Harmful Algal Blooms (HABs)
Algae are vitally important to marine and fresh-water ecosystems, and most species of algae are not harmful. Algal blooms occur in natural waters used for drinking and/or recreation when certain types of microscopic algae grow quickly in water, often in response to changes in levels of chemicals such as nitrogen and phosphorus from fertilizer, in the water. Algal blooms can deplete the oxygen and block the sunlight that other organisms need to live, and some can produce toxins that are harmful to the health of the environment, plants, animals, and people. Harmful algal blooms have threatened beaches, drinking water sources, and even the boating venue for the 2008 Olympic Games in Beijing, China. Cyanobacteria (blue-green algae) and red tides are examples of algae that can bloom and produce toxins that may be harmful to human and animal health. HABs can occur in marine, estuarine, and fresh waters, and HABs appear to be increasing along the coastlines and in the surface waters of the United States, according to the National Oceanic and Atmospheric Administration (NOAA). HSB epidemiologists have led a number of studies to investigate the public health impacts of blue-green algae blooms and Florida red tide. The studies have demonstrated that there is the potential for exposure to potent HAB-related toxins during recreational and occupational activities on water bodies with ongoing blooms.
Although scientists do not yet understand fully how HABs affect human health, authorities in the United States and abroad are monitoring HABs and developing guidelines for HAB-related public health action. The U.S. Environmental Protection Agency (EPA) has added certain algae associated with HABs to its Drinking Water Contaminant Candidate List. This list identifies organisms and toxins that EPA believes are priorities for investigation.
Many states regularly experience harmful algal blooms (HABs), and state public health departments are often are asked to provide guidance about HAB-associated human and animal illnesses. HSB subject matter experts help states to develop their public health responses to HAB events, including providing outreach and education materials and assessing exposure and the potential for health effects.
Cyanobacteria (blue-green algae)
Cyanobacteria, also known as blue-green algae, grow in any type of water and are photosynthetic (use sunlight to create food and support life). Cyanobacteria live in terrestrial, fresh, brackish, or marine water. They usually are too small to be seen, but sometimes can form visible colonies, called an algal bloom. Cyanobacteria have been found among the oldest fossils on earth and are one of the largest groups of bacteria. Cyanobacteria have been linked to human and animal illnesses around the world, including North and South America, Africa, Australia, Europe, Scandinavia, and China.
Cyanobacterial blooms and how they form
Cyanobacterial blooms (a kind of algal bloom) occur when organisms that are normally present grow exuberantly. Within a few days, an bloom of cyanobacteria can cause clear water to become cloudy. The blooms usually float to the surface and can be many inches thick, especially near the shoreline. Cyanobacterial blooms can form in warm, slow-moving waters that are rich in nutrients such as fertilizer runoff or septic tank overflows. Blooms can occur at any time, but most often occur in late summer or early fall.
They can occur in marine, estuarine, and fresh waters, but the blooms of greatest concern are the ones that occur in fresh water, such as drinking water reservoirs or recreational waters.
What a cyanobacterial bloom looks like
Some cyanobacterial blooms can look like foam, scum, or mats on the surface of fresh water lakes and ponds. The blooms can be blue, bright green, brown, or red and may look like paint floating on the water. Some blooms may not affect the appearance of the water. As algae in a cyanobacterial bloom die, the water may smell bad.
Harmful Marine Algae
Harmful marine algae, such as those associated with red tides , occur in the ocean and can produce toxins that may harm or kill fish and marine animals. There are many kinds of marine algae that produce toxins that can accumulate in shellfish. In the US, one of the illnesses that may result from eating algal toxin-contaminated shellfish is neurotoxic shellfish poisoning (NSP). NSP is caused by eating shellfish contaminated with brevetoxins, which are produced by Karenia brevis, the marine algae associated with Florida red tides. NSP is a short-term illness with neurologic symptoms (such as tingling fingers or toes) and gastrointestinal symptoms. There are very few cases of NSP in the US because coastal states carefully monitor their shellfish beds and close the beds to harvesting if high concentrations of brevetoxins are detected in the water or the shellfish. Brevetoxins may also be in the air along the Gulf coast of Florida during Florida red tide events and may symptoms such as eye irritation and a sore throat in healthy people. People who have asthma may have symptoms, such as chest tightness, that last for several days after exposure. Ciguatera tides fish poisoning is another disease associated with toxins produced by marine algae. The toxin responsible, called ciguatoxin, accumulates through the food web, and very high levels may exist in reef fish, particularly (but not only) large carnivorous reef fish.
Background: Algae are vitally important to marine ecosystems, and most species of algae are not harmful. However, under certain environmental conditions, microscopic marine algae called Karenia brevis (K. brevis) grow quickly, creating blooms that can make the ocean appear red or brown. People often call these blooms “red tide.”
K. brevis produces powerful toxins called brevetoxins, which have killed millions of fish and other marine organisms. Red tides have damaged the fishing industry, shoreline quality, and local economies in states such as Texas and Florida. Because K. brevis blooms move based on winds and tides, pinpointing a red tide at any given moment is difficult.
Red tides occur throughout the world, affecting marine ecosystems in Scandinavia, Japan, the Caribbean, and the South Pacific. Scientists first documented a red tide along Florida’s Gulf Coast in fall 1947, when residents of Venice, Florida, reported thousands of dead fish and a “stinging gas” in the air, according to Mote Marine Laboratory. However, Florida residents have reported similar events since the mid-1800s.
Assessing the Impact on Public Health
In addition to killing fish, brevetoxins can become concentrated in the tissues of shellfish that feed on K. brevis. People who eat these shellfish may suffer from neurotoxic shellfish poisoning, a food poisoning that can cause severe gastrointestinal and neurologic symptoms, such as tingling fingers or toes.
The human health effects associated with eating brevetoxin-tainted shellfish are well documented. However, scientists know little about how other types of environmental exposures to brevetoxin—such as breathing the air near red tides or swimming in red tides—may affect humans. Anecdotal evidence suggests that people who swim among brevetoxins or inhale brevetoxins dispersed in the air may experience irritation of the eyes, nose, and throat, as well as coughing, wheezing, and shortness of breath. Additional evidence suggests that people with existing respiratory illness, such as asthma, may experience these symptoms more severely.
Ciguatera fish poisoning (or ciguatera) is an illness caused by eating fish that contain toxins produced by a marine microalgae called Gambierdiscus toxicus. Barracuda, black grouper, blackfin snapper, cubera snapper, dog snapper, greater amberjack, hogfish, horse-eye jack, king mackerel, and yellowfin grouper have been known to carry ciguatoxins. People who have ciguatera may experience nausea, vomiting, and neurologic symptoms such as tingling fingers or toes. They also may find that cold things feel hot and hot things feel cold. Ciguatera has no cure. Symptoms usually go away in days or weeks but can last for years. People who have ciguatera can be treated for their symptoms.
- Page last reviewed: January 13, 2012
- Page last updated: July 24, 2012
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