What Causes Algae in Lakes and Why It’s Worsening

Excess nutrients, primarily phosphorus and nitrogen from human activities, are the root cause of algal overgrowth in lakes. These nutrients act as fertilizer for microscopic algae and cyanobacteria (often called blue-green algae), fueling explosive population surges known as blooms. The problem is getting worse because the main drivers are all intensifying at once: agricultural runoff has increased with modern farming, climate change is warming lake waters to temperatures that favor the most harmful species, and decades of accumulated phosphorus sit locked in lake sediments, ready to recycle back into the water column even after external pollution is reduced.

Nutrients Are the Fundamental Trigger

Almost every algal bloom traces back to an oversupply of phosphorus, nitrogen, or both. Agricultural runoff carrying fertilizers and animal waste is the single largest contributor, delivering huge quantities of these nutrients into waterways that drain into lakes.¹ Urban sources add to the load through stormwater that picks up lawn fertilizers, pet waste, and phosphorus-containing detergents as it washes off streets and parking lots into storm drains. Wastewater treatment plants, even modern ones, discharge some nitrogen and phosphorus, and septic systems near lakeshores can leak nutrients directly into groundwater that feeds a lake.

The balance between nitrogen and phosphorus matters as much as the total amount. An analysis of growing-season data from 17 lakes around the world found that blue-green algae (cyanobacteria) tended to dominate when the ratio of total nitrogen to total phosphorus dropped below roughly 29 to 1 by weight. Above that threshold, cyanobacteria were rare. Below it, they had a competitive edge over harmless green algae.² This is one reason why dumping a single nutrient in excess can be worse than a moderate increase in both: skewing the ratio can flip the community from mostly green algae to mostly cyanobacteria, the group responsible for the toxic, scummy blooms that close beaches and poison water supplies.

Phosphorus Hiding in the Mud

Even lakes where external nutrient sources have been curtailed can still suffer persistent blooms because of phosphorus trapped in bottom sediments. Over decades of pollution, lake sediments accumulate enormous stores of phosphorus. When the water above the sediment loses its oxygen, a condition called anoxia, chemical reactions release that stored phosphorus back into the water. Research on shallow lake zones found that these areas contributed roughly half of the total phosphorus released during sediment anoxia events.³

The scale of this internal recycling can be striking. In chamber experiments at two California reservoirs, anoxic conditions drove phosphorus fluxes from sediment as high as 122 mg per square meter per day at one site and 67 mg at the other, alongside large releases of ammonia nitrogen.⁴ Once oxygen drops even partway, hypoxic conditions can still push meaningful amounts of phosphorus into the water. A study in a coastal brackish lake confirmed that as sediment environments shift toward more reduced (low-oxygen) states, phosphorus release under anoxic conditions increases further.⁵ This creates a vicious feedback loop: algal blooms die and decompose, consuming oxygen near the bottom, which triggers more phosphorus release, which feeds the next bloom.

Why Warmer Water Favors the Worst Algae

Climate change is reshaping lake ecosystems in ways that consistently benefit cyanobacteria over the harmless green algae and diatoms that once dominated. Cyanobacteria grow fastest in warm water. Research has shown that at higher temperatures, species like Microcystis achieve optimal growth rates and outcompete green algae, diatoms, and other phytoplankton groups.⁶ Increased agricultural nutrient use since the mid-twentieth century combined with rising water temperatures and shifts in the physical structure of lakes have together contributed to more frequent harmful algal blooms across Canada and beyond.⁷

Warming also strengthens thermal stratification, the layering of warm water on top of cooler, denser water below. When this layering becomes more pronounced and lasts longer, oxygen at the bottom gets depleted more quickly, accelerating the internal phosphorus release described above. At the same time, stratification actually helps cyanobacteria: many species have internal gas vesicles that let them float to the surface. In a well-stratified lake, rising global temperatures and stable layering allow these organisms to park themselves in the sunlit upper meters, absorbing light and effectively shading out competing algae below.⁸ Certain nitrogen-fixing cyanobacteria like Dolichospermum concentrate heavily in the top few meters during stratified periods, taking advantage of both the warmth and the light to fix atmospheric nitrogen and fuel their own growth.⁹

Rising CO₂ Gives Cyanobacteria Another Edge

There is a common assumption that cyanobacteria, being ancient organisms adapted to Earth’s early low-CO₂ atmosphere, would struggle as CO₂ rises. The reality appears to be the opposite for the species we worry about most. Competition experiments between the cyanobacterium Microcystis and several green algae found that, contrary to the prevailing view, green algae actually won at low CO₂ levels. As CO₂ increased, Microcystis became a stronger competitor, winning or coexisting in matchups where it had previously lost.¹⁰ The explanation lies in Microcystis having high-flux carbon uptake systems that become especially advantageous when dissolved CO₂ is plentiful. Since atmospheric CO₂ continues to climb, this is yet another factor tilting the competitive balance toward the bloom-forming cyanobacteria.

