The organism that causes red tide is back in waters off the coast of Southwest Florida.

Samples collected by the Florida Fish and Wildlife Conservation Commission last week revealed varying levels of the microscopic algae Karenia brevis, offshore of Manatee, Sarasota, Charlotte and Lee counties.

At high enough concentrations, the algae can cause a red tide — a major bloom of K. brevis that can kill fish and other marine life, block out light required by seagrass beds, poison seafood and cause breathing irritation for people — not to mention ruin a day at the beach.

But it’s too soon to say whether the region will see a major harmful algal bloom like those of the past few years, says Robert Weisberg, an emeritus oceanography professor who helped build University of South Florida’s red tide tracking program.

Much like the weather, scientists have gotten better at predicting red tide’s behavior in the short term. USF produces a forecast that can foreshadow the algae’s movements several days in advance. But with so many environmental factors at play, predicting severe blooms far into the future is difficult.

Those factors include currents, water temperature, salinity and nutrient availability.

Another big one? Wind, Weisburg said.

The arrival of cool fronts in the fall may be a relief for Floridians, but the northern winds also play a part in encouraging red tide blooms by stirring up water circulation and triggering a process known as upwelling.

This year, the algae may have gotten an extra boost to shore from the winds brought by Hurricane Ian, Weisberg said.

“So beginning with Ian, whatever was on the bottom in the middle of the continental shelf started moving southwards and onshore,” Weisberg said. “Then more recently we started getting a succession of fronts.”

Ian also sent large amounts of polluted water into waterways, which feeds the algae. While that could potentially feed a strong red tide, it also helps phytoplankton that may keep red tide at bay, Weisburg said.

When it comes to the severity of a red tide, it’s all about whether the bloom reaches the mass and concentration to start killing fish and feeding itself with nutrients.

“We’ll see if it starts killing fish,” Weisburg said. “Now it’s going to be a competition. Who’s going to win? All the other plants, or the red tide?”

The large push of freshwater from Hurricane Ian could make it harder for K. brevis, which thrives in higher-salinity waters, to survive.

“It’s hard to tell whether this is going to develop into a terrible bloom or just be a passing bloom,” Weisburg said. “But certainly we have red tide going on right now. I think it’s going to increase southward from Sarasota Bay. How long it lasts is tough to answer.”

In samples collected in the week ending Oct. 21, the algae’s presence was strongest around Venice and Nokomis; only trace amounts were observed near Longboat, Lido and Siesta Key.

On Wednesday, red tide advisories that had been in place for several Sarasota County beaches were lifted after new water samples showed K. brevis was no longer present in those waters. Farther south, the most recent samples collected in waters off of Charlotte and Lee counties showed that the algae persisted there.

Why was there no red tide before now?

The past several years brought untimely and long-lasting bouts of red tide to the Gulf Coast of Florida.

A devastating red tide event spanned from late 2017 to early 2019. Another bloom started in the winter of 2020 and hung around through the spring of 2021. It then worsened after millions of gallons of contaminated water from the Piney Point industrial plant were released into the Gulf. It lingered through the fall, killing thousands of tons of marine life around Tampa Bay.

But historically, late summer and fall is when red tide blooms most often occur.

“Very rarely do we get a year without a red tide,” said Cynthia Heil, director of Mote Marine Laboratory’s Red Tide Institute in Sarasota. “The last time was 2010. So we’re still eagerly watching what happens.”

Each week, FWC researchers and their partners collect samples in waters up and down the state and test for the presence of K. brevis. The algae, though naturally present in the Gulf, had hardly made a blip on the radar this year.

“It’s been an interesting year, because we have so far had no sign whatsoever,” Heil said in late September. “But we’re not out of the woods yet.”

Before Hurricane Ian, some of the other conditions that help red tide thrive were already setting up in the Gulf, according to Heil, who is studying the blooms with a grant from the National Oceanic and Atmospheric Administration.

One of those conditions is the presence of trichodesmium, a cyanobacteria also known as sea sawdust. It produces large amounts of excess nitrogen that provide red tide with a food source when it’s miles offshore.

There have been large amounts of trichodesmium, which is sometimes mistaken for sewage, in the Gulf this year, Heil said.

Scientists also believe that a particular current, known as the Loop Current, can have a powerful effect on when red tide forms and how bad it gets. The warm water current travels up from the Caribbean, past the Yucatan Peninsula and into the Gulf of Mexico and then loops back down into the Florida Strait. Its position can shift dramatically from year to year and is hard to predict.

It’s when the current comes into contact with the slope of Florida’s western shelf, near the Dry Tortugas, that it can cause upwelling favorable to red tide, USF’s Weisburg said.

“How it behaves is the primary reason why some years we have virtually no red tide, whereas other years we have a huge red tide,” Weisburg said.

Since May of 2021, the current has been locked in a pattern out of contact with Florida’s shelf. That could lessen the likelihood of a severe red tide, Weisburg said.

