The Floating Forests of the Atlantic

Written by Rosemary Wool

Back in June 2025, I took a plane from London to Cancun. I was visiting Mexico for the Association for Tropical Biology and Conservation Conference that was kicking off at the end of the month in Oaxaca. I had decided to spend some time in the country before the conference, so I was on my way to Isla Cozumel to do some world-renowned scuba diving. In a very jetlagged, excited and nervous state, I found a bus at Cancun airport that would take me to Playa del Carmen, the vibrant coastal town that is the gateway to Cozumel. I had never been to Central America and I was preparing myself for truly an experience of a lifetime. But when I hopped off the bus I was hit with a smell so revolting I could have sworn the entire town was crawling with decaying chicken that had been sitting out in the heat all day. It seemed to be everywhere I went. Nauseating as it was, my first priority was finding a bed for the night. Tomorrow, I would set off on the next leg of my journey to Cozumel, and come across the source of the putrid smell.

A lonely piece of Sargassum floating near the surface in Florida © Jstuby (Wikimedia Commons / CC0).

Sargassum is a genus of brown macroalgae that is found across subtropical and tropical regions, with many inhabiting the Sargasso Sea, the Caribbean Sea and the Gulf of Mexico. It is known for the way that it freely floats, creating island-sized clumps of seaweed. Spherical, gaseous bladders, called pneumatocysts, give the fronds buoyancy to keep close to the surface and maximalise on photosynthesis. 

These drifting Sargassum patches function as a refuge for many species, including young loggerhead sea turtles escaping from predators [1]. Sargassum also features as a permanent residence for endemic species [2], including the sargassum crab (Planes minutus), the pipefish (Syngnathus pelagicus) and the angler fish (Histrio histrio). Many of these endemic species are camouflaged in some way, and the angler fish has modified fins that it uses to creep around the weed.

Frequent inhabitants of Sargassum. Clockwise from top left: Juvenile loggerhead turtle (Caretta caretta) © FWC Fish and Wildlife Research Institute (Flickr); Greater pipefish (Syngnathus acus) © Wikimedia Commons; Columbus crab (Planes minutus) © Wikimedia Commons; Sargassum fish (Histrio histrio) © NOAA Photo Library (Flickr).

Sargassum may sound rather idyllic, however there is something much more sinister and catastrophic going on. When I took my morning walk around the harbour of Playa del Carmen that day in June, I had my first introduction to what is the largest macroalgae bloom in the world entitled The Great Atlantic Sargassum Belt (GASB). Since 2011, unprecedented quantities of Sargassum started to appear on the coastlines of Brazil, the Caribbean, the Gulf of Mexico, some parts of Florida and even the West coast of Africa. The belt, which resides in the Atlantic Ocean, weighs around 5.5 million tonnes, extends over 8000 km, and is observable from space [3]. The smell originates from when the Sargassum comes to shore and dies within 48 hours, releasing the toxic gas hydrogen sulphide. Not only does this smell horribly, but this gas can also irritate eyes, nose and throats and affect people with respiratory problems [4]. The decomposition also consumes oxygen, creating large oxygen-depleted zones that result in fish kills [5]. It also acts as a physical barrier in the ocean, preventing corals and seagrasses from attaining adequate light, and entangling marine mammals. 

As well as being dangerous to humans and fauna, GASB also creates a substantial socio-economic crisis. Requiring clean-ups invokes a lot of financial strain on governments and local businesses. In the Caribbean, the total cost of cleaning beaches and disposing of the debris to landfills cost an astronomical US$120 million in 2018. Indeed, this cost does not include the economic losses for hotels, which also suffer hugely from a loss of tourism [6]. 

A beach in Saint Martin covered with Sargassum. © Mark Yokoyama (Flickr / Creative Commons).

But what is causing these Atlantic forest blooms?

Well, it appears that it is from one forest to another. Scientists suspect that nitrates and phosphates from logging, illegal mining operations, sewage disposal and agriculture are leeching into rivers in the Amazon. As the Amazon river has the biggest flow in the world, accounting for around half of Atlantic runoff [7], the addition of these sediments into the ocean are substantial. Through a process known as vertical mixing, the influx of nutrients are being stirred up by ocean currents that have been changing due to climate change [10], and what we see as a result is the formation of the perfect conditions for Sargassum growth. As time has gone on, we have seen a migration of the Great Atlantic Sargassum Belt from the Sargasso Sea to more southerly regions in the tropical Atlantic, which provides more favourable conditions for large populations of Sargassum [11].

The great Atlantic Sargassum belt in June 2021, which spanned an area of about 5 × 10⁶ km². The Sargassum itself was estimated to be about 6000 km² when aggregated together, with a total wet biomass of 18 million metric tons (t). The areal density of 0.1% corresponds to biomass density of 3.3 t/km². (Data courtesy of MODIS and [11]).

Is there any hope?

While the GASB has been accelerating each year, new innovations have been borne from the necessity of shifting the masses of Sargassum that are littering coastlines, and it turns out that Sargassum can actually be pretty resourceful. Firstly, it is being used as agricultural fertiliser in places such as the Caribbean. Seaweed-based extracts are becoming increasingly popular in agriculture because they have been shown to improve yields 10-15% [12]. Barbados has also been exploring the use of the algae as biogas to fuel local vehicles [13]. It is also being used in the skincare industry as a natural emulsifier, and lastly in the fashion industry as a new material deemed as the ‘cashmere of the ocean’ by Vogue [14]. 

