What is Red Tide?

What is Red Tide?

We’ve seen it on the news, experienced it at our local beaches, and heard it mentioned by politicians as they describe their efforts to preserve our waters – but what exactly is “Red Tide”? A few blog posts back, we discussed all the details regarding algal blooms in our post titled, “UNCOVERING ALGAL BLOOMS” – but many people are still uncertain regarding the differences between Red Tide and Algal Blooms.

To help provide factual education regarding Red Tide, we went straight to the experts to answer some of the most commonly asked questions and concerns regarding red tide – The National Oceanic and Atmospheric Administration.

What is Red Tide?

Red Tide events are caused by blooms of the harmful algae “Karenia brevis”. These harmful algae blooms (HABs) are particularly common in coastal regions of Florida and Texas. These blooms produce toxins that can cause fish kills, respiratory irritation, and mortality of sea turtles, manatees, birds, and dolphins.

Why does red tide occur?

Red tide occurs naturally in coastal waters of the Gulf of Mexico with blooms appearing seasonally. Although the Florida red tide organism, Karenia brevis, typically blooms between August and December, blooms often deviate from that time frame. Simply put, a “red tide” or HAB results from the rapid growth of microscopic algae.

Is red tide a new concern, or has this occurred before?

Red tides were documented in the southern Gulf of Mexico as far back as the 1700s and along Florida’s Gulf coast in the 1840s. Fish kills near Tampa Bay were even mentioned in the records of Spanish explorers. The Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute (FWRI) harmful algal bloom (HAB) database is the best place to review the history of these events. This database documents Karenia brevis red tides from 1954 to the present, which is one of longest continually recorded datasets of red tide. The database contains more than 125,000 records provided by more than 190 state and county agencies, private research institutions, universities, and FWC staff. Data include location coordinates, cell counts of Karenia brevis and other HAB species, and a variety of water quality measurements such as temperature, salinity, and dissolved oxygen. You can learn more about these records and the history of harmful algal blooms by clicking here.

Can we prevent red tide or HAB?

While there are currently no means of controlling the occurrence of red tide, research continues to find ways to better address the causes and effects of harmful algal blooms. Currently, during a HAB event, NOAA issues twice-weekly forecasts to monitor bloom conditions and the potential for impacts. The forecasts help people make informed choices about where and when to visit areas that may be temporarily affected by a bloom.

How long does red tide last?

Red tides in Florida can last as little as a few weeks or longer than a year. They may even subside and then reoccur. In 2005, for example, a bloom started off the coast of St. Petersburg, Florida, in January and then spread from there to Pensacola and Naples by October, persisting for the majority of the year. The duration of a bloom in nearshore Florida waters depends on physical and biological conditions that influence its growth and persistence, including sunlight, nutrients, and salinity, as well as the speed and direction of wind and water currents.

Is this red tide dangerous?

The Florida red tide organism, Karenia brevis, produces potent neurotoxins, called brevetoxins, that can affect the central nervous systems of many animals, causing them to die. That is why red tides are often associated with fish kills. Mortalities of other species, including manatees, dolphins, sea turtles, and birds also occur.

Wave action near beaches can break open K. brevis cells and release the toxins into the air, leading to respiratory irritation. For people with severe or chronic respiratory conditions, such as emphysema or asthma, red tide can cause serious illness. People with respiratory problems should avoid affected beaches during red tides.

Red tide toxins can also accumulate in filter-feeder mollusks such as oysters and clams, which can lead to Neurotoxic Shellfish Poisoning (NSP) in people who consume contaminated shellfish. While not fatal, NSP causes diarrhea and discomfort for about three days. Rigorous state monitoring of water and shellfish assures that commercial shellfish is safe, often by closing harvest beds. Recreational harvesters have the greatest risk of NSP, often due to a lack of awareness of the problem. Harvesters should check Florida or Texas websites to determine if it safe to harvest shellfish in areas of these states. To learn more about red tide health concerns in general, Florida and Texas offer online resources.

Why is it called red tide?

Often, the blooms turn the water a deep red. While many people call all such events “red tides,” scientists prefer the term harmful algal bloom or HAB.

To learn more facts about red tide, HAB, and what is being done to address the concerns with these events, we encourage you to visit these websites:




T. (n.d.). HAB Monitoring Database. Retrieved October 10, 2018, from http://myfwc.com/research/redtide/monitoring/database/

US Department of Commerce, & National Oceanic and Atmospheric Administration. (2016, February 18). Red Tide in Florida and Texas. Retrieved October 10, 2018, from https://oceanservice.noaa.gov/news/redtide-florida/