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Understanding Red Tide: A Guide to Florida’s Toxic Blooms

Updated: May 24

Understanding Red Tide: A Guide to Florida’s Toxic Blooms


Florida’s beautiful coastal waters are a major draw for residents and tourists alike, but beneath the waves, a harmful natural phenomenon lurks: red tide. Often misunderstood, red tide refers to toxic algal blooms that can severely impact marine life, human health, and local economies. Understanding what red tide is, why it happens, and how to stay safe during an outbreak is key to protecting both people and the environment.


What Is Red Tide?


Red tide is the common name for a harmful algal bloom (HAB) caused primarily by the microscopic marine algae Karenia brevis. While these organisms are naturally occurring in the Gulf of Mexico, under certain environmental conditions they multiply rapidly, creating dense concentrations that turn the water a reddish-brown hue.


Unlike typical algae, Karenia brevis produces brevetoxins, powerful neurotoxins that can:


  • Kill fish and marine mammals

  • Irritate eyes and respiratory systems in humans

  • Accumulate in shellfish, posing a risk to consumers


Causes and Conditions

Red tides form when several environmental factors align, including:


  • Warm water temperatures

  • High nutrient levels from agricultural runoff and sewage

  • Calm seas and favorable wind conditions


Nutrient pollution plays a significant role in the severity and duration of red tide events. Fertilizers, wastewater, and stormwater runoff deliver excess nitrogen and phosphorus into the ocean, fueling algal growth beyond natural levels.


Impacts of Red Tide

Red tide events can last for weeks or even months, and the effects can be devastating:


Environmental:

  • Massive fish kills

  • Death of dolphins, sea turtles, and manatees

  • Disruption of marine ecosystems


Human Health:

  • Breathing issues (especially in individuals with asthma or lung conditions)

  • Skin and eye irritation

  • Shellfish poisoning if contaminated seafood is consumed


Economic:

  • Beach closures and tourism decline

  • Loss of income for fisheries and local businesses


How to Stay Safe


  • Check local health and beach advisories before heading to the shore

  • Avoid contact with dead fish or discolored water

  • Do not harvest or eat shellfish from areas with active red tide

  • Stay indoors or wear a mask if respiratory irritation occurs


The Florida Fish and Wildlife Conservation Commission (FWC) provides regular updates on red tide conditions at: https://myfwc.com/redtidestatus


People and animals are exposed to marine algal toxins through:

  • Eating shellfish or fish containing toxins

  • Swimming or other activities in the water

  • Breathing in tiny droplets in the air that contain toxins

​ People can get sick and have symptoms

  • Brevetoxins from red tides are airborne and may cause respiratory irritation, difficulty breathing, and increased asthma risk

  • Coughing, wheezing, shortness of breath, sneezing, and sore throat

  • Inflammation in the lungs and possible bronchitis outcome

  • Headache and eye irritation

  • Skin irritation

  • Digestive illness

  • Risk of infections and rashes

  • Neurotoxic shellfish poisoning

  • Immunocompromised people should be especially careful.

​ Exposure to blooms and long-term health effects

  • The long-term health effects of harmful algal blooms are still being studied, but research is showing damage to DNA that can lead to mutations in genes that normally prevent the formation of cancers.

  • Neurotoxicologist has found that exposure to toxins from blue-green algae leads to neurodegeneration and increased expression of gene markers in dolphin brains, similar to the changes found in humans with Alzheimer’s disease and related dementia.

  • ​Chronic dietary exposure to BMAA has been linked to a form of amyotrophic lateral sclerosis/Parkinson dementia complex (ALS/PDC)

​ How to protect yourself from harmful exposure

  • ​Check environmental or state websites for the local beach or lake closures before visiting. Red Tide Current Status

  • Get away from the aerosolized brevetoxins from the red tide.

  • Do not swim in the water if dead fish are on the shore.

  • Do not fish, swim, boat, or participate in water sports in areas experiencing a red tide. This includes swimming and jet-skiing.

  • Children should not play along the shoreline where they might e exposed to clumps of algae or red tide water.

  • Red tide poses the same risk to animals. Pets should not drink affected water and should avoid beach areas with red tide.

  • Wash immediately and thoroughly with clean water and soap if you come in contact with contaminated water.

  • Do not go into the water and you have open sores or cuts on your body; you risk flesh-eating bacteria, which can be associated with bloodstream infections

  • Do not eat shellfish such as clams and oysters that are harvested from areas with an active red tide. Follow local guidance when consuming harvested fish or shellfish.

  • Wear a particle filter mask or stay indoors to help with the symptoms of red tide.

  • People with chronic respiratory problems such as asthma should avoid areas with active red tides or severe algae blooms.

  • Do not drink directly from lakes, rivers, or ponds.

  • Boiling water does not remove or destroy algal toxins.

  • Over-the-counter antihistamines may decrease your symptoms.

  • See a doctor if you think you may have eaten contaminated seafood or are sick from exposure to red tide.

  • Talk with your healthcare provider if you have questions about your health and exposure to a harmful algal bloom.


​Red Tide Frequently Asked Questions



How often does red tide happen, and what is it? Nearly every summer, the gulf coast of Florida encounters Harmful algal bloom or HAB. These HABs are microscopic algae that produce deadly toxins which make the air difficult to breathe and kill local fish. These toxic HABs also usually turn the water red which leads to the common name of “Red Tide.” The type of algae that usually occurs off the gulf coast of Florida is known as Karenia brevis. How long does red tide last? When there is a red tide, it can last for weeks or even months. The longest recorded red tide in Florida lasted for 18 months, from October 2006 to March 2008. How far inland is impacted? Airborne red tide toxins can travel inland by up to a mile from the coast depending on the speed and direction of the wind, plus tide levels. Is red tide natural? Red tides have been around since long before humans. However, certain human activities are making them more frequent.. Since the 1980s, harmful red tide events have become more frequent and widespread. This is a result of pollution from industrial runoff, sewage spills, and overuse of lawn chemicals. Why didn't my hotel tell me this was happening? Red tide has negative impacts on Florida's economy, but the tourism industry has a moral responsibility to educate and warn visitors, putting their health before profit. How can we stop supercharging red tide?


  • Improving wastewater treatment infrastructure

  • Limiting fertilizer use, especially before rain

  • Restoring wetlands and natural filters

  • Advocating for stronger water quality regulations


Research on Brevetoxin


You cannot smell or see brevetoxins, but they become aerosolized and pass into the atmosphere near water, where they are generated, and we then breathe them in. Brevetoxins are tasteless, odorless, and heat and acid-stable. These toxins cannot be easily detected, nor can they be removed by food preparation procedures. These toxins pose a significant risk to sea turtle health. Documented effects of contaminants include potential disease progression and adverse impacts on development and immune function and caused tumors.

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