Once started, blooms typically end by spring at the latest. But recent years have brought exceptions.

“In 2021 and 2018 we had these horrendous holdover blooms through the summer,” Heil said. “That’s unusual, and we’re looking at why those summer blooms are happening.”

One possibility is changes in the timing of the rainy season and rainfall amounts, which could be driven by climate change, Heil said, but more research is required.

“We have one of the longest track records for red tide in Florida,” Heil said. “It’s a bad thing if you live here, but it’s a good thing for science because we can start to point fingers at certain factors that may contribute to these blooms.”

“We have some basic understanding of why we’ll get one in a given year,” Weisburg said, “and why they tend to get worse before the dissipate. A real question now is what terminates a red tide bloom.”

Research finds new harms of red tide

Red tides are not new. The Spanish first documented the natural phenomenon in the 16th century.

While scientists have made great strides in understanding red tide, some of its threats are just beginning to be uncovered.

A new study published by the Roskamp Institute in the science journal “Harfmful Algae” found that simply breathing red-tide laced air can trigger severe illness symptoms in people with certain medical conditions.

“In particular, individuals with a previous history of migraine or chronic fatigue syndrome, extreme fatigue that worsens with physical and mental activity, are more likely to have symptoms,” an overview of the study says.

The Manatee County-based research institute studies a wide variety of neurological disease, including Alzheimer’s, traumatic brain injuries, post-traumatic stress disorder and Gulf War illness.

Previous red tide studies gave researchers hints that there was more to discover about the algae’s affect on human health.

“During previous studies looking at how red tide toxins affected lung function, study participants sometimes reported headaches, and we thought it was important to investigate this further,” said Laila Abdullah, a Roskamp Institute research scientist and the lead author of the study.

Neurological symptoms from red tide had only previously been linked to eating contaminated seafood. But participants in the new study were only exposed to red tide toxins via breathing the air, researchers say.

The study had more than 250 participants from Sarasota, Manatee, Lee, Charlotte and Collier counties, which have all seen severe red tide blooms along their coasts in recent years. Medical data from June 2019 to August 2021 was examined.

In a surprise to researchers, participants reported symptoms similar to those seen with neurotoxic shellfish poisoning (NSP). Those include nausea, vomiting, diarrhea, numbness and tingling, dizziness, headache and a reversal of hot and cold sensations.

“Building on previous findings, we found that migraine sufferers reported headaches during red tide blooms. However, we did not expect that people would report NSP-like symptoms,” Abdullah said.

“Different people respond to the toxins in different ways — from not being affected at all to experiencing a range of severe symptoms,” said Roskamp Institute executive director Michael Mullan in a study overview. “We don’t understand why that is the case nor do we know the dose levels of toxins that are needed to cause neurological symptoms. There is still much to understand about this toxin and the levels of threat it poses to human brain health.”

The study also found that even people without medical conditions can become more sensitive to red tide toxins with repeated exposure.

After intense blooms in recent years devastated wildlife and harmed coastal economies and fisheries, researchers around the state are bent on finding more answers about red tide’s causes and effects.

Another recently published study added more weight to the theory that human pollution sources, including runoff, excess fertilizer, sewage and other nutrient-containing matter, can make naturally-occurring red tide blooms more intense and longer lasting.

And there is now more proof that coastal pollution could be coming at the cost of people’s health.

“These symptoms ultimately cause significant public health safety concerns, primarily among vulnerable populations with preexisting neurological conditions,” the study’s abstract states.

Researchers at the Roskamp Institute say they will continue to seek more answers about red tide’s impacts on the human brain.

“Harmful algal blooms like Florida’s red tide are predicted to become more frequent and longer lasting as our climate undergoes changes, and it is important to understand how our brains are impacted by red tides and what we can do to mitigate any risks they may pose,” Roskamp Institute CEO Fiona Crawford said in the study overview.

Red tide resources

Several online resources make it easy to check whether a particular county, beach or coastline in Southwest Florida are currently affected by a red tide bloom.

NOAA’s Red Tide Respiratory Forecast is updated several times a day with the level of red tide risk at specific beaches along the Gulf Coast, from Pinellas County through Collier County. Visit habforecast.gcoos.org.

Mote Marine Laboratory’s Beach Conditions Report is updated frequently by lifeguards and includes reports of red tide conditions, including respiratory irritation and dead fish, as well as other hazards on local beaches. Visit visitbeaches.org.

The Florida Fish and Wildlife Conservation Commission issues weekly red tide reports with conditions by county and maintains a map of red tide sampling around the state. Visit myfwc.com/research/redtide/statewide.

University of South Florida’s Red Tide Prediction and Tracking center provides short term forecasts of red tide’s movement around Tampa Bay and the west coast of Florida. Visit ocgweb.marine.usf.edu/hab_tracking.

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