What Is Red Tide? NOAA on the Harmful Algal Bloom That Closes Beaches
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How we wrote this: A plain-English summary of NOAA National Ocean Service guidance on red tides, checked October 29, 2026. Follow local beach and shellfish advisories.
Red tide = a harmful algal bloom
"Red tide" is a common name for a harmful algal bloom (HAB). Scientists prefer the term HAB because not every bloom turns the water red, and the tide doesn't cause it.
HABs happen when colonies of algae grow out of control while producing toxic or harmful effects on people, fish, shellfish, marine mammals and birds. NOAA says the human illnesses are rare but can be debilitating or even fatal.
Florida's Gulf Coast bloom
One of the best-known HABs in the U.S. occurs nearly every summer along Florida's Gulf Coast. NOAA explains that, like many HABs, it's caused by microscopic algae that produce toxins which:
- kill fish;
- make shellfish dangerous to eat; and
- can make the air difficult to breathe near the shore.
The water often turns red.
Not all blooms are bad
NOAA notes that most algal blooms are beneficial, because the tiny plants feed ocean animals and power the food web. Only a small percentage of algae produce powerful toxins. Even non-toxic blooms can do harm: when masses of algae die and decompose, they can drain oxygen from the water, so animals leave or die.
HABs have been reported in every U.S. coastal state, and NOAA says their occurrence may be on the rise. NOAA scientists work on detecting and forecasting blooms to give communities advance warning.
Staying safe
- Check local beach advisories before you go, and heed closures; see beach water quality.
- If the air makes you cough or your eyes itch near a bloom, move away from the water.
- Don't harvest shellfish from closed areas.
- Keep pets out of discolored water and away from dead fish.
For freshwater blooms and health symptoms, see harmful algal blooms.
Related: Vibrio, sea foam, what is seaweed.