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Editors note: The majority of this content was authored by a Florida Sea Grant team consisting of Lisa KrimskyBetty StauglerBrittany Hall-ScharfKrista Stump  and has been republished here with permission from Florida Sea Grant. Read the original post at: Understanding Florida’s Red Tide

What is it?  

In Florida, red tide is caused by the accumulation of Karenia brevis, a type of single-celled organism called a dinoflagellate.  

Red tides occur around the world and are not all caused by the same species, nor are they always red. In fact, most dinoflagellates are harmless. Though some, including K. brevis, produce neurotoxins that can cause respiratory problems in humans and attack the central nervous systems of fish and other wildlife. Many scientists refer to blooms of K. brevis as harmful algal blooms (HABs) due to the impacts they can have on the environment, humans, and our coastal economies. 

What does it look like? 

Red tides are not always red. When K. brevis blooms in high enough concentrations, the water can appear red, brown, rusty orange or green. Sometimes, the hue of the water will remain normal, even during a bloom.

What causes it? 

Red tide blooms in Florida begin 10-40 miles offshore in the bottom waters of the Gulf of Mexico, where K. brevis is almost always present at low and harmless concentrations. K. brevis cells that hang out at the bottom are brought to the surface by a phenomenon known as upwelling, a process in which deep, cold and nutrient-rich water rises to the surface. 

K. brevis, like all algae, requires three things to grow and survive:

  • Optimal light
  • Temperature
  • Nutrients, specifically nitrogen and phosphorus.

The sources of these nutrients vary among the offshore, nearshore, and estuarine environment.

While offshore, a small bloom of K. brevis can begin by using the nitrogen produced by the nitrogen-fixing algae, Trichodesmium. This is because Trichodesmium can “fix” nitrogen or obtain it from the atmosphere and convert it to a form that is usable by K. brevis. 

Other sources of nutrients in the offshore environment include:

  •  Zooplankton and microplankton waste
  • Grazing food waste
  • Benthic flux, or the exchange of nutrients from the sediment to the water.  

But, what about other sources of nutrients, such as those originating from the Mississippi River as it empties into the Gulf?

“Transport of nutrients from the Mississippi River plume has also been hypothesized to be one source of nutrients that may contribute to bloom initiation,” said Leanne Flewelling, ecosystem assessment and restoration section leader for the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute, and a member of Florida’s new Red Tide Task Force 

Winds and ocean currents then bring the bloom inshore where it has an impact on our coasts. In fact, these currents play the biggest role in determining which parts of our coastline will be affected. Once the bloom moves nearshore, nutrients associated with decaying red tide and fish kill biomass are among the most significant nutrient sources for blooms. So much so, that red tide has been considered nearly self-sustaining.

Nearshore red tide blooms can also obtain nutrients from: 

  •  Air pollution
  • Water releases from rivers and estuaries
  • Estuarine flux, or the exchange of nutrients to and from the estuary. 

Some of our readers have asked, “What about Lake Okeechobee?”

Runoff from Lake Okeechobee brings nutrients to coastal water that could fuel an ongoing coastal red tide bloom,” Flewelling said. “This could have contributed to the severity and persistence of the red tide in 2018. However, red tide is typically not present in coastal waters during the wet season when Lake Okeechobee releases have occurred.”

Nutrient sources also vary spatially, or north to south. In other words, nutrient sources in Sarasota may differ from those further south in Fort Myers. K. brevis, unlike other phytoplankton species, can feed on a variety of nutrient sources, in a variety of forms. For this reason, it is impossible to link a red tide bloom to one particular source of nitrogen or phosphorus. 

How often do blooms occur? 

Red tides are not uncommon and occur almost annually in the Gulf, particularly in the Tampa Bay to Charlotte Harbor region. The first scientifically documented red tide bloom in Florida dates back to 1844, predating extensive human development. 

Florida’s red tides can appear throughout the year, though they usually peak late summer to early fall and can last from a few days to months. 

Where else does red tide occur? 

Various algae species cause red tides all around the globe. But, K. brevis is found almost almost exclusively in the Gulf of Mexico. Because red tides are transported by currents, some, including last year’s bloom, have even been carried by the Gulf Stream current into the Atlantic Ocean. 

When was the last bloom? How long did it last? 

Over the course of 16 months, from October of 2017 until February of 2019, the last red tide event was recorded as the fifth longest since 1954, and the first since 2007 to impact Florida’s southwest, northwest, and east coasts simultaneously.  

What are the impacts?


K. brevis is considered harmful because it produces a variety of natural toxins, the most important of which are the neurotoxic brevetoxins. Through inhalation, direct contact or ingestion, these toxins, in high enough concentration, can harm and kill fish, birds, and marine mammals. The last red tide resulted in serious impacts to fish, marine mammals, marine birds, residents and coastal businesses. These types of impacts are typical during severe red tide events. However, more fish have died during the most recent red tide in Lee and Charlotte counties compared to past severe red tide events.

“Brevetoxin is very deadly to fish of all life stages. Invertebrates are generally less affected during red tides, but studies have shown that bivalve larvae have a dramatically reduced survival rate when exposed to K. brevis, and this could be the case for other species of invertebrates as well,” Flewelling said.

