

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)


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.


What a cyanobacterial bloom looks like


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.

Red Tide

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




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