HABs, or harmful algal blooms, are rapid overgrowths of algae or bacteria in water that produce toxins, deplete oxygen, or otherwise damage ecosystems and human health. They occur in both freshwater lakes and coastal ocean waters, and they’ve been increasing in frequency as water temperatures rise and nutrient pollution worsens. Some HABs are visible as thick green scum on a lake’s surface, while others are nearly invisible yet still dangerous.
What Causes a Harmful Algal Bloom
HABs form when microscopic organisms in the water multiply explosively, fueled by the right combination of nutrients, sunlight, and temperature. The two biggest nutrient drivers are phosphorus and nitrogen, which enter waterways through agricultural runoff, lawn fertilizers, wastewater discharge, and stormwater. Phosphorus is the primary factor behind freshwater cyanobacterial blooms, while nitrogen plays a bigger role in very shallow lakes less than about 10 feet deep.
Warm water accelerates the process. Cyanobacteria, the organisms behind most freshwater HABs, grow fastest at temperatures above 77°F (25°C). But warmth alone isn’t the deciding factor. Blooms have been documented in polar lakes at temperatures below 54°F (12°C), which means nutrient levels matter more than temperature in many cases. Calm, stagnant water also helps, since it allows the organisms to float near the surface where sunlight is strongest.
The Organisms Behind HABs
In freshwater, the main culprits are cyanobacteria, sometimes called blue-green algae, even though they’re technically bacteria rather than true algae. Common bloom-forming types include species in the genera Microcystis and Nostoc. These are the organisms responsible for the green, soupy scum that closes lakes and beaches every summer across much of the United States.
In saltwater, the picture is more varied. Dinoflagellates are single-celled organisms that cause the infamous “red tides” and produce toxins that accumulate in shellfish. Key genera include Alexandrium, Karenia, Dinophysis, and Gambierdiscus. Certain diatoms also cause marine HABs. Pseudo-nitzschia, for example, produces a toxin called domoic acid that can cause serious neurological damage. Even non-toxic species can be harmful: some Chaetoceros diatoms have serrated spines that lodge in fish gills and kill them through tissue damage alone.
How to Spot a Bloom
The EPA identifies three main visual signs of a cyanobacterial bloom: surface water that’s discolored green, white, brown, red, or blue; reduced water clarity that makes it look like pea soup; and thick, mat-like accumulations of scum along the shoreline or floating on the surface. Some blooms look like spilled paint streaking across the water. Others form dense mats that you might mistake for solid ground near the shore.
Not every green pond has a HAB, and not every HAB looks green. Some marine blooms turn water reddish-brown. Others produce no visible change at all while still releasing dangerous toxins. If the water looks unusual in color, clarity, or texture, treat it as potentially harmful until confirmed otherwise.
Health Risks for People
HAB toxins can enter your body three ways: swallowing contaminated water, breathing in tiny droplets near the shoreline, or simply getting the water on your skin. Acute symptoms include nausea, vomiting, abdominal pain, diarrhea, headache, fever, and skin rashes.
The specific harm depends on which toxin is involved. Microcystins, produced by freshwater cyanobacteria like Microcystis, primarily attack the liver. Over 300 different variants of microcystin have been identified, and the most studied form, MC-LR, has been classified by the International Agency for Research on Cancer as a possible human carcinogen. Animal studies have shown it promotes tumor development in the liver and colon with repeated low-level exposure.
Marine HAB toxins cause four distinct poisoning syndromes, all linked to eating contaminated shellfish. Paralytic shellfish poisoning affects the nervous system and can cause numbness, tingling, and in severe cases, respiratory failure. Neurotoxic shellfish poisoning produces similar but generally milder neurological symptoms. Amnesic shellfish poisoning, caused by domoic acid, can lead to permanent memory loss and brain damage. Diarrheic shellfish poisoning causes intense gastrointestinal illness. Each syndrome is caused by a different toxin from a different organism, but the common thread is that shellfish concentrate these toxins as they filter large volumes of water.
Dangers for Dogs and Livestock
Dogs are especially vulnerable to HAB poisoning because they drink lake water freely, lick contaminated fur, and are attracted to the smell of decomposing algae along shorelines. According to Cornell University’s College of Veterinary Medicine, symptoms can appear rapidly or develop over several hours and include vomiting, diarrhea, weakness, pale gums, drooling, muscle tremors, seizures, difficulty breathing, collapse, and in the worst cases, sudden death. Dogs that have been swimming in or near a bloom need immediate veterinary attention. Livestock that drink from contaminated ponds or stock tanks face similar risks.
What HABs Do to Ecosystems
The ecological damage extends well beyond the toxins themselves. When massive quantities of algae or cyanobacteria die, bacteria break down the organic material and consume dissolved oxygen in the process. This can create hypoxic “dead zones” where oxygen levels drop so low that fish, crabs, and other aquatic life suffocate. These dead zones can persist for weeks or months, devastating local fisheries and disrupting food chains that depend on healthy water.
Fish kills from HABs happen through multiple pathways. Some fish die from direct toxin exposure. Others suffocate as oxygen plummets. And species like Chaetoceros diatoms kill fish mechanically by damaging their gills. The cumulative effect on a lake or coastal ecosystem can take years to recover from, particularly when blooms recur season after season.
Safety Thresholds for Recreational Water
The EPA has set specific toxin thresholds for recreational water. For microcystins, the swimming advisory level is 8 micrograms per liter. For cylindrospermopsin, another common cyanobacterial toxin, the threshold is 15 micrograms per liter. When concentrations exceed these levels on any given day, the water is considered unsafe for swimming and other contact activities. States use these benchmarks to decide when to post advisories or close beaches, though monitoring and enforcement vary widely by region.
Climate Change and the Future of HABs
Warming waters are expected to make HABs more frequent and widespread. As ocean and lake temperatures rise, the conditions that favor bloom-forming organisms expand geographically and last longer into the year. Research using Earth system models projects that warmer, less salty coastal waters with more stable surface layers will increasingly favor HAB growth, particularly at higher latitudes where blooms were once rare.
The effects won’t be uniform across all species. A 2025 study in Nature estimated that along the Norwegian coast, blooms of one toxin-producing species could increase by 50% while a different species could decline by 40% under high-warming scenarios. The net result is a shift in which toxins communities are exposed to, not necessarily a simple increase across the board. For regions at higher latitudes, the HAB season is expected to lengthen as thermal windows expand, meaning beaches and fisheries that historically dealt with blooms for a few weeks may face them for months.