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
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
While there, he was exposed to a red tide (caused by algae) while cleaning up after a hurricane. — Beth Mole, ArsTechnica, 1 May 2026 When Floridians think about water pollution, many picture visible problems like red tide blooms, dead fish or closed beaches. — Iuliia Istratiy, Sun Sentinel, 20 Jan. 2026 When activated, the material diminishes the growth of red tide blooms. — Dinah Voyles Pulver, USA Today, 13 Mar. 2026 For Florida’s Gulf Coast, that means researching red tide, addressing habitat loss and restoring overfished species. — Alice Herman, Miami Herald, 20 May 2025 See All Example Sentences for red tide

Word History

First Known Use

1904, in the meaning defined above

Time Traveler
The first known use of red tide was in 1904

Rhymes for red tide

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Browse Nearby Words

red tick
red tide
red tiercel
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Cite this Entry

“Red tide.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/red%20tide. Accessed 7 Jul. 2026.

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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

More from Merriam-Webster on red tide

Britannica.com: Encyclopedia article about red tide

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