A smelly, sometimes toxic “killer belt of seaweed” might put a damper on Floridians’ Memorial Day weekend plans. Sargassum is back just in time for the unofficial start of summer and this year’s influx of the brown algae would be record breaking at 31 million tons.Â
What is Sargassum?
Sargassum is a genus of large brown seaweed. As a seaweed, it is also a type of algae. It floats along the ocean in island-like masses and does not attach to the seafloor the way that kelp does.Â
According to NOAA, this brown algae is abundant in the world’s oceans. It has many leafy appendages, branches, and its signature berry-like structures. These round “berries” are actually gas-filled structures called pneumatocysts. They are primarily filled with oxygen and add buoyancy to the plant structure and allow it to float on the surface of the water, similar to a life jacket.Â
Importantly, Sargassum provides food and a floating habitat for several marine species including various fishes, sea turtles, marine birds, crabs, and shrimp. Some animals, like the sargassum fish will spend their whole lives around Sargassum’s gas-filled floats and the seaweed is a nursery area for some commercially important fishes, including mahi mahi, jacks, and amberjacks.
Is it harmful to humans?
When Sargassum washes up on shore, it begins to rot. That rotting triggers the production of hydrogen sulfide gas, which smells like rotten eggs.
These odors themselves are not harmful to humans when inhaled in well ventilated areas like the beach. But the gases can accumulate enough to cause harm if they are breathed in within enclosed spaces.Â
“Hydrogen sulfide can irritate the eyes, nose, and throat,” writes Florida’s Department of Health in St. John’s County. “If you have asthma or other breathing illnesses, you will be more sensitive to hydrogen sulfide. You may have trouble breathing after you inhale it.”
Coming into contact with the jellyfish or other stinging organisms embedded in the rotting seaweed can cause rashes on the skin. Any workers for volunteers collecting and transporting the seaweed should wear gloves, boots, and gas-filter half masks for protection.
 [ Related: A stinky, 5,000-mile-long wad of seaweed is about to gunk up Florida’s beaches. ]
2025’s mega bloom
In Florida and the Caribbean, Sargassum season runs from April to August, with June and July as the peak months for setting in along the shoreline. However, the blobs have been spotted along shorelines since March this year. The bloom has already broken its own size record set in June 2022 by 40 percent–and is still growing. The annual bloom now stretches over 5,500 miles of ocean between Africa and the Caribbean and weighs an estimated 31 million tons.Â
“Sargassum goes from being a very beneficial resource of the North Atlantic to becoming what we refer to as … a harmful algal bloom, when it comes ashore in excessive biomass,” Brian LaPointe, a research professor at Florida Atlantic University’s Harbor Branch Oceanographic Institute, told CNN. “What we have seen since 2011 are excessive inundation events all around the Caribbean region, the Gulf, as well as the South Florida region.”
Why is this year’s bloom so big?
Increasing ocean temperatures due to climate change is one of the reasons for such a large bloom. The Atlantic and waters around Florida have seen record-breaking high temperatures in recent years, creating ideal conditions for the seaweed to thrive. The excess nitrogen in the water from the burning of fossil fuels or dust from the Sahara is believed to be one of the forces behind this supercharged bloom.
Scientists can use satellites to track the seaweed and issue warnings if needed. The CariCOOS Sargassum map shows that the bulk of the bloom is currently east of Puerto Rico, but it has already been spotted along Florida’s Atlantic coast.
NOAA encourages anyone who encounters Sargassum on the beach to report it with this form.
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