Algae, or phytoplankton, are seen everywhere because they create the base of both marine and freshwater food webs. Though phytoplankton are small, they can grow explosively, creating something called a "bloom." As normal components of the aquatic environment, the proliferation of algae generally provides the energy source to fuel food webs, so most algae are not harmful even when they form blooms. However, when algae bloom in significant numbers (approximately 1 million cells per liter of seawater equals a "bloom") and produce toxic or harmful effects, such events are termed harmful algal blooms or HABs.
A small percentage of algal species cause harm to humans and the environment through toxin production or excessive growth—these are the harmful algae and they include both microalgae (microscopic, single celled organisms) and macroalgae (seaweeds). Some types of harmful algae produce potent toxins which cause illness or death in humans and marine organisms—fish, seabirds, manatees, sea lions, turtles, and dolphins are some commonly affected animals. Other types of harmful algae are nontoxic to humans but cause harm to fish and invertebrates by damaging or clogging their gills or by forming such large blooms that the death and subsequent decay of the algae lead to hypoxia (oxygen depletion) in the bottom waters of marine environments forcing animals to either leave the area or die.
HABs can also have less lethal effects that range from noxious odors and aerosols to the production of slimes. In some circumstances, due to coastal wind and wave action, algal blooms will produce components that can be transported through the air, causing severe eye, nose, and throat irritation, much akin to pollen and other plant constituents on land. Many of these effects can have serious economic impacts on communities in coastal areas that depend on marine resources for their livelihood.
HABs occur in almost every U.S. coastal state; examples include toxin-producing dinoflagellates in New England, the mid-Atlantic, the Gulf of Mexico, the entire West Coast including Alaska, and tropical and subtropical states and possessions; toxic diatoms in the Gulf and along the West Coast; and toxin-producing flagellates and gill-clogging diatoms in the Pacific Northwest.
Perhaps the best known HAB is the “red tide” that occurs nearly every summer along Florida’s Gulf Coast and that Spanish explorers first observed in the 16th century! The organism that causes the Florida red tide (which may not always appear red), a microscopic alga called Karenia brevis, produces a toxin that makes shellfish dangerous to eat. It also kills fish, and in some instances, dolphins and manatees. It may also make the surrounding air difficult to breathe due to aerosolized toxins. Scientists have been monitoring and studying the phenomenon for a number of years to determine how to detect and forecast the location of the blooms. The goal is to give communities advance warnings so they can adequately plan and deal with the adverse environmental and health effects associated with these red tide events.
The factors controlling HAB development and decline are not well understood for many harmful species. In general, algal growth is enhanced when environmental conditions (such as light, temperature, salinity, and nutrient availability) are optimal for growth. Other biological (e.g., grazing by zooplankton) and physical (e.g., transport by ocean currents) factors determine if biomass will accumulate. Some high biomass HABs have been linked to nutrient over-enrichment of coastal waters. Nutrient over-enrichment from human activities can occur when runoff (e.g., from lawns and farmland), wastewater discharges (e.g., from industry and municipalities), and atmospheric deposition supply nutrients at a rate that "overfeeds" the algae that exist normally in the environment. Some HABs have also been reported in the aftermath of natural phenomena including sluggish water circulation, unusually high water temperatures, and extreme weather events such as hurricanes, floods, and drought.
Human illnesses caused by HABs, though rare, can be debilitating or even fatal. The person grows ill after consuming seafood that is tainted from the ingestion of biotoxin-producing algae.