Do you smell that?
It’s the unmistakable stench of rotting eggs, drifting as far as half a mile inland from parts of the Caribbean coastline.
That smell is hydrogen sulfide, and it’s the unpleasant calling card of massive sargassum blooms now washing up on beaches across Mexico, Honduras, Belize, and other parts of the Mesoamerican Reef region. What was once a seasonal, offshore phenomenon has become a growing environmental, economic, and public health challenge for coastal communities.
So what exactly is sargassum, and why is there suddenly so much of it?
What is Sargassum? And Why is There So Much of it?
Sargassum is a type of brown seaweed that floats freely across the open ocean, often forming large, island-like mats. While drifting offshore, sargassum plays an important ecological role: it provides food, shelter, and nursery habitat for fish, turtles, and other marine life.
The problem begins when those floating mats reach land.
Once sargassum washes ashore, it quickly decomposes. As it breaks down, it releases foul-smelling gases and creates low-oxygen conditions in nearshore waters—conditions that are harmful to marine ecosystems and coastal livelihoods alike.
Although sargassum blooms are natural, climate change and human activity have dramatically intensified their scale and frequency. Today’s blooms are fueled by a perfect storm of environmental pressures:
- Increased nutrient runoff from agriculture and deforestation in the Amazon River basin
- Dust blown westward from mining activity and desertification in the Sahara
- Rising ocean temperatures linked to climate change
Together, these factors create nutrient-rich, warm waters where sargassum can thrive unchecked.
“The seaweed grows quickly,” says Javier Pizaña-Alonso, CORAL’s Program Manager in Cozumel. “Under these conditions, blooms can double in size in about 18 days. It becomes a serious issue depending on how long they drift in the ocean before reaching shore.”
How Sargassum Impacts Coral Reefs and Coastal Communities
The problem with excess sargassum isn’t just the smell.
This seaweed is often laden with nitrogen, sulfur, ammonium, and heavy metals, absorbed from polluted river systems upstream. When it accumulates along coastlines, it can severely degrade water quality.
“When the sargassum washes up on shore, it degrades and creates a brown tide,” says Pizaña-Alonso. “This impacts water quality.”
Poor water quality is especially dangerous for coral reef ecosystems, which depend on clean, clear water to survive. Nutrient overloads can trigger excessive algae growth, smothering reef-building corals and disrupting habitats that support more than one million marine species. In shallow coastal areas, thick mats of sargassum can also physically smother corals and alter the behavior and movement of marine animals.
Photo credit: Teresa Wood
The impacts extend beyond the reef. Coastal economies throughout Mesoamerica rely heavily on tourism, and sargassum threatens both livelihoods and public health.
“Travelers don’t like the smell, and it affects our beautiful beaches,” says Pizaña-Alonso.
What’s Being Done—and What Still Needs to Happen
Efforts to address the sargassum crisis vary by location and capacity.
In parts of Quintana Roo, the Mexican navy has begun intercepting sargassum offshore before it reaches beaches. In other regions, including Roatan, Honduras, governments have formed task forces and are advocating for emergency declarations to unlock funding and resources.
Researchers are also exploring whether sargassum can be safely reused—though this remains controversial. Proposed uses range from animal feed and mulch to experimental building materials, notebooks, and shoes. Any reuse, however, must account for the heavy metals and pollutants the seaweed absorbs.
Looking ahead, Pizaña-Alonso hopes to contribute his marine science expertise directly to government-led solutions. But he’s clear that cleanup alone won’t solve the problem.
Addressing sargassum at its source means confronting the root causes: climate change, deforestation, mining, and nutrient pollution. Reducing carbon emissions, protecting forests, and improving land-use practices upstream are essential steps toward restoring balance in ocean systems.
Support the Reefs and the Communities That Depend on Them
Sargassum is a visible reminder that what happens on land doesn’t stay on land. Our actions ripple downstream all the way to the reef.
CORAL works alongside coastal communities in places like Cozumel to protect water quality, strengthen reef resilience, and address the root causes threatening coral ecosystems.
Want to be part of the solution?
Support CORAL’s programs, learn how clean water supports healthy reefs, and help protect the Mesoamerican Reef for the people and wildlife who depend on it.
