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What is a Harmful Algal Bloom?

A harmful algal bloom (HAB) is the proliferation of a toxic or nuisance algae.

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A harmful algal bloom (HAB) is the proliferation of a toxic or nuisance algal species that negatively affects natural resources or humans.

To further define a harmful algal bloom, each word can be explained more fully.

Harmful is the easiest to describe and understand. The harmful effect might be visible, like floating dead or washed-up fish, or it may be hidden, like the alteration of a food chain (from the loss of prey) or the gradual loss of benthic (bottom) vegetation that is habitat for fishes. It may also impact people and cause human illness through consumption of contaminated seafood.

Algal can apply to either microscopic (cannot be seen without high magnification) plant-like cells or to larger aquatic plants that you can see with the unaided eye, such as "sea lettuce." Both types of algae can be found in seawater, brackish water, or fresh water.

The term algal suggests plants that have chlorophyll, just like land plants. Algae is a broad group of organisms that can be differentiated by pigment type, microstructure, behavior, and other characteristics.

A bloom is difficult to describe. In essence, it is an increased abundance of a species above background numbers in a specific geographic area. For certain species, it may be 200 algal cells per liter of seawater or 2 million algal cells per liter of seawater. It could even be an increased concentration of algal cells that are attached to the substrate, like a blade of seagrass or a piece of seaweed.

There are several main groups that form HABs: flagellates (includes dinoflagellates), diatoms, and blue-green algae. Of the approximately 85 HAB species currently documented, almost all of them can be classified as plant-like microalgae that require light and carbon dioxide to produce their own food using chlorophyll. A few HAB species do not have their own chlorophyll and, thus, do not have the capability to photosynthesize. These species often feed on other organisms and are usually called protists, not microalgae.

Flagellates are single-celled organisms that usually have organic walls. They move about/swim with whip-like appendages and can move up and down in the water column covering up to 15–20 meters in a day. This group includes dinoflagellates; approximately 70 percent of HAB species are dinoflagellates.

Diatoms can be defined as siliceous glass boxes that can be individual cells or attached in chains. They can float at the water's surface or sink to the depths of the ocean.

Blue-green algae (also called Cyanobacteria) are among the oldest bacteria found on earth. They still have plant pigments like algae but do not have the internal microstructure like that of the diatoms and flagellates. Individual cells, filaments, or colonies often float at the sea surface and generally appear blue-green in color.