Several scientific studies show that Lake O discharges to the Caloosahatchee River, which runs into the Gulf of Mexico near Fort Myers, don’t cause red tide, but the discharges do make the red tide worse.

“Caloosahatchee River discharges and nitrogen inputs systematically intensify blooms.”

It can’t get much clearer than that statement in a 2022 study by researchers from the University of Florida, the Sarasota Bay Estuary Program and the Sanibel-Captiva Conservation Foundation published in the journal Science of the Total Environment.

“We discovered that nitrogen-enriched Caloosahatchee River discharges have consistently intensified (red tide) blooms to varying degrees over time,” wrote the study’s authors, led by Miles Medina, lead author of the study and a research scientist at the UF’s Center for Coastal Solutions.

The study then traced the sources of red tide food upstream to Lake Okeechobee, which discharges to the Caloosahatchee, and farther up to the Kissimmee River, which drains into Lake Okeechobee.

The discharges help feed developing blooms, the study stated, and the nitrogen (from runoff fertilizer) helps the blooms keep growing.

Longer-lasting blooms

Next, a study published in 2024 in the journal “Florida Scientist” showed that Lake O discharges to the Caloosahatchee make red tide blooms last longer.

A team led by David Tomasko, executive director of the Sarasota Bay Estuary Program, looked at levels of nitrogen (a nutrient that feeds red tide blooms) flowing into the Gulf from the Caloosahatchee, Peace and Myakka rivers between 2007 and 2023. The research showed that only flows from the Caloosahatchee — which contains water discharged from Lake O — “exhibited statistically significant correlations with red tide event duration.”

That was certainly the case during the most recent red tide bloom, Tomasko told the Sarasota Herald Tribune in a story published Feb. 17.

“One of the biggest things going on is that over the last couple of months we’ve taken over 100 billion gallons of water that was in Lake Okeechobee, now (it) is in the eastern Gulf of Mexico,” Tomasko said. “Why is this red tide so big? Why is it so long lasting? They are lowering the lake by 4 feet, and they are pumping out about 1.3 billion gallons a day. They’ve been doing it for more than two months. … Literally about 100 billion gallons of Lake Okeechobee water is now fueling this red tide.”

Tomasko’s study concludes that “reducing (nitrogen) loads overall — and particularly those loads originating with the Caloosahatchee River and Lake Okeechobee — may significantly reduce the duration of (red tide) blooms along the southwest coast of Florida.”

It’s natural, but we’re making it worse

To be clear: Red tide is a natural phenomenon. Red tides have been reported almost annually along Florida’s Gulf Coast as far back as the 1840s. Fish kills near Tampa Bay were even mentioned in the records of Spanish explorers.

Red tide is a bloom of Karenia brevis, marine dinoflagellates (microscopic, single-celled algae) that produce a neurotoxin that kill fish and other marine animals by affecting their nervous systems.

Waves can break open the algae cells and release toxins into the air that can give people respiratory problems (coughing, sneezing, tearing and an itchy throat), especially those with emphysema and asthma.

People on or near a beach during a red tide bloom may also experience eye, nose and throat irritation, as well as coughing, wheezing and shortness of breath. 

Red tide toxins can build up in oysters and clams (a process called bioaccumulation); and eating affected shellfish can lead to neurotoxic shellfish poisoning, which is not fatal but can cause nausea, vomiting, partial paralysis, slurred speech and dizziness.

Pets can also suffer ill effects from contact with red tide. Dogs often lick themselves after swimming and could consume toxins on their fur.

The blooms typically grow far offshore in the Gulf of Mexico but can be brought by onshore winds to the coast. Studies have also indicated that nutrient-laden water flowing from rivers, particularly the Caloosahatchee, into the gulf can draw the blooms toward shore like a hungry person to an all-you-can-eat buffet.

Tracking the damage done

The most recent bloom started late last summer and at its height stretched from Tampa Bay to the Florida Keys. It started dying off in mid-February.

“Respiratory irritation suspected to be related to red tide” was reported in Sarasota, Lee, Collier and Monroe counties, according to the Florida Fish and Wildlife Conservation Commission website.

Fish kills were reported in Sarasota, Charlotte, Lee, Collier and Monroe counties, the FWC reported.

According to a Feb. 18 report, social media posts showed “moderate levels of dead fish scattered over areas such as Siesta Key, but the fish kills have been relatively small with this bloom.”

It’s been a lot worse.

Red tide season tends to run from October until February. But a bloom that started in the fall of 2017 lasted until the spring of 2019 and was marked by a deadly summer of 2018, when millions of pounds of dead sea life washed up on Gulf Coast beaches including 288 manatees and 600 sea turtles.

A study released in January by the Sanibel Captiva Conservation Foundation, Captains for Clean Water and the Conservancy of Southwest Florida found that the economic impact of another red tide bloom like the one in 2018 on southwest Florida (Charlotte, Collier and Lee counties) would be devastating, with the loss of: 

• Over $460 million in commercial and recreational fishing
• Over 43,000 jobs
• $5.2 billion in local economic output
• $17.8 billion in property values with an associated $60 million in property tax revenue
• $8.1 billion in the value of outdoor recreation or quality of life

That year, the bloom even traveled down the Gulf Coast, around the tip of Florida and up the Atlantic Coast — one of nine times red tide has been documented on the state’s eastern seaboard since 1957.

Indian River County was particularly hard-hit: Beaches were closed while public workers and a private contractor used shovels and tractors to haul thousands, if not millions, of dead fish off beaches.

We broke it, we can fix it

The studies point to the fact that humans are responsible, at least partially, for the intensity and length of red tide blooms; so humans can, at least partially, make the blooms shorter and less intense. The Tomasko study concludes that reducing “land-based nutrient loads” is particularly important because it “is one of only a few levers of control available.”

Those “levers” include initiatives such as the Lake Okeechobee System Operating Manual and projects such as the Everglades Agricultural Area Reservoir. Both are designed to reduce red tide-feeding Lake O discharges west to the Caloosahatchee and blue-green algae-feeding discharges east to the St. Lucie River and send the water south to the Everglades, where it should naturally flow and where it’s needed.

But an even stronger “lever” would be to stop nutrient pollution at its source by keeping farmers from over-fertilizing their fields and homeowners from over-fertilizing their lawns.