What is a Red Tide?

A red tide can occur in the ocean or bodies of fresh water

A "red tide," or harmful algal bloom, occurs when algae in the ocean and freshwater grow out of control and can produce powerful toxins that can harm or kill marine life, birds and even humans.

And while many people refer to the blooms as a "red tide," scientists prefer to use the term harmful algal bloom, according to the National Oceanic and Atmospheric Administration's National Ocean Service.

Pesticides and other chemicals used in farming and factories eventually find their way into oceans and other bodies of water and create harmful algal blooms. And those blooms can kill the fish, shellfish, birds and mammals if eaten.

If humans eat any fish or shellfish that came from a red tide area, they too will ingest the toxins and that can make them sick, and in some cases die. As a result, some places restrict fishing until the conditions improve, and even restaurants will take local fish and shellfish off the menu.

The blooms have been reported in every coastal state, and NOAA says the frequency of red tides has increased. Not only is that harmful to people and marine ecosystems, but local and regional economies may also suffer as a result.

The species that causes the most red tides in Florida and the Gulf of Mexico is called Karenia breves, or K. breves for short. K. brevis usually blooms in the late summer or early fall, and large concentrations can discolor the water red or brown.

Pseudo-nitzschia can be found year-round in Florida waters and produce a neurotoxin called domoic acid, which can kill or sicken marine mammals and birds. It can also be dangerous for people if they consume shellfish that are contaminated with the toxins.

Pyrodinium bahamense typically blooms in Florida waters during the summer months, especially the Indian River Lagoon, Tampa Bay and Charlotte Harbor. Toward the end of those blooms, P. bahamense forms a seed-like cyst that falls to the bottom and settles in the sediment until ready to germinate. It can also produce neurotoxins that can collect on shellfish, posing a risk to humans if ingested.

Non-toxic blooms can have a dangerous effect, too. When a large area of algae, like P. bahamense, dies and starts to decompose, that can deplete oxygen levels in the water causing animals to leave the area or die-off. The smell from the decaying algae can also irritate the lungs of humans and birds in the area.

The National Ocean Service has been studying red tides for several years and is working on ways to detect them before they start, and give regions an advance warning so they can plan ahead for the effects the blooms can have on local ecosystems and economies.