Here’s how climate change could make future Red Tide blooms worse

Red tide blooms have been plaguing Florida for hundreds of years at least, and scientists say they’re going nowhere.

But could they get worse?

It is certainly possible in a warming world. The first problem is that scientists don’t know enough to predict what will happen.

“There is a lot of speculation about the impact of climate change on red tides,” said Cynthia Heil, director of the Red Tide Institute at Mote Marine Laboratory & Aquarium.

Related: Red Tide’s return raises concerns about Tampa Bay’s health

Researchers are “at the very beginning” of studying how changing conditions might affect blooms, Heil said. After years of inconsistent funding, even more immediate questions remain unanswered, like what exactly ends a bloom and why Red Tide’s severity varies so much from year to year.

Meanwhile, the region around Tampa Bay saw its second devastating bloom since the start of 2018.

“It’s always: Well, it’s complicated,” said Don Anderson, senior scientist at the Woods Hole Oceanographic Institution. “And that’s certainly the story here.”

Sarah Klass, principal investigator at the Mote Marine Laboratory & Aquarium’s Red Tide Institute, collects water samples during a research cruise July 21. [ ZACHARY T. SAMPSON | Times ]

Out of tens of thousands of algae, Anderson said, it’s hard to determine how a species will respond to climate change. The red tide that typically devastates the Gulf Coast near Tampa Bay is made up of a microscopic organism called Karenia brevis.

More intense rainfall is one obvious way climate change could affect the red tide. The flowers feed on nutrients, especially nitrogen, which are dumped into waterways in excess through runoff from fertilizer-covered yards and leaky septic tanks or sewers. More rain coming during heavy storms could make the runoff problem worse and provide Red Tide with more nutrients.

“What would have been a natural phenomenon is subject to exacerbation if effective management fails,” said Douglas Rader, chief ocean scientist at the Environmental Defense Fund, an advocacy group.

Related: Red tide isn’t just bad for fish, a Tampa Bay man in the ER learned

Some forecasts suggest Florida will see more severe droughts followed by intense downpours during the rainy season, said Lisa Krimsky, water resources officer at Florida Sea Grant.

“Both could potentially influence Karenia brevis for the negative for us,” she said.

Lower river droughts levels, which means less fresh water is flowing into bodies like Tampa Bay. The estuary becomes saltier and this higher salinity can be conducive to red tide. It is believed that a dry winter and spring are one reason a flower traveled so high in the bay this year.

Warming ocean temperatures are another way conditions could change. Corn Karenia brevis has shown to thrive in a range of temperatures, the researchers said, and slight fluctuations may not have a big effect. Major changes could even make the water so warm that it becomes less hospitable to algae.

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Studies have been inconclusive as to whether rising carbon dioxide, a major contributor to global warming, will directly affect Red Tide, said Krimsky of Florida Sea Grant. Carbon dioxide is emitted into the atmosphere when people burn fossil fuels.

Related: Is the red tide harming tourism in Tampa Bay?

Red tides typically appear off Florida’s west coast in the fall, said Rick Stumpf, an oceanographer with the National Oceanic and Atmospheric Administration. He called it “little concern” that 2021 brought a summer bloom just three years after the last such red tide hit the region.

It is, however, far too early to tell if persistent outbreaks will become common, Stumpf said, or if it has a link to climate change.

“I hope it’s just a fluke,” Stumpf said. “We will look into that. We will try to investigate to see if we can sort out the models.

Sarah Klass, principal investigator at the Mote Marine Laboratory & Aquarium's Red Tide Institute, and Mote staff biologist Cody Cole take water samples during a research cruise July 21.
Sarah Klass, principal investigator at the Mote Marine Laboratory & Aquarium’s Red Tide Institute, and Mote staff biologist Cody Cole take water samples during a research cruise July 21. [ ZACHARY T. SAMPSON | Times ]

These red tide theories will take years to be verified, and they envision just one of the multiple harmful algal blooms troubling Florida. While red tide is a blight on the central Gulf Coast, blue-green algae is a problem elsewhere, particularly Lake Okeechobee and the St. Lucie, St. Johns, and Caloosahatchee rivers.

Cyanobacteria in such blooms thrive in warm conditions, said Anderson, of Woods Hole.

“There will be winners and losers, and for now the only clear winners will be freshwater cyanobacteria,” he said.

Although the science around Red Tides is less established, Anderson said, Florida is in a better position than anywhere else to help researchers find answers. The state has a substantial record of water quality data from past blooms, spanning several decades.

“At some point there will be information that can be extracted from this,” he said.

This story was produced in partnership with the Florida Climate Reporting Network, a multi-newsroom initiative founded by the Miami Herald, South Florida Sun Sentinel, Palm Beach Post, Orlando Sentinel, WLRN Public Media and Tampa Bay Times.

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Red Tide cover

Tampa Bay has questions about Red Tide. Here are some answers.

Is it safe to eat seafood? Here’s how Red Tide affects what you eat.

Can I go fishing? The state limits saltwater fishing.

Piney Point: Environmental catastrophe may be fueling Red Tide.

Red Tide Resources

• The Florida Fish and Wildlife Conservation Commission has a website which follows where the red tide is detected.

• Florida Poison Centers have a toll-free 24/7 hotline for reporting illnesses, including those resulting from exposure to Red Tide: 1-800-222-1222

• To report dead fish for cleanup in Tampa Bay, call the Florida Fish and Wildlife Conservation Commission at 1-800-636-0511 or file a fish kill report online.

• In St. Petersburg, call the Mayor’s Action Center at 727-893-7111 or use the see clickfix website.

• Visit St. Pete/Clearwater, the tourism wing of the county, operates an online beach dashboard at

How to Stay Safe Around Water

• Do not swim near dead fish.

• Those with chronic respiratory problems should be careful and stay away from places where red tide blooms. Leave if you think Red Tide is affecting you.

• Do not harvest or eat distressed or dead shellfish or fish from the area. Healthy fish fillets should be washed with clean water and the entrails discarded.

• Pet owners should keep their pets away from water and dead fish.

• Residents living near the beach should close their windows and run air conditioners with appropriate filters.

• Bathers can protect themselves by wearing masks.

Source: Florida Department of Health in Pinellas County

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