Air Date: Week of March 6, 2026

A sargassum bloom in Costa Rica in February 2026. 2025 saw a record bloom of sargassum in the Atlantic Ocean. (Photo: Carolina Calvo Brenes, Departamento de Comunicación Institucional del Ministerio de Ambiente y Energía, Gobierno de Costa Rica, Wikimedia Commons, Public Domain)

Though the floating seaweed known as Sargassum provides critical habitat for many species in the Sargasso Sea in the North Atlantic, it is now finding a fertile home in southern waters, where it’s wreaking havoc on coastal communities and ecosystems. Teresa Tomassoni, oceans correspondent with our media partner Inside Climate News, she spoke with Living on Earth’s Aynsley O’Neill about impacts to respiratory health, tourism and sea turtles.

Transcript

CURWOOD: It’s Living on Earth, I’m Steve Curwood.

DOERING: And I’m Jenni Doering.

So, maybe you’re dreaming of an escape from the frigid temperatures of the Northern Hemisphere this time of year. But your tropical vacation could be disrupted by something clogging those sandy shores.

CURWOOD: Sargassum is a floating macroalgae, and it saw a record bloom in the Atlantic Ocean in 2025, wreaking havoc on coastal communities. Most of this seaweed is normally contained by ocean currents in the Sargasso Sea in the north Atlantic, where it provides critical habitat for many species.

DOERING: But from 2009 to 2011, an unprecedented and prolonged wind event and possible ocean current changes pushed some sargassum out of the Sargasso Sea towards southern waters, where it’s found a fertile home. Teresa Tomassoni is an oceans correspondent with our media partner Inside Climate News, and she spoke with Living on Earth’s Aynsley O’Neill.

O'NEILL: So, what are these blooms looking like, and how do they affect coastal communities?

TOMASSONI: So the sargassum bloom that has occurred in the tropical Atlantic has actually formed what's called the Great Atlantic Sargassum belt. So this is a giant patch of about 5000 miles that stretches from the west coast of Africa to the Gulf of Mexico in the Caribbean. So this is a giant floating patch of sargassum that can be seen from space. It's the largest macroalgal bloom on Earth. And since 2011, we've seen with some variability, year to year, but overall, an increasing trend of this sargassum flooding beaches. 2025, last year was a record-breaking year. It was a 40% increase in the volume of sargassum in the Atlantic from 2022, the previous record year. And already scientists are saying 2026 is looking to be another record-breaking year. So scientists and governments are dedicating a lot of time and resources to understanding what is causing this bloom and how do we manage it?

O'NEILL: Well, Teresa, what's the primary driver behind these blooms? How much of this has to do with something like climate change versus another factor, something else going on?

TOMASSONI: It's a really interesting and important question that I haven't necessarily gotten straight answers from scientists. They're still trying to figure out what exactly caused this and what exactly is perpetuating this crisis. What we do know is that climate change was likely a culprit for driving that unprecedented wind event, current changes, those strong winds that blew the sargassum outside of the Sargasso Sea. Then this sargassum gets carried away into these currents, into warmer waters in the tropical Atlantic. And we know that sargassum does thrive in warmer waters. It also thrives in nutrient-filled waters. So the scientists that I've been speaking with have said it was sort of this perfect storm of conditions, warm temperatures and nutrient-filled waters from agricultural runoff from deforestation, off the mouth of the Amazon River Basin, off the mouth of the Congo, off the east coast of the US and in the Caribbean. Runoff from farms carries major amounts of nitrogen, phosphorus, iron, all of which help plants grow, and so in the ocean, it's helping sargassum bloom at unprecedented, massive rates. I think for a long time, it was thought that climate change is the primary driver for sargassum blooms, but as I mentioned, that might have been the initial culprit for getting sargassum outside the Sargasso Sea, but it's not necessarily the driving factor for these blooms. Warmer water conditions are favorable to sargassums, so it really thrives. But I'm also told that sargassum thrives within a certain range of temperatures, so if it gets too hot, it may actually stop growing.

O'NEILL: Well, in its original context, the sargassum was, you know, a boon. It was habitat for local fish populations. But what about in this situation? And what are some of the environmental consequences that happen when you have this overabundance of sargassum?

TOMASSONI: You're right. Sargassum is this critical habitat. It's a good thing. We need sargassum in the ocean, but as several scientists mentioned, too much of a good thing in the wrong place at the wrong time can be bad. It can be really bad. So what we've seen with these blooms is very severe environmental consequences as well as economic consequences for the countries or the states that are most affected by these inundations, these floods of seaweed washing upon their shores. Environmentally, these dense mats of seaweed that are getting pushed towards shorelines by currents, by tides, are really devastating near-shore ecosystems. They're so dense that they block sunlight from filtering through the water. Coral reefs and seagrass beds need sunlight to survive, for instance, but that seaweed is blocking that sunlight from reaching them, and when it dies, it's falling and even smothering entire seagrass beds and coral reefs. It also becomes a hazard for some marine life. So it's no longer a constructive habitat for baby sea turtles or other marine life, for instance. It's actually too thick for them to, to get through. I spoke with one sea turtle biologist here in Turks and Caicos, and she said she's actually seen drowned adult sea turtles entangled in these beds of seaweed floating near shore. Also, a few years ago, I was reporting on leatherback sea turtle conservation in Panama, and while patrolling the beaches there with scientists and community members, I watched as they really had to aid these tiny, two-inch baby hatchlings, surpass, climb over, get to the shore, because there were these mountains of sargassum in their way that would either entangle or trap them or just block their path from reaching the ocean.