Nutrients Falling From the Sky

Agricultural fields and urban areas do not just send nutrients downstream. They also send them into the air. Atmospheric deposition of nitrogen and phosphorus, through dust, industrial emissions, and agricultural aerosols, is an underappreciated pathway. Monitoring during the algal bloom season at Lake Taihu in China showed that high atmospheric nitrogen and phosphorus deposition loads helped sustain cyanobacterial blooms throughout the summer and autumn growing period.¹¹ In some regions, atmospheric phosphorus deposition has increased enough to shift certain lakes from being phosphorus-limited, where phosphorus controls how much algae can grow, back to being nitrogen-limited, a condition that once again favors nitrogen-fixing cyanobacteria.¹² For lake managers focused on controlling direct nutrient inflows, airborne deposition is a source they have almost no ability to control at the local level.

How Grazing Pressure Shapes Bloom Timing

In a healthy lake, tiny animals called zooplankton graze on algae and keep populations in check. When this grazing pressure weakens or shifts, blooms can take off. Modeling work has shown that when zooplankton are moderately selective in what they eat, the system stays roughly balanced. But when selectivity becomes too high, zooplankton avoid toxic species and eat only harmless ones, allowing toxic phytoplankton to dominate and bloom.¹³

Field studies paint a nuanced picture. In estuaries in New York, researchers found that low grazing pressure by tiny protozooplankton early in the season allowed harmful algal blooms to get started. Later, when grazing ramped up and exceeded algal growth rates, it helped end the bloom. But an unexpected wrinkle emerged: juvenile copepods (a type of larger zooplankton) appeared to eat the predators of the harmful algae rather than the algae itself, triggering a cascade that actually promoted the bloom in some experiments.¹⁴ Meanwhile, a study in a eutrophic lake found that during peak phytoplankton abundance, zooplankton preferentially grazed on cyanobacteria, including non-toxic strains, suggesting that zooplankton communities can sometimes adapt to feed on bloom-forming species regardless of toxicity.¹⁵ In short, grazing can both prevent and promote blooms depending on who is eating whom, and disruptions to the food web, whether from overfishing, invasive species, or pollution, can tip the balance in the wrong direction.

Zebra Mussels and Invasive Species

Invasive filter-feeders offer a case study in unintended consequences. Zebra mussels are voracious filterers that clear enormous volumes of lake water, and you might expect them to reduce algae. They do reduce total phosphorus somewhat, but their filtering is selective. They preferentially consume harmless green algae and diatoms while rejecting cyanobacteria, which they expel in mucus-bound clumps. A study across U.S. lakes found that the total effect of zebra mussel establishment was a roughly 1.4-fold net increase in microcystin levels, the toxin produced by certain cyanobacteria. This resulted from a combination of slightly increased cyanobacterial abundance, a strong selective shift toward more toxic cyanobacteria strains, and only a partial offset from reduced phosphorus.¹⁶ So an invasive species that makes lake water look clearer can actually make the toxin problem worse.

What Algal Blooms Do to Health and Ecosystems

The concern about blooms is not just aesthetic. Many cyanobacteria produce potent toxins, most notably microcystin and anatoxin, that can harm humans and animals through contaminated drinking water, recreational contact, and even inhalation of spray or mist near affected lakes.¹⁷ Microcystin in particular has become a significant worldwide concern because it can accumulate in irrigation water and enter the food chain through crops like alfalfa, posing risks to livestock and potentially to people who consume animal products.¹⁸ Even benthic cyanobacteria that grow on lake bottoms, rather than floating on the surface, can produce these toxins. One study found benthic strains in Australian drinking water reservoirs producing high concentrations of cylindrospermopsin and microcystin, meaning that monitoring programs focused only on surface scums can miss an important source of contamination.¹⁹

Beyond toxins, massive blooms cause oxygen depletion when they die and decompose, creating dead zones where fish and other organisms suffocate. Harmful algal blooms can kill fish and other economically or ecologically important organisms through a combination of direct toxicity and oxygen starvation.²⁰ For lakeside communities, the economic pain is concrete. A multi-lake analysis across six Ohio counties found that homes near bloom-affected lakes lost between 11% and 17% of their value, with lake-adjacent homes losing more than 22%. At Grand Lake St. Marys alone, total property value losses exceeded $51 million, roughly double the $26 million the state of Ohio spent on cleanup.²¹

