The Decade-Long Perfect Storm Behind Florida’s Viral “Seaweed Blob”

It was all anyone could talk about for a week straight: the smelly seaweed blob straight out of a bad ‘80s sci-fi horror movie.

Earlier this month, headlines from outlets ranging from CNN to The New York Times to The Guardian bemoaned an encroaching “5000-mile” seaweed blob hitting Florida coastlines and ruining your beach vacation with its profound stench of rotting eggs.

But Inverse spoke with experts who said the sensational news headlines failed to capture the true nature of the looming seaweed — known as sargassum — which has actually been ongoing for a decade and has its roots in climate change, among other factors.

In short: experts say you don’t need to personally freak out about sargassum, but it’s important to understand why scientists are concerned about the impacts of seaweed on coastal communities — and how they think we can take action to protect ourselves from it.

“It appears to be a huge threat to tourism and beach community livelihoods,” Clarissa Anderson, executive director of the Southern California Coastal Ocean Observing System (SCCOOS) and an expert in harmful algal blooms, tells Inverse.

What is Sargassum? 

Unlike other seaweed that may be anchored to the sea bottom, rocks, or shoreline, the specific kind of sargassum hitting the beaches in Florida is a type of free-floating “holopelagic” seaweed, which means it completes its life cycle at sea. The National Oceanic and Atmospheric Administration describes it as floating in the sea in “island-like masses.” The two types of holopelagic sargassum are Sargassum natans and Sargassum fluitans.

“Those two different species are the ones that form the sargassum seaweed and have existed for more than 500 years in the North Atlantic,” Ligia Collado-Vides, a marine scientist and associate chair in the Department of Biological Sciences at Florida International University, tells Inverse.

Why is the Seaweed Encroaching on Florida?

So how did this sargassum get from the North Atlantic to the Caribbean and Florida, where it’s currently encroaching on beach communities?

To answer that question, we have to make our way to the Sargasso Sea — the only sea in the world that doesn’t contain a land boundary. This is the region where the highest blooms or “biomass” of Sargassum grows. It’s a region shaped by various ocean currents, including the North Atlantic Current. But over the past decade, sargassum has been expanding its reach into previously uncharted waters.

Researchers wrote in a 2019 Science article that since 2011 “the mats have increased in density and aerial extent to generate a 8850-kilometer-long belt that extends from West Africa to the Caribbean Sea and Gulf of Mexico.”

Collado-Vides says climate anomalies in 2010 impacted the North Atlantic Current, releasing some of the sargassum and sending it south. That sargassum coincided with a “perfect storm” of factors — ranging from temperature to nutrients in the water — and led to the formation of the Great Atlantic Sargassum Belt.

“Atlantic ecosystems need Sargassum”

Oceanographers first detected this belt via satellite imagery in 2011, and we’ve been seeing it ever since, though its size fluctuates in a given year. Some years — like 2018 and now 2023 — have seen more prominent sargassum blooms depending on changing climatic conditions like temperature.

Anderson explains the Great Atlantic Sargassum Belt is a tropical section of the Atlantic Ocean where sargassum thrives and grows in large mats. Scientists also refer to these mats of sargassum as “rafts” and now conducting research trips to collect sargassum samples from the Great Atlantic belt for the first time.

Collado-Vides says that news characterizations of the sargassum as a blob are inaccurate since they liken it to a blanket when it really resembles more of an unpredictable “dynamic” mass of floating pieces. Anderson says that while “blob” is not a technical term and “is subject to a lot of interpretation and exaggeration,” the current mass of Sargassum has nonetheless become more “extreme” and “widespread” over the last five to ten years.

Climate Change and Sargassum

Scientists aren’t quite sure why the sargassum is growing or its size is changing, but Anderson says the Gulf Stream — which transports warm water from the Gulf of Mexico to the Atlantic — may be affecting the sargassum.

Collado-Vides explains that other factors contributing to the growth of sargassum include climate change and nutrients like phosphorous and nitrogen. Fertilizer runoff from industrial agriculture often includes such nutrients, which make their way into the ocean and contribute to so-called “algal blooms.”

Anderson adds that algae also thrive in warmer waters due to climate change. Research published in the journal Nature in 2017 found climate change was causing algal blooms to form in places where they hadn’t previously occurred.

