Fox News Weather Center

What is red tide?


Before planning a visit to a southern U.S. beach, you may want to check if there is a toxic algae bloom invading the waters.

Red tide is a phenomenon that could ruin a beach visit by possibly irritating your respiratory system or making your eyes water. In addition to making beachgoers cough, it also contaminates shellfish and kills fish and larger marine animals, including dolphins and manatees.

"Red tide is a general term for harmful algal blooms (HABs) in the ocean. There are many different algae that are described as red tide," Dr. Richard Stumpf, oceanographer for National Oceanic and Atmospheric Administration's (NOAA) National Centers for Coastal Ocean Science, said.

These harmful algal blooms usually make the water reddish brown. Most of the red tide organisms are able to swim up and down in the water.

"Major red tide species in the U.S. Gulf of Maine red tide is Alexandrium fundyense, which is also found in Puget Sound and north into Alaska. The Gulf of Mexico red tide is Karenia brevis, and it has gone as far as North Carolina," Stumpf said.

Pacific Red Tide

An algae bloom creates a natural red tide. It is sometimes toxic and always organic. (PeakMystique/Getty Images/iStockphoto)


Stumpf noted that there is another algae species that makes water appear bright red, and these are not typically harmful.

Only rarely is the bloom so bad as to make the beach unusable.

"Normally, there are always beaches, days and times of day that are good. For example, under calm weather in the winter, the morning land breeze [wind blowing from land to ocean] makes the morning a better time to avoid the irritation," Stumpf said.

Alexandrium, a specific type of algae, lives in the sediment during winter. Once the temperature warms up in March and April, they pop out of the sediment. According to Stumpf, Alexandrium only rarely reaches concentrations dense enough to change the color of the water. The cells are quite toxic with saxitoxin, which is a potent toxin often known as paralytic shellfish toxin (PST).

Karenia brevis is always present at low concentrations in the Gulf of Mexico.

"If it reaches an area with more nutrients when the water is relatively calm, it grows. In the fall, changes in the wind patterns create upwelling on the Florida coast that, when combined with the swimming, accumulates Karenia at the coast," Stumpf said.

Karenia swim to nutrients or to light, depending on what it needs. If the right amount of nutrients and light meet in combination with the right ocean circulation, then a lot of cells can collect in a small area.

It doesn't always appear as a dark red and brown color, some algae like Karenia brevis can give a yellowish cast to the water when there are moderate concentrations.

In most cases, the highest risk comes from people consuming seafood that ate the toxin: for example, eating whole hard shellfish such as clams, mussels, oysters that have been collected in waters that are not tested.

"Dolphins in the Gulf of Mexico have died from eating small fish that are full of the toxin," Stumpf said.

Red tide fish kill

Massive fish kill caused by toxic harmful algae bloom, or red tide. (dombrowski/Getty Images/iStockphoto)


"Karenia brevis has brevetoxin, which affects nerves. It also affects fish gills, which causes them to die," Stumpf said.

According to Stumpf, if Karenia lasts into winter, it can settle on sea grass in warm water where manatees congregate to stay warm. Manatee illness and even sometimes death is common during blooms during cold winters.

People with asthma may need to use their medications after being around harmful algal blooms because the toxins can even become airborne.

"Brevetoxin is the one of the few toxins that can become an aerosol. It causes upper respiratory irritation, symptoms like a cold, immediately on exposure," Stumpf said.

After breathing the brevetoxin, it can reduce their lung function for hours to days after exposure.

"Healthy people recover rapidly," Stumpf added.


"Most of the toxin blooms last weeks to months. Some Karenia blooms have persisted through an entire year [1995-1996, 2005]. A variety of factors will make the blooms end, and we don't understand this enough to predict when a bloom will end," Stumpf said.

Stumpf said they are able to look at a couple of indicators when trying to predict the bloom's end. The indicators include a lack of nutrients or disease of the algae due to a dense bloom favoring the spread of disease, a change to incompatible weather or the natural life cycle.

Before planning a trip to the beach, visit NOAA's tides and currents forecast. It provides an overview of the Gulf, forecasts, and links to state sites with more information.