Mysterious masses of seaweed assault Caribbean islands

[Judi Lynn]

By Katie LanginJun. 11, 2018 , 5:20 PM

In retrospect, 2011 was just the first wave. That year, massive rafts of Sargassum—a brown seaweed that lives in the open ocean—washed up on beaches across the Caribbean, trapping sea turtles and filling the air with the stench of rotting eggs. “We had some really massive turtle kills,” says Hazel Oxenford, a fisheries biologist at The University of the West Indies in Cave Hill, Barbados. Before then, beachgoers had sometimes noticed “little drifty bits on the tideline,” but the 2011 deluge of seaweed was unprecedented, she says, piling up meters thick in places. “We’d never seen it before.”

Locals hoped the episode, a blow to tourism and fisheries, was a one-off. But a few years later “it came back worse,” Oxenford says. Now, the Caribbean is bracing for what could be the mother of all seaweed invasions, with satellite observations warning of record-setting Sargassum blooms and seaweed already swamping beaches. The Barbados government declared a national emergency on 7 June. “It’s catastrophic,” says James Franks, a marine biologist at The University of Southern Mississippi in Ocean Springs, who is one of many scientists trying to explain why a part of the ocean that was once seaweed-free is now rife with Sargassum. “Right now there’s [another] huge mass impacting Puerto Rico, and that’s the last thing they need.”

Before 2011, open-ocean Sargassum was mostly found in the Sargasso Sea, a patch of the North Atlantic enclosed by ocean currents that serves as a spawning ground for eels. So when Sargassum first hit the Caribbean, scientists assumed that it had drifted south from the Sargasso Sea. But satellite imagery and data on ocean currents told a different story.

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To confirm that the Sargassum fouling Caribbean beaches in 2011 came from the tropical Atlantic, east of Brazil, Franks and his colleagues traced the likely path of seaweed masses backward through time. First, they compiled records of locations where Sargassum came ashore. Then, using information about surface currents, they calculated its likely source. “Invariably, in all of those instances, it tracked back to the [tropical] region,” says Franks, who reported the findings in 2016. “None of it ever tracked northward into the Sargasso Sea.”

More:
http://www.sciencemag.org/news/2018/06/mysterious-masses-seaweed-assault-caribbean-islands