Sargassum seaweed affects nearly every island in the Caribbean, but some are hit far harder than others. Barbados, Guadeloupe, Martinique, Puerto Rico, Saint Lucia, and Saint Martin consistently rank among the most impacted territories. Mexico’s Caribbean coastline, particularly the state of Quintana Roo (home to Cancún and Tulum), also faces massive annual influxes. Since 2011, what scientists call the Great Atlantic Sargassum Belt has turned occasional seaweed landings into a recurring crisis that lasts up to nine months each year.
The Hardest-Hit Islands and Coastlines
The islands that bear the brunt of sargassum sit along the eastern edge of the Caribbean, directly in the path of Atlantic trade winds and currents that push the seaweed westward. Barbados, the most easterly Caribbean island, often serves as the first major landfall. Guadeloupe and Martinique in the French Antilles are among the worst affected: in just the first eight months of 2018, doctors in Martinique reported over 8,000 cases of acute exposure to gases released by rotting seaweed, while Guadeloupe logged more than 3,300 cases in the same period. Three patients in those territories ended up in intensive care.
Saint Lucia and Saint Martin also receive heavy deposits. Puerto Rico, sitting at the northeastern corner of the Caribbean, catches sargassum carried by both Atlantic currents and circulation patterns within the Caribbean Sea itself. Further west, Mexico’s Quintana Roo coast faces some of the largest accumulations anywhere, with volumes reaching tens of thousands of cubic meters per kilometer of beach in severe years.
Atlantic-facing coastlines are generally more vulnerable than sheltered western shores. Islands with long, exposed eastern beaches tend to collect far more seaweed than those tucked behind larger landmasses. That said, once sargassum enters the Caribbean basin, currents can redistribute it widely, so even islands with primarily western exposure are not immune.
Why Some Years Are Worse Than Others
The Great Atlantic Sargassum Belt forms each year in the tropical Atlantic, north of the equator, typically becoming visible on satellite imagery by January or February. Trade winds and ocean currents push it westward, and by June the belt can stretch across the entire central tropical Atlantic. Peak arrivals in the Caribbean occur during the summer months, though local conditions vary. The Gulf of Mexico sees an earlier local peak around April and May.
Nutrient levels in the ocean are the primary driver of how large the belt grows in any given year. Nitrogen and phosphorus from the Amazon, Congo, and Mississippi rivers feed the seaweed as it drifts across the Atlantic. Upwelling of cold, nutrient-rich water off the West African coast adds more fuel, and Saharan dust carried by trade winds contributes a smaller but measurable amount of nutrients. No single source dominates. Instead, the sargassum picks up fertilizer from different inputs as it moves from east to west across thousands of miles of ocean.
Long-term data from 1983 to 2019 shows that nitrogen concentrations in sargassum tissue have risen in step with increasing global nitrogen levels, suggesting that fertilizer runoff and other human-driven nutrient sources are making the problem progressively worse. Water temperature plays a lesser role than initially suspected. Lab studies found that the two main species grow well across a wide temperature range (18 to 30°C), and areas of cooler water with higher nutrients often produce more sargassum than warmer, nutrient-poor zones.
As of early 2026, satellite monitoring by the University of South Florida showed sargassum at record-high levels in most regions, with beaching events likely occurring across the Caribbean.
Health Risks From Decomposing Seaweed
Fresh sargassum floating in the ocean is relatively harmless. The problems start once it piles up on shore and begins to rot. Decomposing sargassum releases hydrogen sulfide gas and ammonia, both of which can cause respiratory irritation, skin rashes, headaches, and neurocognitive symptoms like confusion and difficulty concentrating. The characteristic rotten-egg smell is hydrogen sulfide, and it is more than just unpleasant.
At moderate concentrations (roughly 50 to 400 parts per million), hydrogen sulfide can cause difficulty breathing, agitation, nausea, vomiting, elevated blood pressure, and loss of consciousness. At higher concentrations, exposure can trigger seizures, cardiac arrest, and death. These extreme exposures are rare but not hypothetical: the thousands of medical cases reported in Guadeloupe and Martinique demonstrate that chronic, lower-level exposure is a genuine public health concern for people living or working near affected beaches.
Damage to Tourism and Local Economies
Tourism-dependent Caribbean economies take a measurable financial hit during heavy sargassum years. A study of Mexico’s Quintana Roo coast estimated that sargassum reduced local economic output by roughly 11.6% in affected coastal zones. The damage does not end when the seaweed is cleared: economic effects lingered for up to 12 months after detection, with output still depressed by 5.9% to 9.9% a full year later. Tourists cancel bookings, leave negative reviews, and choose other destinations, creating a ripple effect that outlasts each individual influx.
For smaller island nations where tourism may account for half or more of GDP, even a modest drop in visitor numbers translates to significant hardship. Hotels, restaurants, dive operators, and fishing communities all feel the impact.
Ecological Consequences
Thick sargassum mats floating near shore block sunlight from reaching seagrass beds and coral reefs below. Seagrasses need light to survive and reproduce, and when they die off, the fish, manatees, and invertebrates that depend on them lose both food and habitat. As the seaweed decomposes in shallow water, it consumes dissolved oxygen and releases organic compounds that can stress or kill marine life outright. Low-oxygen conditions, known as hypoxia, are particularly dangerous for corals, which are already under pressure from warming oceans and acidification. Hypoxic zones created by rotting sargassum favor the growth of algae that smother coral, accelerating reef decline.
Sea turtles face a unique set of problems. Piled seaweed blocks nesting females from reaching the sand to lay eggs and prevents hatchlings from making it to the water. In some cases, sargassum covering a nest prevents direct sunlight from warming the eggs properly, which shifts the sex ratio of hatchlings toward more males (turtle sex is determined by incubation temperature). In other cases, decomposing seaweed generates so much heat that it kills developing embryos entirely.
How Islands Are Responding
Caribbean governments and communities use a combination of offshore and onshore strategies to manage sargassum. Floating barriers deployed offshore aim to intercept seaweed before it reaches the beach, redirecting or concentrating it for collection by harvesting vessels. Onshore, heavy machinery and manual labor remove piled seaweed, though this carries its own risks: improper removal can accelerate beach erosion, and workers face hydrogen sulfide exposure.
A growing number of entrepreneurs are finding commercial uses for collected sargassum. After being rinsed and sun-dried, it can be processed into organic fertilizer and biostimulants that deliver nutrients to crops. Its natural thermal properties make it useful in construction: researchers are incorporating dried sargassum into concrete blocks and thermoacoustic panels designed to insulate homes against heat. Other ventures are converting it into livestock feed to supplement grass during dry seasons, extracting natural dyes and fibers, producing activated carbon, or processing it into biofuel. Some producers have even developed flour from sargassum for use in tortillas and other foods.
None of these solutions yet operate at a scale that matches the volume washing ashore each year. The sheer quantity of seaweed, combined with the cost of collection and processing, means that most affected islands are still primarily in reactive mode, clearing beaches as fast as they can rather than converting the problem into a consistent revenue stream.