O'NEILL: And how are these blooms affecting humans?

TOMASSONI: These blooms are not only affecting marine life, they're affecting people too on various levels. There are major health consequences to these blooms. So once sargassum arrives on shore, in about 48 hours, it starts to decompose, and when it starts to decompose, it begins to release toxic gasses, hydrogen sulfide and ammonia. This can cause respiratory issues. Some people develop asthma if they're exposed to these gasses over a longer period of time. It can cause skin irritations, eye irritations. It can cause headaches and nausea as it decomposes. It's also producing this brown, toxic water, one scientist told me. It's filled with arsenic, and in places like the Yucatan Peninsula, where the land is very porous, that water is seeping right into fresh groundwater that's used for drinking water, into fresh water swimming holes like cenotes, so it can have even further health consequences for people.

O'NEILL: And I imagine there might also be some sort of economic consequence going on here too. I mean, there's these toxic mounds all over the beaches. So how does this affect local communities in that way?

TOMASSONI: Absolutely. These blooms are also really taking an economic toll on countries and states impacted in the Caribbean, in Florida, Puerto Rico, US Virgin Islands, some of these places are losing millions, if not billions of dollars each year in lost revenue they would have otherwise gained in various marine sectors: fishing, tourism, hotels, restaurants along the coast. There was just a recent study put out by the Woods Hole Oceanographic Institution, which puts numbers to this impact. The scientists who produced this analysis estimate Florida loses around $5 billion each year due to sargassum and the harmful impacts it causes. As I mentioned, sargassum, as it's decomposing, not only produces these toxic gases, it also produces this rotten egg smell, which is really a deterrent for tourists that want to relax on the coast and enjoy their vacation. I've spoken with people who either live along the coast or visit parts of the Yucatan Peninsula regularly, and they have decided to no longer visit there because they would prefer to go places inshore where that smell and those gases are not bothering them. So people are making real choices to avoid these areas, and that's having large economic impacts.

O'NEILL: Well, so how are local communities dealing with these blooms? I assume they're not just saying, yeah, we'll take those economic and health consequences. So what sort of management tactics are they using here?

TOMASSONI: Communities, really throughout the region, are struggling to cope with these sargassum inundations. You see people doing really arduous labor, using shovels, rakes, wheelbarrows, to dig up the sargassum, put it in a wheelbarrow, transport it off the beach, or dig these giant holes on the beach and just bury it if they have nowhere to transport the sargassum. And some areas are using these deflection barriers called booms, to prevent the sargassum from reaching the shore. But these are expensive, and not every place has access to these booms. There's also a question of what to do with the sargassum once they collect it on the beach. Sometimes it goes to landfills, which can have pretty severe climate impacts, one scientist told me. I mentioned that the sargassum releases hydrogen sulfide and ammonia, but it also starts to produce methane as it decomposes, and we know Methane is a potent greenhouse gas, stronger than carbon dioxide, and so amassing more and more sargassum in landfills is inevitably going to have a climate impact as well. So people are really starting to rethink how we collect the seaweed and where we put it to have the least environmental impacts, the least climate impacts.

TOMASSONI: And well, some people are saying, well, we can't just get rid of it, and we can't just bury it. So some are looking towards harnessing it. Do I have that right? What have you heard about that?

TOMASSONI: You're correct in that a lot of folks, including various entrepreneurs, scientists, governments, startups, are all trying to think creatively about, how can we harness this seaweed? Out in the ocean it was once a good thing. How can we create something good again, out of it, and not leave it on shore to decompose on shore or in the landfills where it could harm people or the environment. So one organization, one UK startup, for instance, called Seafields, is looking to actually create these farms of sargassum offshore. I mentioned the deflection barriers, these booms that basically serve as a blockade to Sargassum reaching the shorelines. So Seafields is actually looking to create what they call these paddocks, or farms offshore as well. So they would use the booms to basically funnel the sargassum into these paddocks, where they would intentionally keep the sargassum alive, because it has less harmful impacts when it's alive than dead, and they would keep it alive until it was ready to be harvested and then used to create other products. Some startups and entrepreneurs are using sargassum to create biochar. Biochar is kind of like a type of charcoal. It stores CO2 long term and is being used to actually be added to concrete to reduce concrete carbon footprint, and also it's been proven to make concrete stronger. Some entrepreneurs in Mexico, I'm told, are using sargassum to add to plastic to make these wave like flexible roof panels to make sandals. Others are using sargassum to make these sort of fertilizer additives, these biostimulants to make the soil healthier and prompt faster growth of plants. We know sargassum grows well, so it's got a lot of properties that can potentially help other plants grow efficiently as well. This problem, unfortunately, is not likely to go away anytime soon. One scientist told me that for the next ten years, at least, unless there's a major climate event that stops the growth of sargassum, this is the new reality, and we have to face it.

DOERING: Teresa Tomassoni of our media partner Inside Climate News spoke with Living on Earth’s Aynsley O’Neill.

 

Links

Woods Hole Oceanographic Institution | “New Study Quantifies Sargassum’s Multi-Million Dollar Impact to U.S. Coastal Economies”

The Guardian | “Scientists Warn of ‘Regime Shift’ as Seaweed Blooms Expand Worldwide”

Oceanographic | “Turtle Hurdles: Florida’s Sargassum Surge Is a Crisis for Sea Turtles”

National Library of Medicine | “Sargassum Seaweed in the Caribbean: A Major Public Health Problem Still Unsolved”

The New York Times | “Shifting Climate Alters Pattern of Atlantic’s Giant Seaweed Blobs”

Learn more about Teresa Tomassoni