Dust from Africa's Sahara Desert may be contributing to Florida's Red Tide problem
Researchers think iron that has traveled all the way across the Atlantic is triggering a process that causes the toxic algae bloom.
Red Tide hits Florida on a regular basis, but rarely is one of the algae blooms as deadly as this year’s. Already declared the worst in a decade, it’s killed a dozen dolphins, scores of manatees, hundreds of sea turtles and untold thousands of fish.
Why is this year’s bloom so bad? The Sahara Desert may be to blame.
"It’s possible," said research scientist Kate Hubbard of the Florida Fish and Wildlife Research Institute in St. Petersburg, which is the agency in charge of monitoring the bloom. "It is a nutrient source."
WHERE (AND WHAT) IS RED TIDE? Visit our Visitor Information section for more about Red Tide, and find a link to the state's map tracking the blooms.
Bolstering the dust theory is the fact that, according to Hubbard, markers showing the dust’s presence have been detected near shore this summer.
The explanation for how this works demonstrates what an incredibly complicated mechanism the Earth’s weather can be.
Every year, over about 100 days from spring through the fall, winds in Africa will pick up tiny dust particles from the desert and carry them thousands of miles across the Atlantic Ocean. The tiny bits float along on the wind, soaring up to 3 miles above the surface of the water. Sometimes they clump together into a hazy configuration as large as the continental United States.
Then they land at last in and around the Caribbean Sea and the Gulf of Mexico.
READ MORE: Answers to questions about Red Tide
Usually the dust particles feed phytoplankton, build Caribbean beaches and fertilize the Amazonian rain forest.
Sometimes the dust creates beautiful sunsets. Sometimes it causes a thick haze to hang in the air over Texas. Sometimes, according to a Texas A&M study, it suppresses hurricanes from developing in the Gulf.
And sometimes it feeds a Red Tide bloom.
Nobody knows what causes the scattered patches of microscopic Red Tide algae to suddenly multiply by the millions and turn the water the color of rust. The blooms start 10 to 40 miles offshore in the gulf, and then winds and currents move them toward shore. The one going on now has been hanging around offshore since November but recently became more intense.
The blooms can be fueled, and their life extended, by feeding on nutrients. Close to shore, the source of such nutrients can be fertilizer in stormwater runoff and fecal waste from leaking sewage lines and septic tanks.
But there’s another source as well — that dust that landed from Africa.
One of the minerals found in the dust is iron. As the dust falls into the gulf, it attracts a plant-like bacteria, called Trichodesmium. The bacteria uses that iron to convert any nitrogen in the water into a form that can be consumed by other marine organisms.
And one of those organisms that absorbs the converted nitrogen is Red Tide algae.
A 2014 study by the National Oceanic and Atmospheric Administration found that, out of 12 possible sources, Trichodesmium provided the most nitrogen for Red Tide blooms.
The way to check for the presence of the dust is to look for the Trichodesmium, said Hubbard, with the Research Institute.
"We have been observing Trichodemium in the gulf for several months," Hubbard said in an interview this week. "A few times we have seen high concentrations close to shore."
At this point, Hubbard said, it’s not proven that the dust made this year’s Red Tide bloom so bad. She just called it "possible."
The National Aeronautic and Space Administration did detect a large plume of Saharan dust blowing across the Atlantic from the final week of June and into early July, noting in a report: "The distinctive yellow-orange haze associated with the dust can be seen on satellite traversing thousands of miles over just seven days reaching the far western Caribbean."
Sometimes the dust attaches to clouds along the way and falls as rain over the Atlantic, said Bowen Pan, who was one of the authors of the Texas A&M study. But if it makes it all the way across, it can disrupt weather patterns in the U.S.
By mid-July the latest plume of dust was causing haze and air pollution problems in Fort Worth, prompting the National Weather Service to urge people to stay indoors.
Pan pointed out that the predictions about the current hurricane season call for fewer storms than normal. The Texas study links the presence of the dust to that phenomenon.
For such tiny particles — Pan said they measure about 2.5 microns, with a micron being 25th of a thousandth of an inch — they have a large impact on this side of the globe.
Where does the dust start? At least half of it comes from a dry lake bed in north central Chad, near the desert’s southern end, according to NASA scientists. The lake bed is known as the Bodélé depression.
It used to be part of a larger water body called Lake Chad. In the mid-1960s, Lake Chad was about the size of Lake Erie. But persistent droughts coupled with increased demand for freshwater for irrigation reduced Lake Chad to about 5 percent of its original size.
As the waters receded, the silts and sediments resting on the lake bed were left to dry in the scorching African sun. Meanwhile a scouring wind blows between a gap in the mountains just upwind of the depression, forcing it to accelerate to a speed that pushes the dust up into the air and across the ocean.
That means the dust that flies so far to land near Florida isn’t like the kind you find tangled up in bunny form under your couch. This dust is made up of the skeletons of diatoms — microorganisms that were once alive, back when the lake bed was full of water.
Tampa Bay Times senior news researcher Caryn Baird contributed to this report. This article originally appeared in the Times on Aug. 17, 2018.