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

The funds have supported a wide range of projects, from shoreline restoration and coral reef rehabilitation to aquaculture research and red tide tracking. —Alice Herman, Miami Herald, 20 May 2025

Sea lions Sea lions have been washing up dead or sick on Southern California’s coastline due to contact with a toxic algal bloom, often referred to as red tide. —Jade Walker, CNN Money, 7 Apr. 2025

State of play: Medium concentrations of red tide were found in samples from St. Pete Beach, Madeira Beach and Dunedin Causeway, according to the county and the Florida Fish and Wildlife Conservation Commission. —Jeff Weiner, Axios, 2 Oct. 2024

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