Scientists say that this year’s bloom could be one of the biggest ever recorded.
A massive 8,000km long blob of seaweed is floating towards Florida, Mexico, and the Caribbean.
The Great Atlantic Sargassum Belt - a raft of biomass stretching from West Africa to the Gulf of Mexico - contains scattered patches of seaweed on the open ocean.
It's not a new occurrence, but scientists say that this year’s bloom could be one of the biggest ever recorded.
The thick, brown seaweed is already carpeting some beaches in Florida, releasing a pungent smell as it decays and entangling humans and animals who step into it.
So what is Sargassum - and is climate change to blame for this massive bloom?
What is Sargassum?
Sargassum is a leafy brown seaweed festooned with berry-like air pockets. The seaweed floats on the open ocean and - unlike other marine plants- reproduces on the water's surface. The air-filled structures help to give it buoyancy.
Sargassum originates in a vast stretch of the Atlantic Ocean called the Sargasso Sea, well off the southeast coast of the US.
The matted brown mass of seaweed stretches for kilometres across the ocean and provides breeding grounds, food and habitat for fish, sea turtles and marine birds, according to the National Oceanic and Atmospheric Administration (NOAA).
"It's a dynamic, constantly changing set of pieces of this large mass," said Rick Lumpkin, director of the Physical Oceanography Division at NOAA.
It is vital to the ecosystem but can pose a serious problem for tourism when it washes ashore. The seaweed piles up on beaches where it quickly decomposes under the hot sun, releasing gases that smell like rotten eggs.
Too much of it can also harm coastal marine ecosystems by coating beaches with nearly a metre of seaweed.
Why is this year’s Sargassum bloom unusual?
Some sargassum has already reached beaches in Key West - much earlier than usual. Scientists estimate there are more than 10 million tonnes of sargassum in the belt this year.
Lumpkin called it "one of the strongest years, but not the strongest" since scientists began closely observing the biomass via satellite imagery in 2011. He said there was more in 2018.
What causes the seaweed to bloom?
Scientists aren't exactly sure what has caused the bloom - in part because the seaweed wasn't closely monitored until 2011. But climate change and human waste could be contributing to the excessive growth.
"We do know that to get a lot of seaweed, you need nutrients, and you need sunlight. Of course, as you get close to the equator, there's going to be more sunlight," said Mike Parsons, a professor of marine science at Florida Gulf Coast University.
Experts say agricultural runoff seeping into the Amazon and Orinoco rivers then flowing out to the ocean could explain the increased growth of the belt on the western side.
According to a 2021 study, sargassum collected in recent years contained nitrogen levels 35 per cent higher on average than 30 years earlier. This was due to sewage and farm runoff.
"It's almost like sargassum is a barometer for how global nitrogen levels are changing,” Brian Lapointe, an algae specialist, told NPR last week.
Warming waters likely help the seaweed grow faster too. Changes in wind patterns, sea currents, rainfall and drought could also affect blooms.
"It may be the entire belt is fed more some years than others by dust that contains iron and other nutrients that comes from the Sahara Desert," said Lumpkin, from the NOAA.