Public Health

In addition to the impact to sea life, red tides can have human health impacts. Exposure to brevetoxins occur through inhalation or ingestion. K. brevis cells are weak, so wave action can break open the cells, releasing the brevetoxins as an aerosol. People in coastal areas can experience varying degrees of eye, nose, and throat irritation. Beachgoers experiencing respiratory irritation are advised to leave the beach or go to air conditioning and symptoms will usually go away. Some people who come in contact with water or sea foam with severe red tide may experience skin and eye irritation, including rashes. 

K. brevis can cause serious illness to people with severe or chronic respiratory conditions such as asthma or emphysema. For example, red tide can trigger asthma attacks and susceptible populations may experience chronic pulmonary symptoms, even after leaving the area. During severe red tide events,brevetoxins can be detected 1-2 miles inland from the beach. For these reasons, at-risk populations are cautioned to avoid coastal areas with active red tides. 


Red tides can also result in significant economic impacts. Economic costs are associated with four main sectors: recreation and tourism, commercial fisheries, public health, and monitoring and management costs. There is also the potential for a decline in residential home value due to red tide, though these costs have not been examined. 

  • Red tides are estimated to cause more than $20 million in tourism-related losses in Florida each year. 
  • The 2015-16 red tide event resulted in a sales loss of $1.33 million to the hard clam aquaculture industry. 
  • Health costs attributed to medical expenses and lost work days associated with HABs cost the United States $22 million dollars annually. According to the Florida Department of Health, treatment of respiratory illness in Sarasota County during the 2015-16 red tide event averaged $0.5 to $4 million dollars. 
  • In 1998, clean-up costs associated with the disposal of millions of tons of dead fish and marine life has been estimated to be nearly $163,000 annually for Florida. However, severe events such as the current one can be significantly costlier where totally cleanup costs for all affected areas can reach in the millions of dollars. (link to FSG) 

Learn more in our post: Red Tides Are Expensive. Here’s Why 

How do we monitor and forecast red tide? 

Many groups of scientists and volunteers work together to monitor and research harmful algal blooms throughout the year regardless of a bloom’s presence. 

“Monitoring is accomplished through looking at water samples under a microscope for Karenia brevis cells, analyzing satellite imagery, and even using autonomous gliders that can travel great distances at both the surface and bottom making measurements,” Flewelling said.

Water samples are collected from over 100 locations throughout Florida on a weekly, bi-monthly, or monthly basis through partnerships with state agencies, county governments and citizens. Collection sites are sometimes randomly selected by those conducting the sampling or taken from pre-selected locations that are visited frequently. 

FWC maintains a daily sample map that shows results from the last 8 days of red tide sampling: Red Tide Current Status 

Forecasting tools have been developed that predict respiratory irritation due to red tide blooms: Gulf of Mexico Harmful Algal Bloom Forecast and where a bloom may move over the next four days: Red Tide Prediction and Tracking on the West Florida Shelf

During a harmful algae bloom event, the National Oceanic and Atmospheric Administration also issues twice weekly forecasts using satellite imagery and other data to monitor blooms and the potential for impacts.  

“Ideally we could predict bloom impacts over shorter time frames, for example, changes over the course of a day and bloom movement over longer periods of time,” Flewelling said. “Improving forecast tools will require more observations and information about what is happening both along the coast and offshore where our sampling is more limited.”

How do we contribute to the problem? 

While the prevalence of these blooms predates human influence, it is likely that human activity has played a role in fueling current blooms. Red tide blooms feed on excess nitrogen and phosporus once they make their way to our coasts. These excess nutrients come from a variety of sources such as stormwater runoff, fertilizer runoff, septic tanks, and/or faulty wastewater systems. Scientists say these excess nutrients play a role in the extent and duration of large bloom events. 

What are researchers working on now? 

University and agency scientists are working to learn more about how the blooms form, why they eventually break down and how animals and humans are affected once exposed. Research has led to new technologies that help identify and monitor toxic substances in the environment. 

In early August, Gov. Ron DeSantis announced the appointments of 11 expert researchers and leading scientists to the recently re-organized Red Tide Task Force.  

“The issues of red tide are complex, but with the appointments of these leading scientists and researchers, we hope to make a difference,” DeSantis said in a press release.  

What can we do? 

Floridians are accustomed to these “natural events,” but the impacts are also becoming more pervasive. If Florida is to progress in a safe and economically viable future, Floridians must address our contribution to the problem by reducing our nutrient inputs and supporting research to further understand red tide causes, impacts, and mitigation strategies. 

Flewelling said there are other actions you can take to help scientific research about red tide:

Where can I find up-to-date information? 

During periods of non-red tide blooms, full reports are posted each Friday by 5:00 PM EST on FWC’s Red Tide Current Status page

When a red tide is in bloom, additional reports in the form of interactive maps are provided by 5:00 PM EST daily on FWC’s Daily Sample Map

Additionally, an interactive map produced by the University of South Florida’s College of Marine Science Ocean Circulation Group, models the transport of the bloom at the surface and at the bottom of the water column. 

Harmful algal blooms are also tracked at the federal level. NOAA has multiple sites for red tide information: 

In addition to these reports, you can also visit the Visit Beaches website to keep up with local conditions before you head out. 

Learn more

Watch our public panel, Beyond Dead Fish: How Red Ride Affects All Floridians, to learn more.