An algal bloom is a rapid overgrowth of algae or algae-like bacteria in a body of water, often triggered by excess nutrients from agricultural runoff, sewage, or urban pollution. Some blooms are harmless, but others produce toxins that can sicken people and animals, contaminate drinking water, and kill fish. These toxic events, called harmful algal blooms (HABs), are growing more frequent worldwide, with bloom rates in large lakes increasing by about 1.8% per year over the past two decades.
What Causes Algal Blooms
Algae exist naturally in every lake, river, and ocean. They only become a problem when something tips the balance and lets them multiply out of control. The biggest driver is nutrient pollution, specifically phosphorus and nitrogen washing into waterways from farms, lawns, and wastewater discharge. These nutrients act like fertilizer for algae, fueling explosive growth. Research on freshwater lakes has shown that phosphorus is the single most critical nutrient behind bloom formation, and reducing phosphorus inputs is considered the most effective way to prevent them.
Warm water temperatures accelerate the process. As surface water heats up, it creates a layered effect where warm water sits on top of cooler, denser water below. This layering, called stratification, traps nutrients near the surface where algae can access both sunlight and food. Calm, slow-moving water makes things worse by letting algae accumulate rather than disperse. This is why blooms tend to peak in late summer, when days are long, water is warm, and rainfall has washed a season’s worth of nutrients downstream.
What a Bloom Looks Like
Not all green water is dangerous, but certain visual cues signal a potential harmful bloom. According to the CDC, harmful algal blooms can resemble scum, spilled paint, foam, or thick mats and globs floating on the surface. The water itself may turn vivid shades of blue, green, brown, yellow, orange, or red. In saltwater, the discoloration tends toward red, brown, orange, or yellow, which is where the term “red tide” comes from.
Other warning signs include dead fish or animals near the shoreline, a strong unpleasant smell, and visible foam or scum collecting along the water’s edge. The tricky part is that you cannot tell whether a bloom is producing toxins just by looking at it. Clear water can still contain dissolved toxins, and a vivid green bloom might be a nontoxic species. If water looks or smells off, the safest approach is to stay out of it entirely.
The Organisms Behind Blooms
Several types of microscopic organisms cause blooms, and each one poses different risks. Cyanobacteria (sometimes called blue-green algae, though they’re technically bacteria) are the most common culprits in freshwater lakes and reservoirs. They produce a range of toxins, including microcystins, which can damage the liver, and cylindrospermopsin, which affects the liver and kidneys. The EPA has set recommended recreational water quality thresholds for both: 8 micrograms per liter for microcystins and 15 micrograms per liter for cylindrospermopsin.
In marine environments, dinoflagellates are responsible for red tides and produce brevetoxins, which cause neurotoxic shellfish poisoning when they accumulate in clams, mussels, oysters, and scallops. Diatoms, another type of marine algae, produce domoic acid, a toxin that can cause memory loss and seizures, leading to what’s known as amnesic shellfish poisoning. Dinoflagellates also produce ciguatoxins, which build up in reef fish like barracuda, grouper, red snapper, and amberjack, causing ciguatera poisoning.
How Blooms Affect Your Health
The symptoms you experience depend on how you were exposed and which toxin is involved. Skin contact with bloom-contaminated water can cause rashes and irritation. Swallowing contaminated water or eating affected seafood leads to gastrointestinal problems like nausea, vomiting, and diarrhea, and in some cases neurological effects like tingling, numbness, or confusion.
You don’t even need to touch the water to be affected. When wind and waves break up a bloom, toxins become airborne as tiny droplets. Breathing in these aerosolized toxins can cause eye and throat irritation, nasal congestion, coughing, chest tightness, wheezing, and shortness of breath. People with asthma or other respiratory conditions are particularly vulnerable. Cyanobacteria exposure specifically has been linked to difficulty breathing, dry cough, runny nose, unusual sneezing, and sore throat.
Some toxin-related illnesses from contaminated seafood are more severe. Paralytic shellfish poisoning, caused by saxitoxin, can lead to muscle paralysis. Neurotoxic shellfish poisoning causes neurological symptoms like dizziness and slurred speech. These illnesses range from mild and self-limiting to life-threatening, depending on the amount of toxin consumed.
Dead Zones and Ecosystem Damage
Even after a bloom dies, the damage continues. When massive quantities of algae sink to the bottom and decompose, bacteria break them down and consume enormous amounts of dissolved oxygen in the process. This creates hypoxic zones, areas where oxygen levels drop so low that fish, shellfish, corals, and aquatic plants cannot survive. These are commonly called dead zones.
The problem is compounded by water stratification. When lighter freshwater sits on top of heavier saltwater in coastal areas, oxygen from the surface can’t mix down to replenish the depleted bottom layers. The result is a suffocating layer of low-oxygen water that can persist for weeks or months, driving mobile species away and killing anything that can’t escape. Dead zones now occur in hundreds of coastal areas worldwide, with some of the largest in the Gulf of Mexico and the Baltic Sea.
Blooms Are Getting Worse
A global study of 962 large lakes found that bloom frequency has been climbing steadily. Of 620 lakes that experienced blooms in at least half the years studied, 504 showed an upward trend. The global rate of increase averaged 1.8% per year between 2003 and 2022, but the pace has accelerated sharply: from 2016 to 2022, the rate jumped to 4.6% per year, roughly four times faster than the period from 2003 to 2015.
Rising air temperatures are a major factor. Globally, annual bloom frequency tracks closely with air temperature trends, and nearly 45% of bloom-affected lakes showed a strong direct correlation between warming temperatures and more frequent blooms. Subtropical regions are seeing the fastest increases at 2.3% per year. Blooms are also starting earlier in the year, advancing by nearly one day per year on average, and lasting longer, with bloom duration growing by about 1.6 days per year. The combination of nutrient pollution and warming water means conditions that once produced occasional blooms are now producing chronic ones.
How to Protect Yourself
If you swim, fish, or boat in freshwater lakes or coastal areas, check for local water quality advisories before heading out. Many states post real-time bloom alerts on environmental agency websites. Avoid water that looks discolored, smells foul, or has visible scum or foam on the surface. Keep children and pets away from affected shorelines, as dogs are especially vulnerable because they tend to drink lake water and lick algae off their fur.
If you’ve been in water you suspect was affected by a bloom, rinse off with clean water as soon as possible. Don’t eat fish caught from bloom-affected waters unless local advisories say it’s safe. For shellfish, pay attention to harvest closures, which are issued specifically to prevent toxin exposure from filter-feeding species like clams and mussels that concentrate algal toxins in their tissue. Boiling or cooking contaminated water or seafood does not destroy most algal toxins.