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

Some of us are floating on top of the water like it’s been a red tide. —Amanda Rosa, Miami Herald, 12 May 2026

In California, the majority of red tides occur between early spring and late summer, according to the California Sea Grant. —Alexiah Syrai Olsen, Sacbee.com, 4 May 2026

While there, he was exposed to a red tide (caused by algae) while cleaning up after a hurricane. —Beth Mole, ArsTechnica, 1 May 2026

The next year, discharges from Lake Okeechobee into the coastal estuaries fueled algal blooms that blanketed the bays in cyanobacteria and red tide. —Michael Adno, Rolling Stone, 3 Mar. 2026

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