A swath of red tide stretching for more than 200 miles has formed from Tampa Bay to Key West and moved ashore along much of Southwest Florida’s coast.
The concentration of the organism that causes red tide, Karenia brevis, is weak in some places. In others, the red tide bloom is so strong it has turned the blue-green seawater burnt orange for dozens and dozens of miles.
The bloom started near Tampa Bay shortly after Hurricane Milton in October. It has been offshore and showing up in the satellite imagery for the last few months.
Now its epicenter is Lee and Charlotte counties, and Sanibel and Captiva islands.
David Tomasko is the director of the Sarasota Bay Estuary Program with more than 30 years experience in coastal sciences and water quality assessments. He is a co-author of a research paper reporting a correlation between extended red tide blooms and excess nitrogen in Southwest Florida waters.
Tomasko says this culprit is 75 miles to the east.
For more than two months, the Army Corps of Engineers has been releasing millions of gallons a day from Lake Okeechobee into the Caloosahatchee River, water polluted with nutrients from decades of excessive use of fertilizers by both family farms and industrial agriculture around the Big Lake.
"The Army Corps is looking to lower Lake Okeechobee by a couple of feet in response to all the rain you've had this past wet season," Tomasko said. “It's been ongoing for pretty much two months. What that means is we've taken about 80 billion gallons of water that used to be in Lake Okeechobee and now it's out in the eastern Gulf of Mexico.”
Tomasko said it’s no surprise the red tide is focused in the waters just off greater Lee County, where the Caloosahatchee River flows into the Gulf of Mexico.
"The key thing here is discharges of nitrogen-rich water coming down the Caloosahatchee River draining Lake Okeechobee don't create red tide," he said. "But they make red tides worse."
Showing on satellite for months
Nearly four months ago, Hurricane Milton made landfall at Siesta Key and power washed the detritus of modern life off streets and buildings with wind-driven sheets of rain moving up to 120 mph.
The current red tide bloom started near Tampa Bay shortly after Hurricane Milton in October. It has been offshore and showing up in the satellite imagery for the last few months.
Rainfall totaling 20 inches soon made its way into the Gulf of Mexico filled with that scum and scuz – dirt, rubber, and oil from roads; grass, leaves, and fertilizers from lawns – and days later, an area offshore and south of Tampa Bay began showing all the signs of setting up to become a sizable red tide bloom.
The billions of microscopic organisms that can bloom into a red tide are often nourished by fertilizers and other gross residue washed into the sea.
A similar set of events occurred during Hurricane Ian in 2022 and the nutrients in all that residue appeared to help “feed” smaller red tide blooms that broke out between Sarasota and Naples, coming and going, for months.
The phenomena are similar this time, but the outcome has not been the same.
In this case, the concentration of red tide organisms drifted south. State and federal red tide researchers tracked the growing areas offshore as environmental satellites sent back images, leading the researchers to suspect some areas were starting to bloom.
Marine scientists from several of Southwest Florida’s coastal nonprofits were collecting samples of seawater in huge areas of the Gulf of Mexico.
Onshore winds kicked up and pushed huge areas filled with red tide organisms that had been offshore of Sanibel and Captiva for many weeks toward the islands, where at some point they mixed with water filled with the nutrient pollution flowing from Lake Okeechobee down the Caloosahatchee River.
Two weeks ago, SCCF reported on red tide impacts starting to show up on the islands.
Portions of the bloom acted like outstretched fingers at times, pushing the acrid, fish-killing red tide ashore in recent weeks.
Local health departments advised people to stay away from more than a dozen beaches in Southwest Florida, including Turner Beach at the southern end of Captiva Island, Alison Hagerup Beach Park on Captiva and Bowman’s Beach on Sanibel Islanders. Warning signs are posted at public access points.
Barefoot Beach State Preserve and Vanderbilt Beach in Collier County have active health alerts issued after water samples showed elevated red tide toxin levels. Respiratory irritation has been felt offshore of Marco Island, and dead fish are floating in Naples Bay.
The Florida Department of Health in Monroe County issued a caution for red tide near Marvin Key and Marquesas Keys.
Samples collected along Lee County beaches showed background concentrations during this time even though the satellite images and reports from aerial photography showed the red tide west of Sanibel for the past month.
Last weekend, Sanibel-Captiva Conservation Foundation scientists found very high levels of Karenia brevis at Algiers Beach and Tarpon Bay Road Beach.
Mark Thompson, a Sanibel-Captiva Conservation Foundation researcher, said their samples were coming in at 22 million cells per liter in the water just off the beach. Concentrations of more than 1 million are considered high.
“The runoff from recent rain events have provided additional nutrients which help sustain the bloom in the waters near the barrier islands,” Thompson said last weekend. “The conditions of Sunday evening and Monday morning — with light onshore winds and no waves in the surf zone — caused it to become more concentrated along the shoreline, leading to more potential adverse effects.”
Satellite imagery as well as aerial photography and video shows that a portion of the bloom is so large it stretches from Sanibel and Captiva islands to Naples.
Duration uncertain
Historical patterns suggest red tide events in this region can last weeks to months, particularly when fueled by hurricane-induced nutrient upwelling likely caused by Hurricanes Helene and Milton last year.
The duration of the current red tide event along Florida's Gulf Coast remains uncertain, as the bloom's persistence depends on environmental factors such as nutrient availability, ocean currents, and weather patterns.
Recent forecasts predict northern movement of surface waters and northwestern transport of subsurface waters over the next 3.5 days, but these models don't indicate an imminent end to the bloom.
A cold front shifting winds to the northwest may temporarily reduce coastal impacts, but satellite data shows high chlorophyll concentrations – a dead giveaway of red tide -- remain that way offshore from Charlotte to Monroe counties.
The Florida Fish and Wildlife Conservation Commission plans to issue its next status update Wednesday, which may provide revised projections.
Environmental reporting for WGCU is funded in part by VoLo Foundation, a non-profit with a mission to accelerate change and global impact by supporting science-based climate solutions, enhancing education, and improving health.
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