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The last recorded red tide event in Delaware occured in late August and early September, 2007, due to the presence of Karenia brevis – a naturally-occurring dinoflagellate (single-celled phytoplankton with two flagella). This was the first documented occurrence of the organism north of Cape Hatteras, North Carolina. The organism is primarily found on the Gulf Coast of Florida, and it is believed that an eddy from the Gulf Stream brought K. brevis to Delaware’s near-shore waters. 

Since this event the Recreational Water Program and the University of Delaware, College of Marine and Earth Studies have initiated a Karenia monitouring program that extends from early summer to late fall, the known potential red tide season.  Weekly microscopy identification of local algal species along with molecular based Polymerase Chain Reaction (PCR) identification has shown that Karenia brevis can be found in Delaware waters during warm weather months, but at cell densities well under any concern to the public.  Another Karenia species, K. papalonaecea, is also found in Delaware waters during warm weather months, but this species is not as toxic as K. brevis and has not been identified in bloom concentrations.

The health risks associated with red tides are caused by the ability of K. brevis to produce brevetoxin, which may be released and aerosolized when the organism is broken up in the surf.  Its effects are well documented in Florida. The aerosolized toxin is 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 Delaware bloom were 14,000 cells per liter. Despite relatively low levels of the organism, anecdotal reports of respiratory irritation, conveyed to the Department after the bloom, raise the possibility of respiratory irritation while on the beach or in the water when levels of the organism are much lower than the recognized threshold levels for expression of symptoms – possibly as low as 5,000 cells per liter.



Karenia brevis (cells/liter)

Possible Effects (K. brevis only)


background levels of 1,000 cells or less



>1,000 to <5,000

Possible respiratory irritation in sensitive individuals


5,000 to 10,000

Possible respiratory irritation in sensitive individuals and shellfish harvesting closures


>10,000 to <50,000

Respiratory irritation more likely in general population; but not widespread


50,000 to <100,000

Respiratory irritation more likely; possible fish kills


100,000 to <1,000,000

Respiratory irritation likely in general population; probable fish kills



As above, plus discoloration


If you experience coughing, asthma-like symptoms, or other respiratory irritation while on the beach or while swimming, simply moving away from the water will generally eliminate symptoms.  If you experience such symptoms, and believe they are a result of your presence on the beach and/or while swimming, please contact the Delaware Department of Natural Resources at 302-739-9939. We are glad to take your call, and will initiate a follow-up investigation.


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