Climate Change Human impacts

The Largest Belt on Earth?


Wang M., Hu C., Barnes, B., Mitchum, G., Lapointe, B., and Joseph M. 2019. The great Atlantic Sargassum belt. Science, 365, 83-87. DOI:


Sargassum seaweed, washed ashore. Image credit: Unknown Author, licensed under CC BY-SA.

Sargassum is a genus of brown algae (seaweed), first reported by Christopher Columbus in the fourteen-hundreds; the presence and abundance of which, led to the naming of the Sargasso Sea. The Sargasso is the only place in the world where this algae grows to such large extents in the open ocean, it has been nicknamed the Great Atlantic Sargassum Belt (GASB).


As the world’s only holopelagic seaweed (spends whole life in the pelagic, or open sea), Sargassum is a unique and intriguing species. Seaweeds, kelps, and grasses are ecosystem engineers, they form habitats in the ocean which are generally diverse and productive because they provide shelter, food, and protection. In general, seaweeds have 3 major requirements: sunlight, seawater, and a place to anchor. Because of these restrictions, most seaweeds are found in shallow, near-shore areas. Since Sargassum does not have the anchoring requirement, it is one of few ecosystem engineers in the open ocean. More than 145 invertebrate species are known to associate with Sargassum including ten endemic species (unique to that location) and various endangered and threatened species. One group of endangered species, the American and European eels, migrate up to 24 months to reach the Sargassum belt – it’s only known breeding grounds.

Sargassum seaweed, washed ashore in Playa Del Carmen, Mexico, May 2019. (Gower and King, 2019).


Although the seaweed creates a highly beneficial environment for many species, it can also be detrimental, especially when large mounds wash ashore. These beaching events cause environmental (decay), ecological (smothering), and economical (cleanup, lost tourism) issues.


Previously, Sargassum existed in small isolated patches; however, since 2011 the density has exploded. Most summers it can now be seen forming a colossal mass all the way from West Africa to the Gulf of Mexico, earning it the name the Great Atlantic Sargassum Belt (GASB). The GASB forms in the summer months and dies off through the winter.

The largest GASB bloom so far was in 2018 when the belt stretched 8,850 km across the Atlantic ocean and weighed and estimated 20 million metric tons!


Due to the pigmentation of chlorophyll (the sunlight absorbing molecule) Sargassum is highly visible to satellite sensors against its dark backdrop – the deep open ocean. Thanks to this, there is great records of Sargassum density and distribution through time. This allows scientists to study it with relative ease. The authors of this study utilized this data, and others, to try to determine which factors may be connected to the increase in Sargassum density. The explanatory variables that they use also come from data that are often available to anyone to view, including the North Atlantic Oscillation index which is a measure of El Niño / Southern Oscillation in the North Atlantic ocean.


A. Mean Sargassum coverage of the Atlantic ocean through time. B. Satellite imagery of the GASB. Wang et al., 2019.
The variables of interest in this study. Wang et al., 2019.















Three main factors were found to be associated with the massive blooms.

  1. Seed population
  2. Climate
  3. Nutrients


Seed Population

In many of the years since 2011, the population has not completely died-off throughout the winter. This allows the belt to start growing earlier and larger in the bloom months. The years when that have the largest seed population going into summer had the largest blooms, particularly the 2018 bloom.



This was a slightly unexpected result; in that we may expect plants to perform better with more sun and heat, but the GASB was actually correlated with lower temperature years. The GASB correlated well with high winter NAO and low sea surface temperature (SST).



Plants also need lots of nutrients to grow and the open ocean is often quite low on this resource. So, it’s no surprise that the GASB was correlated with years of relatively high nutrient inputs. The high winter NAO results in greater upwelling*, and the lower SST was related to higher Amazon river outflow; both of which greatly increase nutrient levels.


*Upwelling is the process by which deep ocean waters are brought to the surface. Deep waters typically contain much higher nutrient levels because of all the decaying matter and excretory products that sink down.


Whether the GASB will remain in the future; whether we will continue to see it grow like in 2018; and whether it will have a net positive or negative impact are all yet to be answered. But one things for certain, the climate is changing and new blooms and shifts like this will certainly continue to occur.



Ten endemic species that inhabit Sargassum
Histrio histrio, Sargassum angler fish. Image credit: Unknown Author, licensed under CC BY-SA.
  1. Sargassum Angler Fish (Histrio histrio)
  2. Sargassum crab (Planes minutes)
  3. Sargassum shrimp (Latreutes fucorum)
  4. Sargassum pipefish (Syngnathus pelagicus)
  5. Sargassum anemone (Anemonia sargassensis)
  6. Sargassum slug (Scyllea pelagica)
  7. Sargassum snail (Litiopa melanostoma)
  8. Amphipod (Sunampithoe pelagica)
  9. Amphipod (Biancolina brassicacephala)
  10. Platyhelminth (Hoploplana grubei)



Other references:
  1. Gower, J., King, S. 2019. Seaweed, Seaweed, Everywhere. Science, 365 (6448), 27. DOI: 10.1126/science.aay0989
  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., 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. 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.






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