Besides finding use for all of this Sargassum, monitoring technology has been improving. The US and European Copernicus Earth Observation programmes have made significant progress in global remote detection of floating Sargassum. Over six different instruments are used to provide ocean colour products that allow the quantification and tracking of Sargassum mats over large regions, including MODIS (Moderate Resolution Imaging Spectroradiometer) [15]. There is still a lot of work to be done however, including attaining satellite information in cloud-prone regions, which impacts the validation of forecasting models. Reducing uncertainties surrounding nutrient dynamics in the Tropical Atlantic are also paramount to improving forecast accuracy, developing efficient response strategies and exploring sustainable uses for Sargassum [15]. 

References

[1] BBC News, 2014. Satellites track turtle 'lost years'. BBC News, 5 March. Available at: https://www.bbc.com/news/science-environment-26414147 [Accessed 27 January 2026].

[2] Laffoley, D.d’A., Roe, H.S.J., Angel, M.V., Ardron, J., Bates, N.R., Boyd, I.L., Brooke, S., Buck, K.N., Carlson, C.A., Causey, B., Conte, M.H., Christiansen, S., Cleary, J., Donnelly, J., Earle, S.A., Edwards, R., Gjerde, K.M., Giovannoni, S.J., Gulick, S., Gollock, M., Hallett, J., Halpin, P., Hanel, R., Hemphill, A., Johnson, R.J., Knap, A.H., Lomas, M.W., McKenna, S.A., Miller, M.J., Miller, P.I., Ming, F.W., Moffitt, R., Nelson, N.B., Parson, L., Peters, A.J., Pitt, J., Rouja, P., Roberts, J., Seigel, D.A., Siuda, A.N.S., Steinberg, D.K., Stevenson, A., Sumaila, V.R., Swartz, W., Thorrold, S., Trott, T.M., & Vats, V., 2011. The protection and management of the Sargasso Sea: The golden floating rainforest of the Atlantic Ocean. Summary Science and Supporting Evidence Case. Sargasso Sea Alliance, 44 pp.

[3] Wang, M., Hu, C., Barnes, B.B., Mitchum, G., Lapointe, B., & Montoya, J.P., 2019. The great Atlantic Sargassum belt. Science, 365(6448), pp.83–87. doi:10.1126/science.aaw7912. Available at: https://www.science.org/doi/10.1126/science.aaw7912 [Accessed 18 March 2023].

[4] Marchante, M., 2023. Miami Beach and the Keys could get loads of seaweed. It's smelly and can cause these symptoms. Miami Herald, 17 March. Available at: https://www.miamiherald.com/news/local/environment/article273532920.html [Accessed 27 January 2026].

[5] Burton, R., 2018. Sargassum: Seaweed or Brown Algae. Florida Museum. Available at: https://www.floridamuseum.ufl.edu/science/sargassum/ [Accessed 29 September 2020].

[6] Nibbs, F., 2024. Sargassum is choking the Caribbean’s white sand beaches, fueling an economic and public health crisis. The Conversation, 4 June. Available at: https://theconversation.com/sargassum-is-choking-the-caribbeans-white-sand-beaches-fueling-an-economic-and-public-health-crisis-230954 [Accessed 27 January 2026].

[7] Dai, A. & Trenberth, K., 2002. Estimates of freshwater discharge from continents: Latitudinal and seasonal variations. Journal of Hydrometeorology, 3, pp.660–687.

[8] Olson, D., 2022. Too much of a good thing: the Atlantic Sargassum Belt. Smithsonian Ocean. Available at: https://ocean.si.edu/ocean-life/plants-algae/too-much-good-thing-atlantic-sargassum-belt [Accessed 14 January 2026].

[9] Zhang, Y., Barnes, B.B., Goodwin, D.S., Siuda, A.N., Schell, J.M., McGillicuddy Jr, D.J., Lapointe, B.E., Qi, L., & Hu, C., 2025. Dramatic decline of Sargassum in the north Sargasso Sea since 2015. Nature Geoscience, pp.1–7.

[10] Hu, C., 2023. Ocean optics illuminates aquatic algae. Physics Today. Available at: https://physicstoday.aip.org/features/ocean-optics-illuminates-aquatic-algae [Accessed 13 January 2026].

[11] Li, J., Van Gerrewey, T., & Geelen, D., 2022. A meta-analysis of biostimulant yield effectiveness in field trials. Frontiers in Plant Science, 13, art. no. 836702. doi:10.3389/fpls.2022.836702.

[12] Rhatwal, B., 2024. Barbados biomass project powers cars with sargassum. Bioenergy-News.com. Available at: https://www.bioenergy-news.com/news/barbados-biomass-project-powers-cars-with-sargassum/ [Accessed 27 January 2026].

[13] Vogue, 2025. Could this invasive seaweed become the ‘cashmere of the ocean’? Available at: https://www.vogue.com/article/could-this-invasive-seaweed-become-the-cashmere-of-the-ocean [Accessed 27 January 2026].

[14] Sargassum Information Hub, 2026. Research – Monitoring and forecasting of floating Sargassum. Available at: https://sargassumhub.org/monitoring/research/ [Accessed 27 January 2026].