A 5,000-Mile-Long Mass of Seaweed is Coming to Shore Transcript

It sounds like science fiction. A 5,000-mile-long belt of seaweed weighing more than 11 million tons is sloshing around in the Atlantic Ocean. When some of it reaches Florida, it threatens to wreak havoc in the coastal waters and on the beaches. But it is very real. It’s called the Great Atlantic Sargassum Belt, so big it can be seen from space, spanning the tropical Atlantic from West Africa to the Caribbean. Earlier I talked with Ajit Subramaniam, an oceanographer at Columbia University’s Lamont-Doherty Earth Observatory.

Sargassum is a seaweed that grows entirely on the surface of the ocean. It is never attached to land, and the Sargasso Sea is called the Sargasso Sea because of the prevalence of sargassum in the northern part. The Great Sargassum Belt is a new population of sargassum that seems to have developed since about 2011. We didn’t see it there in satellite imagery before that, and then we saw this explosion of a new population about then, that seems to basically slosh back and forth between the coast of West Africa and the Yucatan Mexican coast on the other side of the Caribbean on an annual basis.

Is this one bed of seaweed? Is it multiple plants?

Actually, it is made up of individual strands. So when you’re out on a ship, we often will see a band of sargassum that may be a couple of meters wide, so let’s say 10 feet wide, and that stretches disconnected but into the horizon. So you have these streaks that are continuous only to about 200 meters or 300 meters each one, but then they line up one behind the other because of the wind.

And you say that this was first seen in imagery in 2011 and it’s grown a lot since then. Why did it suddenly appear, and why is it growing so fast?

There are a couple of theories on that that have been published. One has to do with the change in circulation and a deep mixing which brought nutrients to the surface in 2010 that could have initiated this new population. The other theory is that it is changes in agricultural or land-use patterns in the Amazon River Basin that has increased the flow of nutrients coming out of the river. I personally think that both may be partially right, but I do not know that either one explains it completely. So for me, it is still a little bit of a mystery as to what caused the new population, but it is obvious it is there, and it has been growing since.

And I also understand that in the open sea, there can be benefits from this. Is that right?

That’s right. So in the Sargasso Sea, for example, where this population has existed since before the times of Christopher Columbus, it is seen as a habitat for fish. When we are out in the ocean, we often see fish like mahi-mahi that are hanging out under these big rafts of sargassum. So it’s a very active ecosystem. I’ve sometimes compared it to an upside-down coral reef, in that it’s a hotspot of biological activity.

What are the threats when it gets closer to land?

There are just multiple threats. One is that when it washes up on beaches… And in Barbados, I’ve seen piles of sargassum five feet high. And you do not want to go to the beach when it is covered with sargassum, both because it really smells very badly when it rots, but then people have now done studies to show that pregnant women are affected by the hydrogen sulfide that is produced due to the rotting of the sargassum.

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Methane is produced when it rots, and that is a very important greenhouse gas. You also have environmental damage, because while in the process of cleaning up sargassum, you have heavy trucks going on the beach and damaging the rather delicate environment that a beach is. There is increased erosion when sargassum is washing upon the beaches and gets washed away. It’s been suggested that hatching turtles have a difficult time climbing their way through the massive sargassum that is beached, and therefore, it might be affecting turtle populations.

Is there anything that can be done about this?

The ideal way to deal with this is to try and prevent it from beaching. The question is where do you put it, because you don’t have enough land area to then go dump it someplace. I have been working with colleagues. We’ve been working on this idea that if we can collect the sargassum when it is still in deep water offshore and sink it, then we are actually coming up with a nature-based solution, basically mitigating against climate change. Because when the sargassum grow, they do photosynthesis, which basically means they take up carbon dioxide from the atmosphere and convert it into biomass. And if you sink the biomass to depths greater than maybe 2000 meters, you are taking the carbon dioxide out of circulation for about a hundred years from the atmosphere.

We’ve seen a lot of discussion about seaweed replacing plastics, and they’re starting to farm it in the oceans. Are there any ecological concerns about farming in the ocean?

If you are doing this in a way to mitigate against climate change or replace plastic from seaweed, you just want to make sure that you’re doing some sort of lifecycle analysis where you’re not spending more carbon in harvesting less amount of carbon. I always say you can’t spend 100 kilos of carbon to get rid of 10. And so that lifecycle analysis is something that needs to be done, and people are doing research on that. But I do not know that that’s a settled fact, that you can actually do this in a carbon-efficient way if your objective is to mitigate against climate change.

Ajit Subramaniam of Columbia University’s Lamont-Doherty Earth Observatory. Thank you very much.

Thank you. Pleasure.