Could South Florida see a major seaweed event by spring 2025? Satellite imagery suggests so

When Dr. Dennis McGillicuddy fished with his grandfather in the waters off the coast of Miami, he said they looked for sargassum–because seaweed was a sign of life. 

“Our objective was to find sargassum, because if we could find a weed line of sargassum, that meant we would find the fish,” he told NBC6 Thursday from the Woods Hole Oceanographic Institution in Cape Cod, Massachusetts, where he works as the senior scientist in the Department of Applied Ocean Physics and Engineering.

In pictures McGillicuddy took in March of last year, a range of animals can be seen hiding in the vegetation: crabs, shrimp and even various small fish, some of which would grow into an animal emblematic of South Florida–the dolphinfish, or mahi-mahi.

“So when my grandfather and I were fishing together decades and decades ago, we were looking for the weed lines where not only the juvenile mahi hide, but where the adult mahi forage for food,” McGillicuddy shares excitedly. 

But despite playing what McGillicuddy describes as an essential role in a balanced environment, “like many other things, too much of a good thing can be a bad thing, and that’s what we’ve seen in the recent blooms of sargassum.”

This year’s bloom, captured on satellite imagery by the University of South Florida’s (USF) Optical Oceanography Laboratory, could turn out to be more than South Florida bargained for.

What does new satellite imagery show?

USF’s Sargassum Watch System uses satellites to provide a general outlook of the density of sargassum, a brown macroalgae commonly called seaweed, on the ocean’s surface. The warmer the color, the greater the amount of seaweed in that spot. 

And images from December 2024 show a statistically remarkable amount of it in the Central and Central East Atlantic ocean. 

“It’s higher than the 75th percentile in the past 15 years for the same period,” Dr. Chuanmin Hu, professor of Oceanography at USF’s College of Marine Science says. “Now, why it matters to us? It’s still far away, hundreds of miles away, [but] some of that eventually will come to Florida.”

Sargassum will keep growing if it gets adequate sunlight and nutrients, and though experts can’t yet predict exactly how much it will expand, the bulletin published by the Optical Oceanography Laboratory states that “because of the relatively high amounts of Sargassum in the tropical Atlantic, 2025 is likely another major Sargassum year.”

McGillicuddy adds: “When it’s actively growing, [sargassum] doubles about once every 10 days. So if it continues to grow at its maximal rate between now and when it inundates, yeah, it’s going to be a major inundation event. However, if it runs out of nutrients, then it’s possible the bloom could dissipate.”

Why does it matter how much seaweed washes up on the beach?

Well, there’s the obvious. It can be an eyesore, and makes going to the beach a little more uncomfortable, and yes, smelly.

“The inundation of large amounts of seaweed onto our coastlines affects recreation, tourism [and] the decay of the sargassum on the coastlines actually can release hydrogen sulfide gas, which is very poisonous. We have to be careful about that, but it’s also affecting the local environments as well,” McGillicuddy says.

Then, there’s what too much seaweed can do to the environment. Specifically, if the bloom is too large, it could block sunlight and have negative consequences for sea grasses and coral reefs.

“Another direct consequence we see of this in the coastal ocean has to do with animals that use the coastal environment for reproduction,” McGillicuddy says. “As you probably know, loggerhead turtles their preferred nesting environment of beaches. And so we’ve seen situations when the adults can’t get ashore to lay their eggs, and then those that are lucky enough to lay their eggs, the hatchlings have difficulty getting back out into the ocean because of the seaweed barrier that’s created by sargassum.”

When could it arrive in South Florida?

Hu says, depending on ocean currents, South Florida could see sargassum start to wash up on the beach anywhere from April to June.

Now, experts don’t want you to panic.

Again, sargassum is important to the environment, and “has been with us as long as human beings have been sailing the oceans. Actually, the very first written account of sargassum in the North Atlantic comes from the log books of Christopher Columbus in 1492,” McGillicuddy says.

What is fairly new is how much of it is out there.

“The emergence of the Great Atlantic sargassum belt in 2010 to 2011 was really an unprecedented event, as best as we can tell, in the records of sargassum distribution throughout history,” McGillicuddy says. “There’s been a lot of interannual variability, year-to-year variability in the magnitude of the Great Atlantic sargassum belt, but generally speaking, the trend has been upward, that biomass is increasing over time.”

Researchers are trying to figure out exactly why that is, what the longterm effects could be and how to public health officials could potentially mitigate concerns.

But for now, beachgoers can “pay attention, and keep an eye out,” but Hu emphasizes, “don’t panic, this is not like a hurricane.”