Off the eastern coast of North America, free-floating brown seaweed called sargassum makes up what’s known as the Sargasso Sea — the only sea without a land boundary, bounded instead by four ocean currents.
“The Sargasso Sea was named, as far as we know, by [Christopher] Columbus, as he was traveling over to the new world,” said Brian Barnes, a research assistant professor at the University of South Florida College of Marine Science.
You may know sargassum as the stinky algae that periodically washes ashore, but in the Atlantic, it’s been an important breeding habitat for many marine species, including critically endangered sea turtles.
“There are tons of little critters, juvenile fish, invertebrates, crabs, shrimps and so forth, that use it for … a place to hide, and some for food,” he said. “Some people call it like a floating rainforest out in the middle of the ocean.”
But something changed a decade ago.
"Prior to 2015, we saw an average of something like 175,000 tons of sargassum in the Sargasso Sea, broadly. Starting in 2015, that number dropped to about 14,000 tons,” Barnes said.
Only about 10% of the sargassum is left from what was recorded prior to 2015 in the north Sargasso Sea.
Although they don’t know why exactly this sudden shift occurred, scientists believe it could have something to do with nutrients and temperature.
“We didn't know that something was happening at the time, so we don't have a good sense of the nutrient regime at the time in 2014, 2015 or what exactly led to this change,” he said.
And the sea’s annual sargassum supply from the northwestern Gulf has also been dwindling.
"So as the Gulf, especially, has grown warmer and warmer, we think that may be one of the reasons why that it no longer can serve as the source for the Sargasso Sea," he said.
Sargassum in the Gulf prefer temperatures between 68 to 82 degrees Fahrenheit, but summer water temperatures there have recently exceeded 86 degrees, so experts believe the sargassum is weak and struggles to survive once it arrives in the Sargasso Sea.
But as this historic dynamic fades, sargassum is actually flourishing 600 miles south of the Sargasso Sea in a 5,000-mile stretch between West Africa and the Gulf in the tropical Atlantic.
It’s called The Great Atlantic Sargassum Belt.
“The thought is … in 2011, the Sargasso Sea actually fed the belt. It started the belt, and now the belt has overtaken,” Barnes said. “It’s now completely dominant in the tropical Atlantic, and there's essentially nothing in the historical footprint.”
Barnes’ team, including scientists with USF and the Sea Education Association, was funded by NASA to officially document this decline using satellite imagery.
"Our study documents the changes in the patterns of transport and in the abundance and distribution patterns throughout the Atlantic,” he said.
Their findings were peer-reviewed and published in the journal Nature Geoscience earlier this month.
“The impacts to the broader ecology are, on some level, straightforward, but also unknown. We don't know if patches are farther apart," he said. "Can species or individuals get from one patch to another? How interconnected do they need to be in order to sustain a growing population?”
“We've made some hypotheses as to why it changed. It has changed, but the downstream applications' impacts are really hard to know," he continued.
