In October 2017, about one month after Hurricane Irma wreaked havoc on Florida, researchers detected the initial whispers of red tide.
It first appeared, as harmful algal blooms normally do, in trace amounts. A little bit here, a little bit there.
But by the second month after Irma’s landfall, right around November, red tide had peaked in the Charlotte Harbor area, where it spread, it smelled and it stuck around. Federal ocean scientists referred to the event as “unusually persistent” — it ultimately flowed north to the Panhandle and over to the Atlantic Coast.
With Hurricane Irma then, and Ian now, scientists are trying to piece together what impact storms have on red tide. They know the storms don’t cause red tide (the organism typically originates offshore, and then is pushed east toward land). But hurricanes may contribute to red tide’s intensity and location based on which way the current or wind moves water and, crucially, what’s in that water.
“The real question is going to be: With all the runoff stirring up all the sediments, how will Hurricane Ian influence red tide development — will it make it worse? Will it make it last longer?” asked Michael Parsons, director of the Water School at Florida Gulf Coast University. “That’s the big question.”
It’s an important question for Tampa Bay, where red tide is becoming too close for comfort. Bloom concentrations were detected in 18 water samples across Southwest Florida last week: four in Sarasota County, nine offshore of Charlotte County and five offshore of Lee County, according to the latest Florida Fish and Wildlife Conservation Commission data.
At least one report of somebody having trouble breathing, likely because of red tide, was documented last week in Sarasota County, according to the commission.
The latest red tide forecasts from the University of South Florida show very low concentrations of the red tide-causing karenia brevis drifting north from the Sarasota area over the next few days. But whether red tide will make it to Tampa Bay remains unclear, according to Kate Hubbard, director of the Florida Fish and Wildlife Conservation Commission’s Center for Red Tide Research.
“Each year is different, and the uncertainty with hurricane impacts make it extremely difficult to predict more than a few days out,” Hubbard wrote in an email. “At the present time, it’s hard to say whether the cells we are seeing now will remain localized.”
It’s also hard to say what impact the billions of gallons of runoff left in Ian’s wake will have on red tide formation, according to Hubbard. Karenia brevis can feed on a diversity of nutrients and use them as fuel. But too much freshwater can also deter red tide because its organism survives best in saltier marine environments. In short, Ian’s runoff doesn’t cause red tide, but can intensify it if algae cells are already present.
On Sept. 30, two days after Ian’s landfall, satellite imagery captured murky runoff accumulating in the middle-lower Tampa Bay area after rainfall flooded from the bay’s tributaries, according to visuals provided by Chuanmin Hu, a professor of optical oceanography at the University of South Florida. The root beer-like color originates from discharges from waterways like the Little Manatee and Alafia rivers, and other runoff, Hu said.
The lack of saltier water from that runoff could also determine whether Tampa Bay sees any significant rise of red tide. The algal blooms won’t grow when a given body of water is around one-third freshwater, according Parsons.
“If there’s still a lot of runoff coming into Tampa Bay, the water might be too fresh. That’s another thing to think about,” Parsons said. One of the few continuous monitoring stations that tracks salinity levels in the bay was knocked out by the storm and was offline through last week.
For now, at least, Florida’s manatees have been spared by red tide’s return in Southwest Florida, according to Fish and Wildlife Research Institute spokesperson Carly Jones. That’s good news for the species that saw a record die-off in 2021, leaving 1,100 animals dead due to pollution-fueled seagrass loss, mainly on Florida’s east coast.
There’s also another variable at play with red tide: ocean currents. Red tide typically flares up when there’s a persistent ocean current from the southeast. A storm can add to that coastward current, or it can reduce it. In the aftermath of Hurricane Ian, “the deep circulation has been favorable for onshore transport since Ian’s passage,” Jones said.
In other words: Currents are helping to move red tide closer to shore.
Conditions are most favorable for red tide from September to January, which happens to coincide with some of peak hurricane season. In any given year, scientists can detect red tide cells before, or after, a storm. Because of that, it’s hard to say whether this winter is shaping up to be a rough winter for red tide, according to Yonggang Liu, director of the Ocean Circulation Lab at the University of South Florida.
The bad red tide that flared up beginning in 2017 wasn’t caused by Irma, but it could have still had an impact, according to Liu. There was an almost two-month lag between Irma and the arrival of the red tide that year, which may have been ushered in by repeated cold fronts.
Red tide blooms, Liu noted, were already predicted earlier this year. The first stage of red tide, a lack of deeper and cooler ocean water rising to the surface in late spring, was an indicator for what was to come. If the northward currents continue in the following weeks, red tide cells currently to the south could be brought to the Tampa Bay area. But Liu said he doesn’t think that will happen.
“Hurricane Ian’s heavy rainfall and drainage may contribute the nutrients to the ecosystem in the coastal ocean, but they are not the game changer,” he said.
So, what’s Liu’s prediction for red tide in Tampa Bay in the coming months?
“Let’s keep our fingers crossed.”