Red tide

Two dead fish lying on a beachReaders may have seen images of large numbers of fish, marine mammals and sea turtles washing ashore on beaches in southwest Florida. While these images are disturbing, the animals’ deaths are not related to discharges from Lake Okeechobee, and are, sadly, not uncommon. They are related to a red tide event in the Gulf of Mexico. To learn more about other algae blooms that are related to Lake O, check out Lisa Krimsky’s blog post on that topic.

What is red tide?

Red tide is the name given to a type of harmful algal bloom. The organism that causes these events in Florida is a type of plankton called a dinoflagellate. Dinoflagellates are neither plant nor animal, but they have characteristics of both. They belong to the group of organisms referred to as protists. Dinoflagellates can photosynthesize, like plants, but many of them can also eat prey. They have two whip-like appendages called flagella. They can use these flagella to swim and orient themselves. Some dinoflagellates, like those that cause red tides, can produce toxins. Most dinoflagellates live in salt water. The species of dinoflagellate that causes red tide blooms in Florida is Karenia brevis.

What is a bloom?

When conditions are just right, dinoflagellates can reproduce rapidly. Their main way of doing this is to divide into two, by a process called mitosis. Thus the population can quickly double, and double again, and so on. When there is a higher than normal amount of microscopic plant-like organisms in a water sample, this is referred to as a bloom. Blooms can be caused by planktonic algae, or by dinoflagellates.

What causes a red tide bloom?

Scientists do not know what causes red tide blooms. Florida red tide blooms typically form 10-40 miles offshore in the Gulf of Mexico, often off southwest Florida. Red tides have been documented in the Gulf of Mexico since the mid-1800’s. A Spanish explorer noted reddish water and lots of dead fish as early as the mid-1600’s. The blooms often drift close to shore, where nutrient runoff from land may prolong the duration of the red tide event.

How do red tides affect wildlife and humans?
three loggerhead sea turtle hatchlings
These loggerhead sea turtle hatchlings washed up on Marineland Beach during the 2007 red tide event.

Karenia brevis produces a toxin called brevetoxin. This toxin gets into the water and can also get into the air. Brevetoxin can affect the central nervous system of fish and other marine life. This is what causes extremely large numbers of dead fish, manatees and sea turtles to wash ashore along Florida’s Gulf coast during a red tide event. People who visit the coast during one of these times may suffer from respiratory irritation. People with asthma or other respiratory conditions should avoid red tide areas. Oysters and other filter-feeders will contain brevetoxin in their guts during one of these blooms. People should not eat recreationally-collected shellfish from an affected area, as the toxins can cause neurotoxic shellfish poisoning. Commercially-collected shellfish are safe to eat, as they are tested for red tide toxins. People should avoid swimming in areas where there are lots of dead fish. Some people may experience skin irritation or burning eyes from swimming during a red tide bloom.

How long do red tides last?

A bloom can last for several weeks, months or even more than a year. The locations affected by the bloom may change as it shifts with winds, tides and currents. Occasionally, if a bloom gets caught in the Florida current, it will make it to Florida’s east coast, but that is an uncommon event. A red tide affected the coast of northeast Florida for several weeks in October 2007.

How can I find out where red tides are occurring?

You can read a status report and see a map showing the extent and severity of blooms in Florida at the Florida Fish and Wildlife Conservation Commission’s website. The FWC also has a website that contains more information about Florida red tides.

How does this year’s red tide event compare to past years?

It is difficult to compare red tides from year to year. Available data for sea turtles and fish mortalities do not specify the cause of death. One set of data that is available is for manatees. The Florida Fish and Wildlife Conservation Commission has data for manatee mortalities, separated by cause of death, dating back to 1996. I have graphed the data for statewide manatee deaths attributed to red tide for each year. The year 2013 was a particularly bad year for red tide-related manatee deaths, with a total of 277. So far this year (through July 19), there have been 67 manatee deaths caused by red tide.Graph of manatee deaths caused by red tide shows spikes in 1996, the mid 2000's, 2013, and the past three years.


Posted: July 31, 2018

Category: Coasts & Marine, Natural Resources
Tags: Red Tide

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