So Much Seaweed in Key Biscayne 2018: Why? How Long? Is It Hazardous?

Large accumulations of seaweed were observed in the summer of 2018 along the Key Biscayne beach, and more is predicted to arrive. The seaweed is comprised of two species of free-floating (pelagic) brown macroalgae: Sargassum natans and Sargassum fluitans found in the Atlantic Ocean.

Sargassum naturally floats onto Atlantic and Caribbean beaches in varying amounts during the year. Residents and visitors are advised to expect large quantities during the summer/early fall, roughly coinciding with the June to November hurricane season. However, the seaweed piling up in 2018 is unprecedented for Key Biscayne.

Why are these huge drifts of seaweed accumulating along much of the Village beach? How long will the floating invasion last? Is it a hazard to beachgoers and marine life?

Marine scientists have been investigating the cause since 2011-2012, when an unusually large Sargassum mass accumulation event was observed in the Caribbean, Atlantic and Gulf of Mexico. Another major event was observed in 2014-2015 and the 2018 onslaught continues (Jones, Jr., 2018).


According to Jones, Jr. (2018), Sargassum beaching “is the result of ‘movement’ from elsewhere, rather than of ‘growth’ in place”. Satellite imagery plus wind and current data are used by University of South Florida scientists to model transport of Sargassum in the Western mid-Atlantic and Caribbean Sea (Wang and Hu, 2017). Maps based on the models show Sargassum moving into “the Gulf of Mexico, the Florida Straits, and the east coast of Florida” (USFOOL, 2018) from the Sargasso Sea source area in the western North Atlantic. The Loop Current moves some sargassum from the Gulf via the Loop Current back to the Atlantic and Caribbean. Local currents, winds and tides move the seaweed into the Key Biscayne area.

How Long?
The August 2018 map, and on the ground reporting from affected areas, indicate a high probability that Sargassum will continue to accumulate on beaches through at least October 2018 (USFOOL, 2018). A University of Miami scientist interviewed by Jones, Jr. (2018) says the accumulations may continue into 2019. In the long term, researchers believe warmer ocean waters and increased ocean pollution may cause more frequent, larger Sargassum events. The timing may “be like hurricanes, where in some years it’s devastating and other years you don’t see it” (Linthicum, 2018).

Sargassum is not toxic and is an important component of our marine ecosystem. It provides food and shelter for marine life and birds, including endangered species and commercially important fish, and provides stability and nourishment to the beach (Doyle and Franks, 2015). However, the rotten egg smell emitted during its decay on the beach, due to the release of hydrogen sulfide, can harm sea life and may cause headaches and nausea in certain people with prolonged exposure (Conover, 2017; Conover, 2018). Pile-ups of seaweed can affect seagoers’ enjoyment of the beach and access of sea turtles to and from nesting areas.


Conover, J. (2017), Tracking Sargassum Seaweed via Satellite – and How Boaters Can Help, Caribbean Compass, 64, 13.

Conover, J. (2018), Atlantic, Caribbean Sea and the Gulf of Mexico: Major Seaweed Invasion Underway,

Doyle, E. and J. Franks (2015), Sargassum Fact Sheet. Gulf and Caribbean Fisheries Institute.

Jones, R.C., Jr. (2018), Holy Sargassum! Seaweed Invasion, U News@TheU

Linthicum, K. (2018), A stranger imperils paradise, Los Angeles Times (Sept. 8, 2018).

University of South Florida Optical Oceanography Lab (2018), Outlook of 2018

Sargassum Blooms in the Caribbean Sea, Bulletin 8, Univ. South Florida.


Wang, M., and C. Hu (2017), Predicting Sargassum blooms in the Caribbean Sea from MODIS observations, Geophys. Res. Lett., 44, 3265–3273, doi:10.1002/ 2017GL072932.



(Written by H. Groschel-Becker on September 11, 2018)


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