediments, and underwater structures can cause their own ecological and aesthetic problems. The filamentous green alga Cladophora, for instance, forms dense mats across shallow lake bottoms in the Great Lakes and other systems. Its growth drivers overlap with but are not identical to those of planktonic algae.

For Cladophora, light is often the primary limiting factor rather than nutrients. In eastern Lake Erie, biomass tracked light availability more closely than phosphorus inputs.⁸ However, laboratory work on Cladophora’s reproductive biology reveals that nutrient regime governs how the organism reproduces: high nitrogen and phosphorus, acidic conditions, and warm temperatures favor the mass release of reproductive cells, while neutral to alkaline and moderate-temperature conditions promote the accumulation of standing biomass.²⁶ Temperature and nutrients, in other words, still matter for benthic algae, but they act through different pathways than for floating blooms, making management a distinct challenge.

When Many Causes Converge at Once

Century-long reconstructions of lake history illustrate how multiple causes can align to produce abrupt ecological shifts. Sediment core analysis of China’s Lake Taihu revealed that the lake existed in a relatively stable state for decades despite gradual nutrient enrichment. Then, in the 1980s, the system crossed a threshold: primary production surged, Microcystis-dominated cyanobacterial blooms took hold, and microcystin production grew exponentially. The shift was driven not by a single cause but by the convergence of nutrient enrichment, rising temperatures, and cascading changes in the food web.²⁷ That pattern, years of gradual change followed by a sudden tipping point, has played out in lakes around the world and serves as a reminder that algae blooms are rarely the product of any single cause acting alone.