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The term “red tide” is used to describe a type of algal bloom. A certain species of phytoplankton, known as dinoflagellates, contains pigments that range in color from red to brown to green, and when that species is present in high quantities, the water it occupies will take on a hue within that range. In the most severe cases, coastal waters appear to turn red.
The dinoflagellate responsible for red tides in the Gulf of Mexico is a microscopic alga called Karenia brevis. During a bloom, K. brevis produces a dangerous neurotoxin that can kill marine life, contaminate shellfish and can even affect humans in the vicinity.
NOT SO FAST, MY FRIEND
Though many refer to the phenomenon as a “red tide, ” researchers tend to avoid the name for a couple reasons: algal blooms are not always red, and they are unrelated to the flow of tides. The preferred term is “harmful algal bloom” for dangerous algal species and simply “algal bloom” for non-harmful species.
The cell walls of K. brevis are so thin that the cells will burst when tossed around in the surf zone. This releases neurotoxins into the air that can cause coughing, itching and sneezing for humans in the area. But the biggest threat that a harmful algal bloom poses to humans is through contaminated shellfish, which, if eaten, can cause nausea, vomiting and disorientation.
Neither cooking nor freezing a contaminated oyster will kill the red tide toxin. Fish caught in the affected area, however, are usually safe to eat if filleted. That said, avoid eating any fish that appears sick.
The neurotoxin released during a harmful algal bloom can be deadly to many species of marine life, including manatees, dolphins, fish and seabirds. Since the toxin disrupts the central nervous system, affected fish are often described as swimming in a lazy circle. Shellfish, such as oysters, are particularly vulnerable since they feed on the algae responsible for red tides.
The Red Tides of History
- In 1542, Spanish explorer Álvar Núñez Cabeza de Vaca wrote of a large fish kill off the Gulf Coast of Florida, but, without detailed information, researchers are hesitant to attribute the event to a red tide. In 1793, men under the command of Captain George Vancouver in British Columbia experienced sickness and, in one case, death after eating mussels from a cove for breakfast, making it one of the first red tides, and worst breakfasts, on record.
- Red tides occur naturally throughout the world, in places as far-flung as Scandinavia, Japan, Papua New Guinea and Chile.
- Although a yearly event on the beaches of south Florida, red tides are actually pretty rare for Alabama. In December 2015, the state experienced its worst harmful algal bloom in more than a decade, resulting in fish kills and the state closure of shellfish growing beds in Mobile and Baldwin counties.
- While there is no singular cause for red tides, there are several factors that can contribute to an outbreak. Warm water temperatures combined with low salinity, calm seas and high nutrient content is the recipe for a harmful algal bloom. In some cases, human activity, in the form of sewage spills and fertilizer runoff, has been blamed for contributing to algal blooms.
- Because of a red tide’s negative effect on the fishing industry, researchers hope future studies will allow them to more accurately predict impending harmful algal blooms to help shell fishermen plan for the event.