Red Tide: What It Is And Why You Need To Avoid Going Near It

A harmful algae bloom can be toxic to humans and pets, and totally mess up your beach vacation.
A type of microscopic algae called Karenia brevis has grown rapidly in Florida, leaving dead fish on the shore.
Octavio Jones via Getty Images
A type of microscopic algae called Karenia brevis has grown rapidly in Florida, leaving dead fish on the shore.

Harmful “red tide” is wreaking smelly havoc on Florida’s Gulf Coast, canceling some beachgoers’ spring break vacations, causing burning eyes and breathing irritation in humans, and killing wildlife.

In recent weeks, a type of microscopic algae called Karenia brevis has grown rapidly there in what is popularly known as red tide, due to the way that so-called harmful algae blooms change the color of ocean water.

These blooms develop offshore and are brought closer to land by currents and winds, usually from an upwelling of nutrients from the ocean floor. And they don’t just happen in Florida; red tides can occur in ocean bodies around the world, and as the result of other kinds of algae, too, though the ones caused by K. brevis are particularly harmful to humans and animals.

There is no certain answer for when this latest red tide event in Florida will end. “If we’re really lucky, it will fade out or die off in the next few weeks,” said Richard Stumpf, an oceanographer at the National Oceanic and Atmospheric Administration, or NOAA, who researches harmful algae blooms. “If we’re not so lucky, it may go on longer.”

Because red tide conditions can change day by day, it’s important to stay informed if you’re planning to spend time on the water or at the beach in any place. Here are some answers to common questions about red tide:

What causes red ride?

Red tides are naturally occurring and have been documented around the Gulf of Mexico specifically since the 1500s, said Richard Pierce, an ecotoxicologist at Mote Marine Laboratory in Sarasota, Florida.

The algae blooms caused by K. brevis in particular require a complex variety of factors aligning at the right time.

“The first is biology — the organism must be present in the water and it must out-compete other phytoplankton,” explains Mote Marine Laboratory on its website.

“The second is the correct chemistry — this includes the appropriate temperature, salinity, and nutrients that it needs to grow and multiply. The third component is the right physical conditions to concentrate and transport K. brevis. The fourth component is ecology – the presence or absence of other life forms, such as other marine algae that may encourage or inhibit K. brevis blooms.”

When these conditions are ripe, blooms strike and the K. brevis organisms multiply out of control.

Nutrient-laden runoff from sewage spills and fertilizer does not cause red tide, Pierce said, but this can intensify it. “If red tide is in the area, it is going to make it worse and make it stay longer,” he said.

How long does red tide last?

Generally, harmful algae blooms form in the fall and can last anywhere from a few weeks to longer than a year, Pierce said.

“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,” states the Florida Fish and Wildlife Conservation Commission, or FWC.

A bloom will eventually “burn itself out” if the water is cooler, if the red tide gets dispersed, and if the bloom gets moved to an area with fewer of the nutrients it needs for reproduction, Pierce said.

A red tide in 2021 in China's Xianglu Bay.
VCG via Getty Images
A red tide in 2021 in China's Xianglu Bay.

Can red tide make you sick?

Harmful algae blooms have been reported in every U.S. coastal state. Those caused by K. brevis are concentrated in Florida and Texas around the Gulf of Mexico, but they have also been found along the Atlantic coast up to North Carolina.

K. brevis red tides are a big deal not only because the reddish-brown water looks unpleasant; the algae can also be toxic to people and wildlife. K. brevis produces tasteless and odorless neurotoxins called brevetoxins that can become airborne and cause eye and respiratory irritation in humans.

You don’t even necessarily have to go in the water to be affected. Just being outside near a red tide can cause symptoms.

“If I go out in a beach with it, I feel like I’ve suddenly come down with a cold. My nose is running. I’m coughing. My eyes are watering. My sinuses sometimes will hurt. I’ll leave the beach, [and] 15 minutes later I’m fine,” said Stumpf.

“People with asthma, they run the risk of more significant health risks,” he added, noting that anyone with chronic obstructive pulmonary disease also has to be careful about exposure.

If you do become affected by red tide, Florida’s Poison Control Centers recommends moving inside to an air-conditioned room to relieve discomfort and seeking medical attention if your symptoms are more severe. Stumpf noted that wearing a mask can also help mitigate the risks of inhaling brevetoxins.

For animals, exposure to brevetoxins can be fatal, which is why red tides often produce smelly piles of dead fish on the shore. The brevetoxins paralyze fish gills, killing them, Stumpf said. “The toxin, if enough is ingested, it can kill birds and mammals,” he added. Just this month, a manatee was rescued in Florida due to distress caused by red tide.

Can you eat eat fish and seafood during red tide?

The neurotoxins produced by K. brevis can build up in shellfish and cause neurotoxic shellfish poisoning when ingested.

But Stumpf said that as long as you buy seafood from a reputable dealer and not off the back of somebody’s truck, it is safe to continue eating fish and shellfish because the industry is closely monitored.

“Commercially available shellfish are often not locally harvested and, if harvested locally, are tested for red tide toxins before they are sold,” the FWC notes.

Can pets go outside where red tide is happening?

With pets, it’s better for their health if you keep them away from a red tide.

“They’re just like people. If they are along the beach when the wind is blowing onshore, they are inhaling the neurotoxins, and that is not good for them,” said Pierce.

Watch out for the foam that collects on the beach during red tide, too, because that can be high in toxins.

“A lot of times, dogs especially will lick a maybe dead fish, lick the foam, and they can become very sick. And actually, they can die from that,” Pierce said.

I’m planning a beach vacation. How far in advance can red tide be predicted?

Red tide events have seasonal patterns. “If you want to avoid red tide, generally the time to vacation [in Florida] would be May, June, July, August,” Pierce said. “Statistically, blooms would start in September-October and last through the winter, and then lessen at the end of April.”

But if you are planning a vacation weeks in advance, understand that it’s not yet possible to predict exactly when and where red tide may be impacting certain beaches. Your best bet is to check beach and weather forecasters for up-to-date information on red tide concentrations.

The FWC, for example, releases regular updates on red tide conditions based on K. brevis sampling. NOAA also forecasts potential respiratory irritation at individual beaches in Florida and Texas, based on field samples of K. brevis concentration and factors like wind speed.

Stumpf said wind direction can also help determine whether it is safe to be on the shore.

“You could be out on the beach and never notice it. I’ve had lunch on a beach during a severe red tide and had a wonderful lunch, because the wind was blowing the toxin over the gulf, not inland,” he said.

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