“Scientists are trying to understand how climate and changes in the temperature and nutrient makeup of the ocean are leading to a positive expansion of sargassum, i.e. increase,” Anderson adds.

Is Sargassum a Threat?

Experts explain that sargassum isn’t inherently bad — in fact, the Ssrgassum mats provide refuge to a host of marine animals, including sea turtles. It’s just the excess quantities of sargassum in recent years that have become concerning.

“Atlantic ecosystems need sargassum. We need to get back to the Goldilocks Zone of just enough, but not too much,” Anderson says.

Marine biologists worry about how the algae impact the environmental health of ocean communities. Collado-Vides explains that large quantities of sargassum effectively consume oxygen when bacteria decompose or break down the algae, creating low-oxygen or “anoxic” conditions in the ocean that can affect the wider marine ecosystem. Sargassum mats also block light, which sea grasses and other oceanic life need to grow.

“It's very, very difficult to know where going to land.”

For humans, the sargassum is largely concerning because of its profound funk. Collado-Vides explains sargassum’s odorous smell — akin to rotten eggs — is due to toxic gases like hydrogen sulfide, which emits when the algae decompose.

According to the CDC and the Occupational Safety and Health Administration (OSHA), exposure to hydrogen sulfide can affect the eyes and respiratory system. Symptoms like headaches, asthma, dizziness/sleepiness, irritability, and stomach upset can occur at lower levels of hydrogen sulfide exposure starting at two parts per million — though much higher levels of exposure can cause comas or worse. A 2022 paper found a risk of preeclampsia in pregnant women living near sargassum blooms on the island of Martinique. The rotten egg smell becomes noticeable at even lower levels of exposure to hydrogen sulfide at 0.01 parts per million.

“The toxic gases are not healthy for anybody and they can be smelly,” Collado-Vides says.

Collado-Vides says you would need large amounts of decomposing algae to get to the point where you’re noticing rotten egg smells, so people visiting or living in these areas won’t be exposed to toxic gases all the time. But the smell could affect your enjoyment of the beach if you’re taking a short trip while the algae is decomposing. Animals could also get trapped in the algae.

“I think the greater threat is to the ability to use beaches that are covered in sargassum,” Anderson adds.

Governments and private companies are organizing efforts to clean up sargassum from beaches, but these initiatives can become quite expensive, causing financial stress in coastal communities. One such cleanup in the Caribbean cost $120 million in 2018. But Collado-Vides says smaller-scale and lower-cost cleanup efforts are still good to attempt, though challenges remain around where to dump the sargassum after removing them from the beaches. (Sargassum contains arsenic, so a landfill may not always be a safe option).

“They don't need to do this super huge cleaning, but at least reducing to certain amounts that are going to stop decomposition to the level of creating anoxic conditions — that could be a good strategy of management,” Collado-Vides says.

Can Citizen Science Help Us Track Sargassum?

It’s notoriously difficult to predict where, exactly, sargassum will end up or how much we will get. Scientists like Collado-Vides frequently monitor seaweed on the coastline but could leave before winds bring in the sargassum. They may head into the field thinking the sargassum will land in Key West but shifting winds could actually bring it much farther north to Fort Lauderdale.

“It's very, very difficult to know where going to land,” Collado-Vides adds.

But with the help of local residents — in addition to using satellite imagery — scientists are hoping to understand the extent of seaweed and get a better handle on where the sargassum eventually winds up.

Researchers like Lowell Iporac — a Ph.D Candidate at the Marine Macroalgae Research Lab at Florida International University in Miami — who study seaweed are relying on citizen scientists to get the data they need.

They have amassed a volunteer network spanning Florida and the Caribbean to record environmental conditions, arrival times, and levels of sargassum algae accumulating on shores. Interested volunteers can download the Epicollect5 app on their smartphone and search the ‘Sargassum Watch” project within the app to start logging in observations.

Iporac explains that observational data of sargassum by citizen scientists can “can further refine large-scale satellite forecasting models of sargassum arriving across the Caribbean.”

But one thing’s for certain: wherever it ends up, the smelly seaweed is here to stay.

“There does not seem to be an end in sight to the Sargassum problem plaguing the Caribbean and Florida,” Anderson concludes.