Toxic Red Tide Is Back in Florida—Here’s What to Know

Caused by an overgrowth of algae, the blooms can be harmful to humans, pets and marine wildlife

Dead fish floating in water
Killed by red tide, thousands of dead fish float in the Boca Ciega Bay in Madeira Beach, Florida, in July 2021. The harmful algae blooms are once again killing fish along Florida's southwest coast. Octavio Jones via Getty Images

Dead fish are washing ashore and beach-goers are facing respiratory problems as harmful algae accumulate along the southwest shores of Florida in a phenomenon nicknamed “red tide.”

Named for the brownish-red hue it gives the water, red tide occurs when toxic algae proliferate. In Florida, that problematic algae is known as Karenia brevis. In early March, the Florida Fish and Wildlife Conservation Commission detected K. brevis in high concentrations at several sampling locations along the state’s southwestern coast.

Though K. brevis and other types of algae inhabit all bodies of water, they typically exist in such small numbers that they go unnoticed. However, under the right conditions, these tiny organisms can rapidly multiply. Scientists call this out-of-control overgrowth an “algal bloom.”

These blooms can cause major problems for humans, pets and marine wildlife. Some, like K. brevis, produce toxins that can kill fish and sicken humans. Others are nontoxic, but they block sunlight and deplete oxygen levels in the water, which can lead to fish die-offs.

When K. brevis grows excessively, it can cause skin irritation, eye irritation and respiratory issues—such as coughing, sneezing and asthma attacks—in humans and pets that swim in affected water or breathe in its airborne toxins. The toxins can also make their way into shellfish, and if people eat those shellfish, they may develop neurotoxic shellfish poisoning, which causes stomach problems and other symptoms. When red tide is present, Florida officials recommend staying out of the water—and keeping pets away, too.

Local business owners are concerned the red tide may drive tourists away, just as they’re gearing up for the spring break rush. Some beach-goers, though, appear undeterred by the algal blooms.

“It’s got my asthma going,” says Shawn Snook, who was visiting a beach in Clearwater, Florida, to Bay News 9’s Trevor Pettiford. “But you know, it’s the beach, so we’re going to push through it.”

This year, Florida’s Poison Control Centers received 36 reports of red tide exposure as of Monday, reports Max Chesnes of the Tampa Bay Times. Last year at this point, there had only been two reports; and 15 such reports had been filed by this time in 2021.

Red tide is not just limited to Florida—it can occur all over the world. Last August, a red tide caused by the algae species Heterosigma akashiwo killed off thousands of fish in the San Francisco Bay Area. And it’s not a new problem, either. Florida officials say records of red tide along the state’s Gulf of Mexico coast date back to the 1840s.

In the Gulf of Mexico, red tide most commonly occurs between August and December, but it can also happen at other times of year, notes the National Oceanic and Atmospheric Administration (NOAA).

Scientists aren’t completely sure what causes K. brevis to accumulate in such large numbers at some times but not others. They do know that the organisms follow nutrients as they move through the water—so when currents and weather systems bring nutrients up from the seafloor and toward the shore, K. brevis tends to follow.

This year, it’s also possible there’s a link between Hurricane Ian, which made landfall last September, and the current red tide. But scientists urged caution when connecting the two incidents.

“We cannot draw a simple line connecting one to the other,” says Michael Parsons, a marine scientist at Florida Gulf Coast University, to the Tampa Bay Times.

Human-caused climate change may also be contributing to red tides. Rising ocean temperatures and higher levels of carbon dioxide may be spurring rapid algal growth. On top of that, extreme weather events, such as heavy rains, could be causing more agricultural runoff into the oceans, per the Environmental Protection Agency. Filled with nutrient-dense fertilizers and other chemicals, this runoff feeds toxic algae like K. brevis.

Climate change may also be altering weather patterns in the Gulf of Mexico, which could be affecting the timing of red tide in Florida. In a normal year, K. brevis tends to accumulate along the state’s Gulf Coast in the fall. Then, come winter, strong winds sweep down and push those algal clusters south into the gulf. But this year, the state did not experience those typical wind patterns, which meant K. brevis “can hang around” into winter and spring, says Richard Stumpf, a NOAA oceanographer, to the Washington Post’s Amudalat Ajasa. That’s happened twice in the last five years, which is a higher-than-normal frequency, he tells the publication.

“If we tend to get fewer persistent northerly winds in the winter because of climate change, we will tend to see longer-lasting blooms,” Stumpf tells the Post.

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