A silent threat is growing in our freshwater systems worldwide, fueled by a changing climate and human activity
Imagine turning on your tap to fill a glass of water, only to find it smells foul and is tinged a sickly green. This isn't a scene from a dystopian novel; it was the reality for nearly half a million residents in Toledo, Ohio, in 2014, when a massive bloom of cyanobacteria—often called blue-green algae—contaminated the city's water supply from Lake Erie 7 .
People affected in Toledo water crisis
Optimal growth temperature for cyanobacteria
Cyanobacteria are ancient, simple life forms that have existed for billions of years. While usually harmless in small numbers, they can rapidly multiply into dense, visible "blooms" that wreak havoc on aquatic ecosystems and human health. These blooms are not just a natural nuisance; they are being supercharged by human activity. From agricultural runoff feeding them a nutrient-rich diet to climate change warming the waters they love, human actions are rolling out the welcome mat for this green menace, turning our vital freshwater resources into potential health hazards 7 2 .
Despite their common name, cyanobacteria are not true algae but are bacteria that perform photosynthesis. This unique combination allows them to grow rapidly and, in some cases, produce potent toxins known as cyanotoxins 1 .
These toxins can cause a range of health issues, from skin rashes and respiratory irritation upon contact to serious liver or neurological damage if ingested in contaminated water 7 .
Pets are especially vulnerable, often suffering fatal consequences after swimming in or drinking from contaminated lakes 1 .
The primary drivers are an overabundance of nutrients, particularly phosphorus and nitrogen, which act like fertilizer. These often wash into water bodies from agricultural fields and urban landscapes 7 .
Cyanobacteria thrive in warm water, with optimal growth often occurring above 20-25°C. Climate change is steadily raising lake temperatures around the globe, extending the bloom season 7 .
Calm, stable water allows cyanobacteria, many of which can regulate their buoyancy, to congregate on the surface, forming dense scums 7 .
When these conditions align, a bloom can explode, sometimes covering entire lakes in a green, paint-like slick.
Human-induced climate change is intensifying the very conditions that cyanobacteria love. Rising global temperatures are creating warmer lake environments that favor cyanobacteria over other, less harmful types of algae 7 .
Rising temperatures create ideal growth conditions for cyanobacteria
Heavy storms flush more nutrients into waterways
Lower water levels concentrate pollutants
Furthermore, climate change is disrupting rainfall patterns, leading to more extreme storms. These heavy rainfall events flush larger amounts of nutrient-rich runoff from the land into waterways, providing a feast for waiting cyanobacteria 2 .
Compounding the problem, climate change is also driving unprecedented water scarcity worldwide. A groundbreaking 2025 study in Science Advances revealed that continents are drying at an alarming rate, with groundwater being depleted faster than it can be replenished 9 . This scarcity creates a vicious cycle: as water levels drop in lakes and reservoirs, the same amount of nutrients becomes concentrated in a smaller volume of water, further promoting bloom formation and increasing their toxicity.
As the threat from cyanobacterial harmful algal blooms (cyanoHABs) grows, scientists are racing to predict where and when they will strike. A crucial step in this effort is the development of a national forecasting system. Researchers with the Cyanobacteria Assessment Network (CyAN) project, a multi-agency effort including the EPA, set out to create a model that could provide an early warning for thousands of lakes across the United States 1 .
The research team faced a daunting challenge: how to monitor the health of thousands of lakes spread across the country efficiently. Their ingenious solution combined satellite technology with sophisticated computer modeling in a multi-step process 1 .
| Lake Name | State | Forecasted Bloom Probability | Risk Level |
|---|---|---|---|
| Lake Erie (Western Basin) | OH, MI |
|
High |
| Lake Okeechobee | FL |
|
High |
| Utah Lake | UT |
|
Moderate |
| Clear Lake | CA |
|
Low |
| High | Avoid swimming, boating, and fishing. Keep pets away from the water. Follow local health advisories. |
| Moderate | Be cautious when recreating. Look for signs of a bloom (discolored water, scum). Do not swallow water. |
| Low | Low risk, but remain aware that conditions can change rapidly. |
The dangers of cyanoHABs are not confined to the water. Recent research has uncovered a new and alarming exposure route: airborne toxins. When waves break on a lake surface, they generate what scientists call Lake Spray Aerosol (LSA)—tiny droplets of water ejected into the air 8 .
| Tool or Reagent | Function in Research |
|---|---|
| Lake Spray Aerosol (LSA) Samplers | Specialized devices that collect airborne particles from the air at various distances from the shoreline. |
| Medium Resolution Imaging Spectrometer (MERIS) | A satellite sensor used to measure large-scale chlorophyll-a concentrations and identify bloom locations from space 7 . |
| Liquid Chromatography-Mass Spectrometry | A highly sensitive laboratory technique used to identify and quantify specific cyanotoxins (like microcystin) in water and air samples. |
| Cyanobacteria Index (CI_cyano) | A standardized metric derived from satellite data that allows scientists to consistently track and compare bloom severity across different lakes and years 7 . |
These droplets can encapsulate cyanobacterial cells and their toxins. Studies near Lake Michigan have detected microcystin, a potent liver toxin, in aerosols collected kilometers inland from the shore 8 . This means that simply living or spending time near a contaminated lake could expose you to cyanotoxins through inhalation, potentially leading to respiratory issues. This discovery significantly expands the potential health impact of cyanobacterial blooms, moving the risk beyond direct water contact 8 .
The evidence points to a future where cyanobacterial blooms become more frequent and severe. The first global emergence of unprecedented "Day Zero" drought events—driven by a combination of prolonged rainfall deficits and increasing water consumption—is projected for the 2020s and 2030s in hotspot regions like the Mediterranean, southern Africa, and parts of North America 4 . These extreme droughts concentrate pollutants and create ideal conditions for blooms upon the return of rain.
The EPA's forecasting tools provide critical early-warning systems to help communities prepare for and respond to blooms 1 .
Efforts to reduce nutrient pollution from agriculture and wastewater are more important than ever to prevent blooms from forming.
Reduce nutrient runoff from lawns and gardens
Prevent nutrient contamination of waterways
Create natural buffers to filter runoff
The story of cyanobacteria is a powerful example of how interconnected our planet is. Our actions on land, the emissions we release into the atmosphere, and the ways we use our water are all reflected in the health of our lakes and rivers. By understanding this green menace, we can begin to make the changes needed to keep our water clean and safe for generations to come.