red tide

noun

: seawater discolored by the presence of large numbers of dinoflagellates (as of the genera Karenia, Gymnodinium, and Alexandrium) which typically produce a toxin poisonous especially to many forms of marine vertebrate life and to humans who consume contaminated shellfish compare saxitoxin

Examples of red tide in a Sentence

Recent Examples on the Web

Nutrient pollution from freshwater runoff also can fuel toxic red tide, which occurs naturally. —Bill Kearney, Sun Sentinel, 22 May 2025

There’s no public health warning associated with the events, but some people have reported health effects from the red tide. —Grace Toohey, Los Angeles Times, 22 Oct. 2024

Algae with domoic acid is sometimes called red tide. —Carter Evans, CBS News, 7 Apr. 2025

To track algal blooms, including red tide, NOAA maintains a monitoring system using satellite imagery. —Alice Herman, Miami Herald, 20 May 2025

Word History

First Known Use

1904, in the meaning defined above

Rhymes for red tide

• abide
• allied
• applied
• aside
• astride
• azide
• backside
• bankside
• bastide
• bayside
• bedside
• belied

Kids Definition

red tide

noun

: seawater discolored and made poisonous by the presence of large numbers of dinoflagellates

Medical Definition

red tide

noun

: a large area of seawater discolored by the presence of large numbers of dinoflagellates (as of the genera Karenia, Gymnodinium, and Alexandrium) which typically produce toxins poisonous especially to many forms of vertebrate marine life and to humans who consume contaminated shellfish

In simple terms, a red tide is a dinoflagellate population explosion, caused when certain species "bloom," or multiply many times faster than usual.

—Ellen Girardeau

Red tides, the blooms of toxic Karenia brevis algae that appear almost every year in Florida, often lasting for months and turning the ocean red, may be dangerous to asthma sufferers—even if they don't go swimming.

—Nicholas Bakalar

see brevetoxin, saxitoxin, shellfish poisoning