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What is a Red Tide?



“Red Tide” is the common term for a particular type of harmful algal bloom made up of large concentrations of toxic red dinoflagellates called Karenia brevis (K. Brevis). These are tiny red-colored, naturally-occurring aquatic microorganisms which, in sufficient concentrations, can cause a reddish tint to the water. At very high concentrations, they can cause toxic or harmful effects on people, fish, shellfish, marine mammals, and birds.

If you believe you have observed a potential harmful algal bloom, contact the Recreational Water Program, at 302-739-9939.

Delaware last experienced a red tide in late August and early September of 2007. It was the first documented occurrence of Karenia brevis north of Cape Hatteras, North Carolina. The organism is primarily found on the Gulf Coast of Florida. Scientists believe that an eddy from the Gulf Stream brought K. brevis to Delaware’s near-shore waters.

Although Delaware has not experienced another bloom of K. brevis, the State continues to monitor for harmful algae blooms such as those which cause red tides.

K. brevis produces brevetoxin, which may be released and aerosolized when the organism is broken up in the surf. The aerosolized toxin has been documented by Florida officials to cause respiratory irritation in the general public when levels reach 100,000 to 200,000 cells per liter. Effects may include coughing and/or asthma-like symptoms.

Maximum confirmed densities during the 2007 Delaware bloom were 14,000 cells per liter.

Red Tide-Related Symptoms

LevelCells Per Liter of K. brevisPossible K. brevis Effects
PresentBackground Levels (1,000 or fewer)None
Very Low-aFrom 1,001 to 4,999Possible respiratory irritation in sensitive individuals
Very Low-bFrom 5,000 to 10,000Possible respiratory irritation in sensitive individuals
and shellfish harvesting closures
Low-aFrom 10,001 to 49,999Respiratory irritation more likely in general population;
but not widespread
Low-bFrom 50,000 to 99,999Respiratory irritation more likely; possible fish kills
MediumFrom 100,000 to 999,999Respiratory irritation likely in general population; probable fish kills
High1,000,000 or more Respiratory irritation likely in general population; probable fish kills;
and discoloration of the water



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