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Cyanobacteria and Algae Blooms

Click here for more information on HSB’s studies of cyanobacteria and harmful algal blooms.

What are algae?
Algae are simple plants that range in size from microscopic to large seaweeds more than one hundred feet in length. The most common microalgae are green, brown, red, algae and cyanobacteria (called blue-green algae). Green, brown, and red microalgae are chlorophyll-containing, eukaryotic organisms, similar to plants; cyanobacteria are photosynthetic prokaryotic organisms, similar to bacteria. Algae are found almost everywhere on the planet, in sea water, fresh water, and in brackish water, which is a combination of fresh and sea water.

What is an algal bloom?
An algal bloom is a rapid increase in the population of algae in a water system. Algal blooms result when water temperatures are warm and when nutrients, such as nitrogen and phosphorus, are present in the water.

What is a harmful algal bloom (HAB)?
Not all algal blooms are harmful. However, blooms can be harmful when they are so thick that they block sunlight that other organisms need to live. When bloom organisms die and decompose, they deplete the oxygen in the water and starve fish and plants, causing fish kills and damaging local ecology. Some algae produce toxins and release them into the water. During a bloom, the amount of toxin present in the water can poison people, wild animals, and pets that go near the water, consume the water, or swim in the water. Cyanobacteria (blue-green algae) blooms and other algal blooms 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. On this website, the term harmful algae bloom (HAB) refers to the fast growth of any algae or cyanobacteria that results in harm to animals, people, or the local ecology.


What are cyanobacteria?
Cyanobacteria are microscopic organisms found in all types of water. They are more like bacteria than plants, but because they live in water and use sunlight to create food (photosynthesis) they are often called "blue-green algae." Cyanobacteria are important to marine and freshwater ecosystems because they make oxygen as a by-product of photosynthesis and they are a food source for other organisms.

Why is a cyanobacteria bloom harmful? 
Under the right environmental conditions, such as high levels of nutrients, cyanobacteria can multiply quickly and form a bloom. Some species of cyanobacteria produce toxins. Toxins can cause harm to people, wildlife, and pets when they drink contaminated water. There is some evidence that these toxins can accumulate in freshwater fish and shellfish.

Red Tides

What is a Florida red tide?
When the microscopic algae called Karenia brevis grows quickly, it can create blooms, called Florida red tides, that make the ocean appear red or brown.

Why is a Florida red tide harmful?
K. brevis produces powerful neurotoxins called brevetoxins, which can kill fish and other marine organisms. Florida red tides damage local fishing industries, shoreline quality, and local economies.

HSB epidemiologists have led a number of studies to investigate the public health impacts of blue-green algae blooms and Florida red tide. They have shown that there is the potential for exposure to potent HAB-related toxins during recreational (eg., boating) and occupational (eg., lifeguard) activities on water bodies with ongoing blooms.