Why do harmful algal blooms occur?

Research continues on the causes of harmful algal blooms.

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Satellite image of Lake Erie showing a mild harmful algal blom in the southwestern portion of the lake.

Harmful algal blooms (HABs) occur when colonies of algae—simple plants that live in the sea and freshwater—grow out of control while producing toxic or harmful effects on people, fish, shellfish, marine mammals, and birds.

While we know of many factors that may contribute to HABs, how these factors come together to create a 'bloom' of algae is not well understood.

Studies indicate that many algal species flourish when wind and water currents are favorable.

In other cases, HABs may be linked to 'overfeeding.' This occurs when nutrients (mainly phosphorus, nitrogen, and carbon) from sources such as lawns and farmlands flow downriver to the sea and build up 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 like sluggish water circulation, unusually high water temperatures, and extreme weather events such as hurricanes, floods, and drought.

People often get sick by eating shellfish containing toxins produced by these algae. Airborne HAB toxins may also cause breathing problems and, in some cases, trigger asthma attacks in susceptible individuals.

HABs can also be costly in economic terms as well. At present, HABs cause about $82 million in economic losses to the seafood, restaurant, and tourism industries each year. HABs reduce tourism, close beaches and shellfish beds, and decrease the catch from both recreational and commercial fisheries.

NOAA scientists continue to monitor and study HABs to determine how to detect and forecast the location of the blooms. The goal is to give coastal communities advance warning, so they can adequately plan and deal with the adverse environmental and health effects associated with a harmful bloom.

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