The Strange Sargassum Bloom: How it May Provide Answers to Responsibly Sequestering Carbon in the Ocean

Patches of sargassum seaweed have been spotted in the Atlantic Ocean for centuries.?However, since 2011, for reasons that are still unclear, masses of sargassum have been piling up annually onto the shores of the Caribbean islands, wreaking havoc on their coastal ecosystems and crippling tourism. The low season of the cycle is now higher than the high point of the cycle five or six years ago. These disconnected patches stretch over 5,000 miles and weigh over 10 million tons.?The right combination of currents and wind push the sargassum ashore, and this year, there is more sargassum than ever.

Sargassum is a type of seaweed that floats on the surface of the ocean without any contact with land or the sea floor.?Berry-like structures filled with oxygen provide buoyancy and as they lose oxygen, the sargassum sinks to the seafloor.?This is usual. What's unusual is the relatively recent occurrence of masses of sargassum landing on the shores of the Caribbean islands. When they come close to shore, they smother coral reefs, upset the pH balance, and can keep newly hatched baby turtles from swimming back into the ocean. After landing on the beach, the sargassum decomposes, releasing hydrogen sulfide, which has the smell of rotten eggs and can causes respiratory problems for sensitive populations. Decomposing sargassum also releases methane, a potent GHG. Removing it from the coastlines requires heavy machinery which impacts the fragile beach ecology.

Dr. Ajit Subramaniam, Research Oceanographer at Columbia University’s Lamont-Doherty Earth Observatory, has been studying sargassum for several years now. His findings have been featured in the Guardian (https://www.theguardian.com/environment/2023/mar/07/great-atlantic-sargassum-belt-seaweed-visible-from-space) and PBS (https://www.pbs.org/video/costal-threat-1679253609 /). ?His interest in researching sargassum lies in finding a solution that protects island communities through a net positive approach, one that also safeguards the surrounding ecosystem.?

Ajit has studied sargassum both in the open ocean and with satellite imagery. In June of this year, an opportunity arose to gain a greater understanding of sargassum in the coastal ecosystem and on the beaches, in Antigua, where a startup went to address the issue from a scientific perspective. Their approach: sinking the floating seaweed to great depths before it approached land.?Ajit served as an independent observer.

I had the opportunity to interview Ajit about his recent trip to Antigua.

Sapna: Ajit, can you tell me about the sargassum problem in Antigua?

Ajit:?The prevalence of the problem was evident the second I landed on the island. The customs official whisked my boxes of mysterious-looking research equipment through customs when I told her I was there to better understand the sargassum problem.? Once there, I attached video cameras to the underside of the sargassum rafts (large mats of floating seaweed) to view the sargassum ecosystem directly.??

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Scoping sargassum along the coast of Antigua, June 2023. The white device floating inside the brown sargassum bloom is a video camera, filming the underside of the sargassum ecosystem. Photo courtesy of Ajit Subramaniam.

Sapna: What did the local community have to say about the problem??

Ajit: A great deal. I really valued being able to talk to local people to understand how the sargassum was impacting them. ?I spoke to homeowners and hotel owners, who have been deeply impacted by the accumulation of sargassum on the beaches. The smell of hydrogen sulfide being released from the rotting sargassum was so intense that it made the beaches uninhabitable. A homeowner who lived three blocks away from the beach told me all her silver jewelry had turned black from the hydrogen sulfide.?Electronics with copper chips were also impacted because the copper was getting corroded as well.?Preliminary measurements of methane emissions from the beach where the sargassum was rotting indicated levels higher than those from wetlands. Hotel owners have been hiring backhoes to gather the piles of rotting sargassum and dumping it away from the beach. Because the sargassum piles contain so much sand, once the sargassum has completely rotted away, only sand is left, and further expenses are required to haul the sand back and replenish the beach. A lot of the local businesses in Antigua were quite upset that the government wasn't really helping them out in much of this work.?

