Algae

What a harmful algal bloom is

A harmful algal bloom, often shortened to HAB, is a rapid overgrowth of algae or cyanobacteria in a body of water. Most of the blooms that affect fresh water are cyanobacteria, once called blue-green algae, which are not true algae but bacteria that behave like them.

These organisms live in nearly every lake, river, and reservoir at low levels most of the time. The problem starts when conditions let them multiply out of control, forming dense mats or a paint-like scum on the surface. The EPA notes that not every bloom produces toxins, but you cannot tell a toxic bloom from a harmless one by looking at it. That uncertainty is why health agencies treat any visible bloom as a potential hazard.

Three ingredients drive a bloom: nutrients, warmth, and slow-moving water. When all three line up, cyanobacteria multiply fast.

Nutrient pollution is the main fuel. Nitrogen and phosphorus wash off farm fields, lawns, and wastewater into lakes and rivers, and the EPA identifies this runoff as the leading cause of the nutrient overload that feeds blooms. Warm water accelerates growth, which is why blooms peak in late summer and why climate change is extending the season. Still or slow water lets the algae concentrate at the surface instead of dispersing. Lake Erie sees a major bloom nearly every summer for exactly these reasons, fed by farm runoff draining into its shallow, warm western basin.

The toxins these blooms produce

Cyanobacteria can release several toxins, and each one targets the body differently. Microcystin is the most common in US fresh water. It damages the liver, and it was the toxin behind the 2014 Toledo shutdown.

Other toxins raise separate concerns. Cylindrospermopsin also harms the liver and kidneys. Anatoxins attack the nervous system and can kill an animal within minutes of a large dose. In salt water and along the coast, different organisms produce their own toxins. The blooms known as red tides off Florida and the Gulf Coast release brevetoxins, which the National Oceanic and Atmospheric Administration links to respiratory irritation in beachgoers and large fish kills. These toxins do not always clear when the visible bloom fades, and some persist in the water after the scum is gone.

How the toxins reach people

Toxins from a bloom reach you through 3 main routes, and drinking water is the one that affects the most people at once. When a bloom forms near a public water intake, toxins can pass into the supply, which is what happened in Toledo. The EPA has issued health advisory levels for microcystin in drinking water, set at 0.3 micrograms per liter for children under six and 1.6 micrograms per liter for everyone else.

Recreation is the most common route for direct contact. Swimming, wading, boating, and water skiing put people in contact with toxins through the skin, and swallowing even a small amount of contaminated water raises the risk. Seafood is the third path. Shellfish filter large volumes of water and concentrate toxins in their tissue, which is why states close shellfish beds during coastal blooms. The FDA and state agencies monitor commercial harvests, but shellfish gathered privately during a bloom carry real risk.

What the toxins do to the body

The health effect depends on the toxin and the exposure. Contact with bloom water often causes a rash, along with irritation of the eyes, nose, and throat. Swallowing contaminated water can bring stomach pain, vomiting, diarrhea, headache, and fever, the CDC reports.

Higher exposure does deeper harm. Liver toxins like microcystin can damage liver function, and neurotoxins like anatoxin can cause numbness, tingling, dizziness, and in severe cases difficulty breathing. Children face greater risk than adults because they swallow more water while playing and weigh less, so the same dose hits harder. People with liver conditions are also more vulnerable. Along the coast, NOAA notes that brevetoxins from red tide can become airborne in sea spray, triggering coughing and breathing trouble in people on the beach, especially those with asthma.

Dogs are the most frequent victims of harmful algal blooms, and the outcomes are often fatal. A dog that swims in bloom water and then licks its fur can swallow a heavy dose of toxin, and its smaller size means a lethal amount is far lower than for a person.

The speed is what makes it deadly. Neurotoxins can kill a dog within 30 minutes to an hour of exposure, the CDC reports, often before an owner can reach a veterinarian. Dogs are drawn to the water, drink from it freely, and do not avoid scum the way people do. Every summer, veterinary and news reports document dogs dying after swimming in blooming lakes and ponds, which is why health agencies urge owners to keep pets out of any water with visible algae.

No single agency owns the problem, so several share it. The EPA sets drinking water health advisories for cyanotoxins and helps states manage nutrient pollution. The CDC runs the One Health Harmful Algal Bloom System, which tracks bloom-related illnesses in people and animals across the country.

NOAA focuses on the water itself. It operates a Harmful Algal Bloom forecast system that predicts blooms in Lake Erie, the Gulf of Mexico, and other waters, giving communities advance warning. State and local health departments do the frontline work, testing water, posting advisories, and closing beaches and shellfish beds when toxins climb. Many states publish bloom maps and advisory pages online during the summer season, which is where a household can check before heading to the water.

What a household can do

Check before you go, because a current advisory is the clearest warning you will get. Many state health and environmental agencies post bloom reports and interactive maps online during bloom season, and NOAA publishes forecasts for the most affected waters. When in doubt, stay out.

Learn to read the water. Avoid any water that looks like spilled paint, pea soup, or a green, blue, or brown scum, and keep away from mats and foam along the shore. Do not swim, wade, or let children play in it, and never drink or cook with untreated lake or river water, since boiling does not remove cyanotoxins and can concentrate them. Keep pets out entirely, and if a dog does get in, rinse it with clean water right away before it licks its fur and watch for symptoms. If you are on a public water system, your utility monitors for these toxins and will issue a notice if levels rise, so follow any do-not-drink advisory exactly. Private well owners near a bloom-prone lake should not assume their water is unaffected. If you develop a rash, stomach illness, or breathing trouble after contact with bloom water, contact a doctor, and call a veterinarian immediately if a pet shows signs after swimming.