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
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Other sources that lead to beach closures and advisories are harmful algal blooms − such as red tides − sewage, and chemical spills, according to the National Ocean Service.
—
Thao Nguyen, USA Today, 1 Sep. 2025
But too much of one specific type of algae can be toxic, causing a harmful algal bloom, also sometimes known as a red tide.
—Lex Harvey, CNN Money, 26 July 2025
Information on how red tide affects sawfish and other marine life will be shared, along with details on the MISS organization (Minorities in Shark Sciences) which is helping further shark research.
—Orlando Sentinel Staff, The Orlando Sentinel, 12 June 2025
Nutrient pollution from freshwater runoff also can fuel toxic red tide, which occurs naturally.
—Bill Kearney, Sun Sentinel, 22 May 2025
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Thao Nguyen, USA Today, 1 Sep. 2025
But too much of one specific type of algae can be toxic, causing a harmful algal bloom, also sometimes known as a red tide.
—Lex Harvey, CNN Money, 26 July 2025
Information on how red tide affects sawfish and other marine life will be shared, along with details on the MISS organization (Minorities in Shark Sciences) which is helping further shark research.
—Orlando Sentinel Staff, The Orlando Sentinel, 12 June 2025
Nutrient pollution from freshwater runoff also can fuel toxic red tide, which occurs naturally.
—Bill Kearney, Sun Sentinel, 22 May 2025
See All Example Sentences for red tide
Cite this Entry
“Red tide.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/red%20tide. Accessed 9 Sep. 2025.
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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
Ellen GirardeauRed 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
Nicholas BakalarLove words? Need even more definitions?
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