Sargassum levels stayed near historic highs in the Caribbean in June, even as algae blooms began to slow across several of the regions monitored for the seaweed, according to the University of South Florida’s Optical Oceanography Lab. However, coastal inundations are still expected to continue along shorelines across the region.
The OOL at USF uses satellite imagery to monitor the presence of sargassum across the tropical Atlantic, Caribbean Sea, and the Gulf.
“As predicted in the sargassum bulletin in May 2025, total sargassum amount in three of the five regions continued to increase in June,” according to an update from a sargassum outlook report released on July 1 from the OOL at USF. “These regions include the Gulf, the western Caribbean, and the eastern Caribbean. In contrast, total sargassum amount decreased slightly in the central western Atlantic, and dropped substantially in the eastern Atlantic,” the report said.
“Such a disparity indicates that most increases in the first three regions were due to physical transport, while sargassum growth in the tropical Atlantic lost momentum. Nevertheless, most regions still reached their historical highs,” USF explained.
“On the other hand, the spatial distribution patterns in June remained stable from May, and significant sargassum inundation events have continuously been reported around most Caribbean nations and islands, including the Mexican Caribbean coast,” USF stated. A small amount of sargassum continued to reach the Straits of Florida.”
A Possible Decrease in Seaweed Across Some Regions
The USF report explained that while there has continued to be an enormous amount of sargassum across several monitored regions, certain areas monitored by satellites may begin to see a reduction in the seaweed, including the tropical Atlantic region.
“June 2025 appears to be the month of a turning point for the tropical Atlantic, after which the total sargassum amount will decline, although the absolute amount will continue to be higher than most of the previous years,” the USF report said. “The amount in the Caribbean Sea may remain stable as local decrease may be compensated by physical transport, but more sargassum is expected to be transported to the Gulf through the Yucatan,” the report noted.
However, unfortunately, areas across the USVI and Puerto Rico will still need to endure the seaweed for at least a few months longer before seeing a noticeable difference across local beaches. Several areas across the USVI, including Coral Bay on St. John and the Northside of St. Croix, have experienced massive amounts of sargassum this year.
“Sargassum inundation of variable degree will continue to occur in most of the Caribbean nations and islands as well as along the southeast coast of Florida,” USF explained. “However, 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.”
Sargassum Impact to Continue for Months Across USVI
The Source connected with Brian Barnes, Ph.D., an assistant research professor at USF, to obtain more details about the large amount of sargassum and when it may begin to abate across the U.S. Virgin Islands.
“Sargassum biomass overall in the entire Tropical Atlantic will decrease as summer progresses, but the impact of this on Puerto Rico, the U.S. Virgin Islands region and the Caribbean overall is still a few months away,” Barnes predicted.
Regarding potential steps individuals can take to help manage algae, Barnes offered the following advice.
“Booms may be installed to protect certain coastlines, particularly those that are ecologically sensitive, but these are not perfect,” Barnes acknowledged. “However, rapid removal can minimize environmental consequences and can provide a result that is more useful to downstream valorized uses. As such, having removal equipment or personnel staged for rapid removal is important.”
While the noxious algal mats are not particularly dangerous to humans, reducing or eliminating as much of the sargassum is crucial to help protect marine life.
The Environmental Protection Agency shared the following details regarding the possible effects on marine life due to the seaweed:
“Sargassum inundation events can harm seagrasses and corals by blocking the light the grasses need for health, reproduction, and survival,” the EPA said. “This in turn impacts the availability of food and habitats for other aquatic species and reduces the amount of oxygen in the water. The seaweed mass can stress or kill plants and animals in the water and makes it easier for invasive species to establish or grow,” the EPA stated.
“Sargassum may also affect aquatic species and their ecosystems by accumulating marine debris, including microplastics, and leaching heavy metals, pesticides, and other pollutants, some of which may be generated from its decomposition,” the EPA added.
Reasons for Increased Sargassum are Being Studied
As was reported in a previous Source article, Barnes stated that more research is needed to understand the cause of the increase.
“Unfortunately, we do not have a good explanation for the significant increase of sargassum; this is an active area of investigation,” Barnes acknowledged. “In a broad sense, whenever a seed population meets appropriate conditions for growth, such as the temperature, nutrients, light regime, etc., there can be rapid biomass expansion,” Barnes explained. “The exact confluence of environmental factors is unknown,” he said in the previous article.
Barnes had also explained that another area of ongoing research is the potential connection between cyclone development and the abundance of the algal mats, including how factors such as ocean conditions and atmospheric changes might influence the amount of sargassum in the ocean.
“This is an active topic of research,” Barnes said. “Following previous hurricanes, we’ve seen both increases and decreases in sargassum abundance,” he added.
“Inundations are really dependent on very local winds and currents, so I cannot give a solid answer here that applies to the Puerto Rico and Virgin Islands regions.”
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 does not expect any cyclonic development across the Atlantic basin over the next seven days. However, the National Weather Service in San Juan, Puerto Rico, has warned that very warm temperatures will affect the region and a tropical wave is anticipated to pass through the Caribbean on Thursday of this week, possibly raising the chance of showers. Saharan dust will also be prevalent in the atmosphere, causing a hazy sky and reduced air quality.
In addition to obtaining weather information 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. Individuals can also find helpful weather information and alerts from the Virgin Islands Territorial Emergency Management Agency.
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