The Legacy Phosphorus Problem

Perhaps the most frustrating aspect of lake algae is how slowly the problem reverses, even when pollution controls are put in place. Phosphorus that has accumulated in catchment soils and lake sediments over decades acts as a long-term nutrient reservoir. One study modeling a heavily impacted watershed found that legacy phosphorus could take on the order of a thousand years to be fully exhausted.²² That is an extreme case, but the general pattern holds across many lakes. Research on Azorean volcanic lakes showed that even under a hypothetical scenario of completely stopping all external phosphorus inputs, reactive sediment phosphorus could continue to influence lake condition for decades depending on individual lake characteristics.²³

Modeling of Lake Mendota in Wisconsin illustrates the two-phase nature of recovery. There is typically an initial, relatively fast improvement as the water column flushes out dissolved nutrients. Then comes a much longer, slower phase as the massive pool of sediment phosphorus gradually declines. Recovery to a healthier state may require decades even after catchment loading drops, because the ecosystem has a “memory” built into its sediment chemistry.²⁴ This is why many lake restoration projects appear to stall: managers have done the hard work of reducing inflows, but the lake’s own sediments keep feeding the blooms.

Shoreline Development and the Cumulative Effect

Residential and commercial development along lake shorelines contributes in less obvious ways. Removing natural vegetation from the shore eliminates a buffer that once absorbed runoff nutrients. Impervious surfaces like docks, patios, and driveways increase stormwater volume. A study comparing developed and undeveloped shorelines on an otherwise clean, deep lake found that developed sites had significantly higher nearshore productivity, with elevated chlorophyll and organic material in the sediment compared to undeveloped sites, particularly in spring.²⁵ The effect is not dramatic on any single property, but multiplied across hundreds of lakefront homes, it adds up to a meaningful nutrient load that compounds the agricultural and atmospheric sources.

What Lake Managers Can Actually Do

Given the complexity of the problem, restoration strategies tend to combine reducing external nutrient loads with treating what is already in the lake. On the external side, the priorities are straightforward in concept if difficult in execution: tighter controls on agricultural runoff through buffer strips, cover crops, and precision fertilizer application; upgraded wastewater treatment; and stormwater management in developed areas. Sediment cores from Lake Ontario illustrate that this can work. After nutrient-control policies took effect, diatom records in the sediment showed a clear shift from nutrient-tolerant species back toward species associated with cleaner water, documenting a real, if gradual, recovery from eutrophication that had accelerated from roughly the 1920s through the 1980s.²⁶

For internal loading, one approach is aluminum dosing. In a Danish lake, adding aluminum to the water at a specific ratio to the mobile phosphorus in the sediment reduced internal phosphorus loading by 93% over two years, and average summer total phosphorus in the water dropped by more than 90%.²⁷ Aluminum binds with dissolved phosphorus and locks it into the sediment in a form that does not re-release under low oxygen. Other in-lake interventions include aeration systems that pump oxygen to the bottom to prevent the anoxic conditions that trigger phosphorus release, dredging to physically remove phosphorus-rich sediment, and biomanipulation such as stocking fish that eat the zooplankton predators to boost grazing on algae. Each has trade-offs in cost, scale, and how long the benefits last.

Satellite Monitoring and Early Warning

One area where the fight against blooms has genuinely advanced is in detection. Satellite imagery now allows researchers and water managers to track chlorophyll-a concentrations, a proxy for algal biomass, across entire lakes without needing to collect water samples everywhere. Work using Landsat satellite images on a Chilean lake system demonstrated that spectral indices could predict chlorophyll-a levels with strong accuracy, using a floating algal index that explained about 87% of the variation in ground-truth measurements.²⁸ Similar approaches are now being applied to lakes globally, enabling managers to spot blooms forming in real time rather than reacting after complaints come in. This does not prevent blooms, but it can shorten response times for beach closures, drinking water advisories, and targeted treatment.

A Historical Pattern That Keeps Repeating

Algal blooms are not new. What is new is their frequency, geographic spread, and severity. Sediment core records from Lake Michigan spanning roughly 160 years reveal a clear before-and-after story. Sediments from before the twentieth century contained low abundances of diatoms associated with nutrient-poor conditions. Starting in the early to mid-twentieth century, the diatom community reorganized, with phosphorus-inferred concentrations rising as cultural eutrophication set in.²⁹ The pattern has since been documented in lakes on every inhabited continent: a period of clean water, followed by nutrient enrichment tied to population growth and agricultural intensification, followed by some improvement where policies intervened, but rarely a full return to baseline. The sediment record is blunt about timescales. Lakes that took decades to degrade are not going to recover in a few years, and the warming climate is now layering new pressures on top of the old nutrient ones, making the path back to clear water longer and more uncertain than it has ever been.