An increase in sargassum seaweed has been observed across the Atlantic Ocean and the Caribbean Sea. The seaweed could soon arrive at coastlines across the region, including the U.S. Virgin Islands.
“As in previous years, we expect continuous increases of sargassum in the central west Atlantic, the Caribbean Sea, and the Gulf of Mexico over the next month [in June],” according to a sargassum outlook from the University of South Florida (USF) Optical Oceanography Lab released on May 31.
“Many Caribbean nations and islands will see increased sargassum inundations in June, including the regions along the Mexican Caribbean coast,” the update continued.
The USF report explained that all regions where sargassum is monitored indicated an increase in the seaweed during May, except for the Eastern Atlantic Ocean. The report also stated that while there has been a notable rise in the amount of sargassum in the ocean and that it will likely arrive at locations including Caribbean islands, it is currently not a significant reason for concern.
“In May 2024, while continuous increases in sargassum abundance were observed in most regions, they are not at an alarming level when comparing with major sargassum years in the past decade,” the report said.
Plentiful Sargassum Areas
The USF update provided the following details regarding where sargassum (measured in million metric tons) has become more prevalent and how much seaweed has been observed.
“As predicted last month, [the amount of] sargassum increased in May in every region except the Eastern Atlantic, with a total biomass of 10.0 million metric tons. The Western Atlantic region showed the largest increase of 1.9 million tons in the last month, making it in the 75th percentile of historical levels for the month of May. The Western Caribbean Sea experienced the largest relative increase (400%) to 0.4 million tons, but it is still below the 75th percentile of historical levels.”
The update continued, “The total sargassum amount in the Eastern Caribbean Sea doubled to 1.0 million tons, slightly above the 50th percentile of historical levels. Sargassum abundance in the Gulf of Mexico remained low, but small amounts were found near the Mississippi River delta and in the Straits of Florida.”
Chuanmin Hu, a professor of Oceanography and Director of the Optical Oceanography Lab at the University of South Florida, provided the Source with more information about his work studying sargassum and details about when and where the seaweed may start to appear.
“Total biomass in a month [for a particular area] means that at any moment in that month if you collect all sargassum and weigh them, you get a number, [and] that is total biomass,” Hu explained. “[Total biomass is] determined by satellites calibrated by field measurements,” he added.
“We have been using field and laboratory measurements as well as satellite remote sensing to map, monitor, track, and understand sargassum in the past ten years,” Hu said. “We have established the Sargassum Watch System (SaWS) to help people monitor and track sargassum in many different regions across the Atlantic. We are doing this work to [better] understand this seaweed, to help the people [affected by its arrival] in various aspects, [and] to educate our next generation of scientists.”
Hu explained that seaweed could arrive across the USVI in June and July.
“The USVI was [largely] spared [from sargassum impacts] in May 2024 as compared with other [islands, such as] Puerto Rico,” Hu explained. “This situation will likely change in June and July, [because] more sargassum will come from the east and southeast to reach the USVI. [However,] the exact amount [of the sargassum that arrives] is unknown,” he added.
Still, the USF report described there could be an “inundation” of sargassum. Hu clarified what an inundation of seaweed means.
“There is no strict definition of [sargassum] inundation,” Hu acknowledged. [However], anything resulting in a continuous band [without] gaps of sargassum accumulations on the beach or in nearshore waters parallel to the shoreline may be called an inundation,” he said. “In [terms of] numbers, it may be equivalent to several tens of kilograms of sargassum biomass per meter of shoreline accumulated in a few days.”
Sargassum — Explained
A previous Source article included additional information about sargassum through an interview with Yuyuan Xie, Ph.D., a research scientist at the University of South Florida. Xie is also involved with the university’s Optical Oceanography Laboratory.
“Pelagic sargassum seaweed is a brown macroalgae floating on the ocean surface,” Xie said. “It was first reported in the 15th century by Christopher Columbus, and a regional sea in the North Atlantic Ocean, the Sargasso Sea, was named after this plant. [Sargassum] serves as a habitat for many marine animals, such as turtles, fish, shrimp, crabs, and so on. These macroalgae can grow to a length of several meters and form floating mats on the ocean surface,” Xie continued.
Health Impacts
Sargassum has positive and negative benefits for the environment. Fortunately, the arrival of the algae is not extremely dangerous to people. However, there are some significant health risks to be aware of.
“Most of the time, moderate amounts on beaches would not represent a risk factor for humans. However, there are exceptions,” cautioned Xie. “After a couple of days onshore, sargassum starts to decompose and release noxious and stinking gases such as ammonia and hydrogen sulfide. The bad smell can cause respiratory problems. There are reports that in some of the Caribbean Islands, the hospitalization rate has gone up during the sargassum season,” he said.
“Sargassum can be both good and bad for the environment. In the ocean, it is a critical habitat for many animals, so they should like to see increased sargassum. Sargassum on beaches can also stabilize sand dunes, thus helping to avoid beach erosion. But too much of a good thing can also make it bad — excessive amounts of sargassum can also cause environmental and economic problems,” Xie added.
“There is no scientific consensus on exactly what caused the sargassum increases in the past decade in the Atlantic Ocean, but climate change may be part of the reason, as it affects precipitation, ocean circulation, and dust events, among others. This is still a research topic.”
Sargassum Updates
Visitors and residents in the USVI can follow the progression of the current mat of seaweed traversing the Atlantic and stay up to date each month on where sargassum may be headed.
“For the general audience, we’re generating a Sargassum Outlook Bulletin on a monthly basis, which can be downloaded via accessing our SaWS page, stated Xie. “This bulletin provides a general picture of the current bloom conditions and future bloom probability for the regions under watch. The SaWS system also provides satellite imagery every day for the current sargassum situation, where a user can download the images and surface currents,” Xie concluded.
