Harmful Algal Blooms

What is a harmful algal bloom?

A harmful algal bloom (HAB) occurs when toxin-producing algae grow excessively in a body of water. Algae are microscopic organisms that live in aquatic environments and use photosynthesis to produce energy from sunlight, just like plants. The excessive algal growth, or algal bloom, becomes visible to the naked eye and can be green, blue-green, red, or brown, depending on the type of algae.

Algae are always present in natural bodies of water like oceans, lakes, and rivers, but only a few types can produce toxins. In these algae, toxin production can be stimulated by environmental factors such as light, temperature, and nutrient levels. Algal toxins released into the surrounding water or air can seriously harm people, animals, fish, and other parts of the ecosystem.

Why do HABs occur?

Scientists are still learning why HABs occur. They know that environmental conditions trigger HABs, such as warmer water temperatures in the summer and excessive nutrients from fertilizers or sewage waste that are brought by runoff. As climate change gradually warms the earth’s climate, scientists expect HABs to become more frequent, wide-ranging, and severe.

How are people exposed?

During a HAB, people can get exposed to toxins from fish they catch and eat, from swimming in or drinking the water, and from the air they breathe. In recent years, there have been numerous instances of HABs in lakes that provide drinking water, like Lake Erie.

Exposure to HABs can be prevented by following local health advisories regarding the safety of recreationally caught seafood and drinking water sources. Importantly, cooking contaminated seafood or boiling contaminated water does not destroy the toxins.

People rarely get sick from HAB-related toxins in commercial seafood, however, because state health officials closely monitor fisheries for HABs and close them during blooms.

What are the health effects of harmful algal blooms?

Depending on the type of algae, HABs can cause serious health effects and even death. For example, eating seafood contaminated by toxins from algae called Alexandrium can lead to paralytic shellfish poisoning, which can cause paralysis and even death. The algae Pseudo-nitzschia produces a toxin called domoic acid that can cause vomiting, diarrhea, confusion, seizures, permanent short term memory loss, or death, when consumed at high levels.

HABs that occur in freshwater, like the Great Lakes and other drinking water sources, are dominated by the cyanobacteria Microcystis. This organism produces a liver toxin that can cause gastrointestinal illness as well as liver damage.

As with many environmental exposures, children and the elderly may be especially sensitive to HAB toxins. Populations that rely heavily on seafood are also at risk of long term health effects from frequent, low level exposures to HAB toxins. Scientists are still learning about these potential effects.

The table below summarizes the most common HABs affecting US waters and their health effects.

Organism	Water Type	Color	Toxin	Target tissue	Health effects
Alexandrium sp.	Salt	Red or brown	Saxitoxins	Nerves and muscles	Paralytic shellfish poisoning, paralysis, death
Karenia brevis	Salt	Red	Brevetoxins	Nervous system, respiratory system	Gastrointestinal illness, muscle cramps, seizures, paralysis, and respiratory problems, especially for asthmatics
Pseudo-nitzschia	Salt	Red or brown	Domoic acid	Nervous system	Amnesiac shellfish poisoning, vomiting, diarrhea, confusion, seizures, permanent short term memory loss, or death
Microcystis	Fresh	Blue-green	Microcystin	Liver	Gastrointestinal illness, liver damage

Other impacts from HABs

In addition to health concerns, HABs can cause fish kills and other damage to the environment. HABs can deplete oxygen in the water or simply block necessary sunlight from reaching organisms below the surface.

Economic impacts of HABs on fisheries and recreational areas can also be extensive. Closed fisheries can lose millions of dollars in revenue each week.

What NIEHS is Doing on this Health Topic

Studying health effects from low doses over time
Since the immediate effects of harmful algal blooms are well known, NIEHS-funded scientists are now investigating potential long term effects of HABs. For example, researchers are studying whether consuming trace amounts of neurotoxic domoic acid over time damages brain function, especially among children or the elderly. In animals, scientists have observed that chronic, low level exposure to domoic acid altered the expression of genes controlling the nervous system and impaired cell function.

Developing better detection
Rapid detection of HABs can help state officials protect public health with minimal economic impacts to fisheries and recreational areas. The NIEHS-funded Woods Hole Center for Oceans and Human Health in Massachusetts designed a robotic system called the Environmental Sample Processor that can be anchored in a body of water to remotely test seawater samples for HABs as they begin to bloom, and send the results to scientists. Other NIEHS-supported researchers are developing instruments that can detect HABs by counting algal cells in water samples or use satellite images to monitor HABs from afar.

Scientists are also developing new methods to detect HABs at water treatment plants, and are testing ways to remove HAB toxins from drinking water.

Improving prediction
With the frequency and severity of HABs increasing worldwide, improved prediction enables public health officials to be one step ahead of blooms. Researchers at the Woods Hole Center for Oceans and Human Health found triggers for algal blooms can vary with water depth. In deep waters, the algae Alexandrium fundyense may bloom based on an internal annual clock, while in shallow waters, conditions such as temperature may trigger blooms. These researchers have also shown that the number of dormant cysts formed after a summer Alexandrium fundyense bloom can predict the extent of the next year’s bloom.

A promising cystic fibrosis treatment
HABs of the organism Karenia brevis can produce airborne toxins that cause respiratory irritation, yet sometimes the toxic effects suddenly go away. This observation led to the discovery of brevenal, a compound Karenia brevis produces to counteract its toxins.

Scientists have also found that brevenal can help clear mucus out of the lungs, which may benefit people with cystic fibrosis. Efforts are currently underway to make synthetic brevenal for use as a therapeutic drug.

Where can I get more information?

The NIEHS Oceans and Human Health Program is actively studying HABs and human health. This is the focus of one of the NIEHS Centers for Oceans and Human Health and a number of individual studies around the U.S.

The National Oceanic and Atmospheric Administration studies HABs and provides regional HAB forecasts.