red tide

American  

noun

1. a brownish-red discoloration of marine waters caused by the presence of enormous numbers of certain microscopic flagellates, especially the dinoflagellates, that often produce a potent neurotoxin that accumulates in the tissues of shellfish, making them poisonous when eaten by humans and other vertebrates.

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red tide British  

noun

1. a discoloration of sea water caused by an explosive growth in phytoplankton density: sometimes toxic to fish life and, through accumulation in shellfish, to humans

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red tide Scientific  

1. A population explosion of certain species of dinoflagellates, a kind of protozoan found in plankton. The dinoflagellates color the water red or reddish-brown and secrete a toxin that kills fish. Red tide usually occurs in warm coastal waters.

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Etymology

Origin of red tide

First recorded in 1900–05

Example Sentences

See Examples For:

red tide

Yet Mr. Jolly is also swimming against the red tide in his new party.

From The Wall Street Journal ● Aug. 19, 2026

Sawfish necropsies have not revealed any pathogen or bacterial infections, nor problems with low water oxygen levels or contaminants such as chemicals, or toxic red tide.

From Seattle Times ● Apr. 12, 2024

This behavior mirrors the natural processes at play when the dinoflagellates cause bioluminescence in the ocean during red tide events.

From Science Daily ● Oct. 20, 2023

A red tide in the San Francisco Bay Area is killing thousands of fish and other marine life, creating a foul odor as a heat wave moves in.

From Los Angeles Times ● Aug. 5, 2023

Sierra leapt toward the nearest tree, caught a branch and swung up onto it as Robbie’s red tide passed.

From "Shadowshaper" by Daniel José Older