If you live near a coastal area, then you may have heard about red tides from time to time. Although they sound pretty, red tides usually bring dire warnings with them.

Red tide is a common but deceptive term for a natural phenomenon that occurs regularly along most coastal areas. Instead of red tide, scientists call these events harmful algal blooms or HABs.

Seawater always contains tiny plant-like organisms called algae. Usually algae can only be seen with a microscope. Under certain conditions, algae can begin to reproduce rapidly. This causes the algae population to increase rapidly or “bloom.” During an algal bloom, a single gallon of seawater can contain millions of algae.

Not all algal blooms are harmful. Some algae, however, produce harmful substances called toxins. For example, the Florida Gulf Coast often experiences harmful algal blooms of a type of algae called Karenia brevis, which produces toxins called brevetoxins.

Blooms of Karenia brevis can discolor the water, turning it yellow, orange, pink, brown or red. This discoloration is probably what led some people to use the term red tide. Scientists don’t like the term red tide, however, for several reasons.

Harmful algal blooms don’t always discolor the water and, when they do, it doesn’t always appear red. Moreover, harmful algal blooms have nothing to do with tides. This is why scientists prefer the more accurate term harmful algal bloom instead of red tide.

Harmful algal blooms are called harmful because the toxins produced by the blooming algae can kill millions of fish and other marine organisms. One tell-tale sign of a harmful algal bloom is a beach covered with dead fish.

Harmful algal blooms can also be harmful to humans. People have been known to get sick from eating shellfish that feed on the toxic algae. Some experts also believe there may be negative effects associated with breathing the air near or swimming in water during a harmful algal bloom.

Because of the effects on marine and human health, as well as shoreline quality and the fishing industry, scientists are trying to learn more about the causes of harmful algal blooms. Some believe they’re simply natural occurrences caused by the seasonal shifting of ocean currents. Others believe human factors, such as pollution, may also play a role.

Algal blooms of non-toxic algae are not necessarily harmful. In fact, they’re often beneficial because they produce more food — the algae — for the marine life that feeds on them.

Scientists have learned that even harmful algal blooms may have benefits, too. For example, even though some fish are killed, scientists have noted that other marine populations seem to increase. This has led some scientists to theorize that harmful algal blooms may be one of nature’s ways of balancing the populations of various marine species.


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    • Hi, Maya! Although some of the algae blooms aren’t harmful to fish, scientists believe certain ones are (even harmful enough to kill fish). Thanks so much for visiting today’s Wonder of the Day®! :-)

  1. The video was really interesting. I love the water, that’s why I think the video was really AMAZING!!!!!!!!!!!!!!!!!!!!!!!

    • Thanks for letting us know what you thought about the video for this Wonder of the Day®, Kendall! We appreciate your comment! :-)

    • That’s a WONDERful idea for a future Wonder of the Day®, WowowoW! Thanks so much for suggesting it and for letting us know you liked today’s Wonder, too! :-)

    • Thanks for letting us know you thought today’s Wonder was cool and that you like the Valentine’s Day background, Paige! We appreciate your comment! :-)

  2. I love the water, but it was so sad to see all of the fish that had died and had been washed up on shore. I think that tomorrow’s wonder is about candy, or some other kind of dessert.

    • Hello, Liddie! We think it is sad to see all the fish wash up on shore, too, but learning some reasons why this happens can make us smarter about our environment and help scientists find ways to keep it from happening more frequently. Thanks so much for visiting this Wonder of the Day® and guessing what you think the next Wonder will be about…we think that’s an AWESOME guess! :-)

    • We’re proud of you for thinking about ways to help people with food, Clayton! Thanks so much for visiting this Wonder of the Day®! :-)

    • Hello, Sienna! Thanks so much for leaving us this GREAT comment today! We hope you enjoyed learning about red tides (or HABs) by exploring this Wonder of the Day®! :-)

  3. I thought today’s wonder was so cool! I never knew that there was a red tide. In Spanish class we were watching a clip about these children who lived in Mexico and what their family would do every single day. Thank you so much for giving me this link!

    • We thought you might like exploring some of the other ocean-related Wonders, Emma! Have a GREAT day and THANKS for being an AWESOME Wonder Friend! :-)

    • Emma,

      We’re so glad you enjoyed the Wonder of the Day® on red tides. It’s pretty interesting, isn’t it?

      Thanks for coming by to comment. Keep up the good work in your Spanish class! :)

  4. I think I left something out. This one girl, their graveyard was on another side of the water so they had to wait for the tide to go down.

  5. We have recently visited Rhode Island. At the beach, in Point Judith there was a red tide. I WONDER if harmful algal bloom can be dangerous to humans. I made sure I washed really well in the shower to get it all off. :-)

    • What a WONDERful place to visit, Charlie! We think it’s SUPER that you spotted a red tide and have been WONDERing about its impact on humans. Usually humans are affected by red tide when they consume fish that have been living in those red tide toxins. We hope you continue to WONDER on your own about the habitats of sea life! Thanks for sharing your comments! :)

  6. I was WONDERing if the red seaweed can kill people just like the fish. Can it kill sharks? I do not like swimming in red tide because it is kind of gross. :-I

    • What a great WONDER, Helena! We WONDER if you’ve been doing some research of your own on this fishy topic? While some algae isn’t harmful, scientists believe HABs (red tide) have toxins that can harm sea life. Humans can get sick when they eat that have been contaminated. We hope you continue to WONDER about keeping yourself and sea life healthy and happy in their habitats! Thanks for commenting! :)

  7. It is a tide that has a lot of blood in it, warning the people that a heard of man-eating sharks that can run on land are coming.
    I’m sooooo kidding. They aren’t real.
    :) :) :)

    • Hey there, Tyler J! We love your imagination, but we hope you learned something new from our Wonder, too! Thanks for sharing your comment! :)

    • We are glad you’ve been thinking about the ecosystem and the wildlife affected by a red tide, Wonder Friend Nike. Thanks so much for sharing your comment and thinking outside the box! :)

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Have you ever wondered…

  • What is a red tide?
  • What term do scientists prefer instead of red tide?
  • What causes red tides?

Wonder Gallery

Try It Out

Although there are many negative effects caused by a red tide, there is a unique positive side effect. In some areas, phytoplankton that are bioluminescent put on a special light show after dark.

As the waves toss them about, phytoplankton emit flashes of light. When the surf rolls in after dark, beach walkers are treated to a natural light show courtesy of Mother Nature.

Here are some videos of bioluminescent phytoplankton in action:


Still Wondering

In National Geographic Xpeditions’ Changing With the Tide lesson, children learn about and act out the functions of the salt marsh habitat.


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