Red Tide

Ocean water is blue or green, right? Not always. During a red tide, waters near a coast line can appear red, orange, brown, pink, or yellow. But what causes a red tide? Is it dangerous?


Fun Facts

  • Ocean water is filled with microscopic algae. Fish and other sea life eat these organisms.
  • Sometimes, algae begin reproducing at a rapid rate. When this occurs, the water might appear red because of all the algae in it.
  • Certain types of algae are harmful. They produce a toxin that can harm both humans and animals. During a red tide, it is common to find dead fish on the beach.
  • Scientists aren’t sure what causes red tides. Red tides might simply be a natural phenomenon. Some scientists think pollution might play a role.
  • Scientists don’t like the term “red tide” because the situation has nothing to do with the tides, nor does it always cause the water to turn red. They prefer the term “harmful algal blooms.”
  • Not all algal blooms are harmful. Some algae do not produce toxins. These algae provide food for ocean animals.
  • During a red tide, you might notice lights in the water at night. This is because the algae is sometimes bioluminescent. In other words, it glows in the dark.



  1. Microscopic: so tiny it can’t be seen with the naked eye
  2. Toxin: harmful substance
  3. Bioluminescent: naturally capable of producing light

Learn More

Watch a video about red tides.