Sargassum piling up and rotting along the shore of Antigua, June, 2023.?As it rots, it releases hydrogen sulfide, devastating the quality of life and local economy. ?The equipment at the forefront measures the amount of methane, a potent GHG, released from the rotting seaweed. Preliminary measurements indicated very high levels of emissions. Photo courtesy of Ajit Subramaniam.

Sapna: Is sinking the sargassum before it reaches shore a viable solution?

Ajit: Potentially. However, we need a better understanding of the ecology of the sea surface and of the seafloor, before we begin sinking seaweed at scale. From there, we need to understand how the seaweed impacts the seafloor ecology once it has sunk. Next, we have to make sure that we are actually sequestering the carbon.?We must conduct modeling studies to determine how much carbon we are actually offsetting. While flying into Antigua, I observed dark lines in the coastal waters along the routes the sargassum was traveling. These lines, I discovered, were made up of dissolved organic matter (DOM), which the sargassum was exuding.?DOM includes carbon, indicating the sargassum was not incorporating all the carbon it fixes into biomass.?The amount of carbon incorporated as biomass versus released as DOM must be calculated properly to account for the amount of carbon actually sequestered by sargassum.

In this aerial view?of the waters off the shores of Antigua, the lines of white patches are sargassum. The dark blue lines, Ajit discovered, consisted of dissolved organic matter, which the sargassum was exuding as it travelled towards shore. Photo courtesy of Ajit Subramaniam.?

Sapna: There are a lot of companies interested in sequestering carbon using marine carbon dioxide removal (mCDR) techniques.

Ajit: There is a big rush right now by private capital to find ways to sink carbon to the ocean floor with very little understanding of the ecological impact.??There are companies proposing to actually farm sargassum in the open ocean in the South Atlantic Gyre which I must confess I have a fairly serious problem with, because I don't believe we have evidence that it can be done at scale in a way that does more good than harm.?Sinking if off the islands of the Caribbean is in some fashion justifiable because we are balancing one potential harm (causing harm to the seafloor) with another.?And there are co-benefits – sinking the sargassum sequesters the carbon and keeps it from harming the island ecology and economy.?

Sapna: We have heard a lot about this sargassum traveling to Florida. Is it there yet?

Ajit:?No, it has not hit Florida beaches to any extent and it won’t.?The amount reaching Florida beaches is very similar to what has always washed up there.?The ocean currents don’t flow in a way that the sargassum would hit Florida to any major extent. It stays in the Caribbean.?At this point the US does not see the massive growth of sargassum in the Caribbean as a US problem even though the US Virgin Islands declared a state of emergency last fall and Puerto Rico is having problems with it.?This completely goes to the issue of environmental justice.?Only when someone in Florida who's going to sit on a beach cares, does this problem get the attention it has received over the past few months.?

Sapna: In the best-case scenario, if you could have everything that you wanted to tackle this problem, what would you want?

?Ajit: I’d like the funding to properly study the many aspects of this problem. ?My priority is to understand how the ecosystem of the ocean floor is affected after we sink the sargassum, because I do believe that there's power in this business of sequestering carbon, if you are doing it to prevent it from coming ashore.?I’d also want to address the operational aspects, of finding ways to sink large quantities of the sargassum on the order of one or more ton per day, which would include robotic platforms, AI, nets to pull in the sargassum, and remote sensing modeling to be able to target the rafts of floating sargassum with the highest efficiency. We must also account for the amount of carbon sequestered through the process.?

Sapna: This is a very complex problem because you are looking at ecology, ocean biogeochemistry, carbon modeling, engineering, and economics.?Addressing it properly is so important because what you learn from your research will not only help the island communities being impacted by the sargassum, but those who want to geoengineer using mCDR.?If there’s a cautionary tale, we should be aware of it.?

Ajit: If you don't know the right question, you're destined to find the wrong answer. ?