June's Saharan dust setting up right conditions for red tide, but still too soon to tell

In June, a massive plume of Saharan dust made its way from Africa to Florida, bringing hazy skies and vibrant sunsets while warding off storms. But months after touching down, this dust could have an adverse effect.

According to researchers, this year’s iron-rich Saharan dust is setting up the right conditions for the start of a red tide bloom — but it’s still too soon to tell if there will be another devastating red tide in the near future.

“The fact that there are measurable amounts of iron dust means that it is setting up the conditions offshore for red tide right now,” said Cynthia Heil, director of Mote Marine Laboratory’s Red Tide Institute. “It doesn’t necessarily mean there will be one, but the odds are fairly good.”

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Saharan dust is exactly what it sounds like – fine dust from the Sahara desert that gets kicked up into the upper atmosphere by storms in the late spring or early summer, Heil said. The dust rides the trade winds across the Atlantic Ocean to the Gulf of Mexico and other places.

But it’s not the dust itself that’s important, it’s the iron in the dust that’s key.

When deposited in open water, the iron essentially dissolves off of the dust, Heil said.

“In open ocean waters, especially tropical and subtropical waters like the Gulf of Mexico, the algae that live there can be limited by iron,” Heil said. “If you feed them iron, then they’ll grow.”

Trichodesmium, a marine cyanobacteria, has iron-based enzymes that take nitrogen from the air and turn it into fertilizer, said Michael Parsons, professor of marine science and director of the Vester Field Station at Florida Gulf Coast University.

“Trichodesmium is everywhere in the water, but it doesn’t grow a lot, because once you get far enough away from land, there’s not enough iron in the water,” Parsons said.

A majority of the enzyme is made up of iron, so for the enzymes to take nitrogen and produce ammonia, iron must be present. And with a lot of iron, a lot of nitrogen is fixed.

“When it has enough iron, it’s fixing like crazy and making lots of ammonia from this nitrogen gas, and it leaks it out,” Heil said. “Up to 50% of what it fixes, it just doesn’t need, so it just chucks it out into the water. It becomes a nutrient source for red tide.”

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Red tide, a toxic microalgae, is always around at some background level, Parsons said. Red tide blooms usually start in the early fall.

Southwest Florida’s most recent red tide bloom began October 2017 and lasted until February 2019. It ravaged Tampa south to Collier County and Florida Bay, the east coast and the Panhandle, leaving wildlife dead and local tourism-based businesses crushed.

The blooms go through a cycle — they start offshore and as they move near shore, they intensify and can be detected and monitored, Heil said. Florida sees a red tide bloom nearly every year, but not every bloom is devastating.

“Kind of one of the unknowns is what makes it a very bad year,” Heil said. “That’s one of the open research questions and whether climate change is going to contribute to that. The fact that we are seeing dust deposited means that the conditions are there for a red tide.”

But it’s still too soon to say whether or not June’s Saharan dust will actually lead to another bad red tide bloom. The dust isn’t required for a bloom to occur, and its presence doesn’t have a single cause and effect in contributing to red tide.

Another condition that plays a role in a red tide bloom's development is upwelling, where deeper water comes up from the ocean’s depths to move a red tide bloom closer to shore.

“It’s like finding that Goldilocks, right? You know, finding the optimal condition,” Parsons said. “You need to have trichodesmium out there to take advantage of the Saharan dust, and then you need to have enough of the red tide cells out there to take advantage of the trichodesmium.”

It’s a common misconception that humans cause red tide, Parsons said. Before red tide even reaches shore, it blooms naturally.

“We’re not really involved in how a bloom starts,” Parsons said. “The real question is once that bloom starts moving closer to shore, is it utilizing any nutrients that we’re putting in the water? The real question is not, did we cause red tide, the real question is, are we making it worse?”

Saharan dust is also a natural process that plays a crucial role in how nutrients are cycled in the ocean and climate, Heil said. It does affect Florida, but it also has global effects.

“The fact that dust coming from Africa is playing a role in Florida red tides is kind of amazing, when you think about the global connections,” Heil said.

Anything residents can do to reduce the amount of nutrients they put in the ocean is good, Parsons said. That might mean examining your fertilizer use or inspecting your septic tank. And for those out on the water, reporting a fish kill or discolored water can be helpful in identifying a red tide bloom.

“There is no active red tide that we know of right now, but it’s always around in background levels,” Parsons said. “People need to just keep in mind that it’s always around a little bit, and it will be back, unfortunately.”

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