Sargassum levels declined across the region in September, but totals remain above historical norms, leaving the U.S. Virgin Islands and neighboring islands at continued risk of beach inundations, according to a recent report from the University of South Florida’s Optical Oceanography Lab.
The OOL at USF uses satellite imagery to monitor the presence of sargassum across the tropical Atlantic Ocean, Caribbean Sea, and the Gulf.
A sargassum outlook report released on Sept. 30 from the OOL at USF noted that the total amount of seaweed has diminished considerably across nearly all of the areas monitored.
“As in previous years, the momentum of sargassum decline continued in September, while total sargassum amounts in each of the five regions decreased,” according to information included in the USF report.
“The decline was substantial in all but the East Atlantic region, with total sargassum amount more than halved. Most sargassum is now in the eastern Caribbean region, followed by the East Atlantic,” USF said.
“In the Gulf, sargassum amounts decreased from 1.9 million metric tons in August to 0.6 million metric tons in September, yet due to changes in ocean circulation, the amount of sargassum reaching the Straits of Florida increased, likely causing more inundations along the southeast coast of Florida,” the report indicated.
“Despite the overall declines, total sargassum amounts in every area except the East Atlantic region still exceeded the historical record for the month of September, and inundation pressure around many Caribbean regions is still high,” the report noted.
Inundations Still Possible Across Areas Including USVI, Puerto Rico
The trend of decreasing seaweed totals is welcomed news, and a reduction in the amount of sargassum across monitored areas was also reported in a USF report released in August. However, USF cautioned that more inundations should be expected, and the exact locations of algae arrivals are challenging to predict. Still, the report specifically noted the U.S. Virgin Islands and Puerto Rico as areas where sargassum may end up.
“Total sargassum amounts in all regions are likely to continue to decline, possibly reaching the annual minimum between October and November,” USF stated. “However, because the absolute amount is still much higher than any previous September, sargassum inundation of variable degrees will continue to occur, especially along the Mexican Caribbean coast and around Haiti, Dominican Republic, Puerto Rico, U.S. Virgin Islands, among others.”
“Importantly, whether a beach or small region receives a large amount of sargassum depends on local factors that are difficult to predict, including winds and ocean currents,” USF acknowledged.
The Source contacted Chuanmin Hu, a professor of optical oceanography at USF after receiving the September update. When asked about the potential for more inundations across the USVI, Hu emphasized that while additional seaweed arrivals are possible, they do not automatically mean an extreme inundation will occur.
“Although the amount of sargassum in September is much lower than August, it is still higher than previous September months,” Hu said. “Inundation events may continue to occur around the areas mentioned in the report, but it does not necessarily guarantee that more events or more severe events will occur,” he noted.
As reported in a recent Source interview with Amy Dempsey, a marine biologist and founder of the consulting firm Bioimpact, Inc., officials in the U.S. Virgin Islands have begun using floating barriers to divert incoming mats of sargassum before they reach the shoreline. The booms have shown promise in keeping beaches clearer, but they require constant upkeep due to strong currents and storm damage, and must also be removed ahead of any approaching cyclones.
Sargassum Explained
A previous Source article included additional information about sargassum in an interview with Yuyuan Xie, Ph.D., a research scientist at USF. Xie is involved with the university’s OOL.
“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.
“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,” according to Xie. “The current predictions for a busy year of sargassum are based on statistical models rather than the exact causes of why the amount of seaweed is expected to increase.”
Follow Sargassum Updates and the Weather Forecast
Individuals can follow the progression of the current mat of seaweed and stay up to date each month on where sargassum may be headed.
Finally, in addition to tracking the occurrence of sargassum, residents and visitors across the U.S. Virgin Islands are encouraged to continue monitoring the local weather forecast.
The National Hurricane Center is monitoring a tropical wave moving across the Atlantic Ocean with a medium risk of development over the next seven days. The wave, currently designated as “Disturbance #2,” could potentially develop into at least a tropical depression as it travels west to west-northwest across the Main Development Region between the west coast of Africa and the Caribbean. Residents are urged to follow the forecast closely.
The NHC has also flagged “Disturbance #1,” an area of low pressure in the Gulf; however, the system is not expected to develop.
Meanwhile, the National Weather Service in San Juan, Puerto Rico, has warned that very warm temperatures will persist across the area, and heat alerts may be issued. Showers and thunderstorms are also possible, and marine conditions will remain hazardous for the next few days due to a powerful swell affecting the local islands. Multiple marine alerts are currently in effect for areas of Puerto Rico and the USVI.
Weather information is available from the NWS, the NHC, and NOAA. The local weather forecast for the U.S. Virgin Islands is regularly updated on the Source Weather Page and VI Source YouTube Channel, and individuals can also find helpful weather information and alerts from the Virgin Islands Territorial Emergency Management Agency.
