Life in the North Atlantic depends on this floating seaweed

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[Many thanks to Peter Jordens for bringing this item to our attention.] In the June 2019 issue of National Geographic magazine, James Prosek writes, “Sargassum plants are a nuisance on some beaches, but they are a shelter and a feast for a complex and amazing variety of marine life.” For full article and spectacular photos by David Doubilet and David Liittschwager, see National Geographic.

‘THERE’S NOTHING LIKE it in any other ocean,’ says marine biologist Brian Lapointe. ‘There’s nowhere else on our blue planet that supports such diversity in the middle of the ocean—and it’s because of the weed.’

Lapointe is talking about a floating seaweed known as sargassum in a region of the Atlantic called the Sargasso Sea. The boundaries of this sea are vague, defined not by landmasses but by five major currents that swirl in a clockwise embrace around Bermuda. Far from any mainland, its waters are nutrient poor and therefore exceptionally clear and stunningly blue.

The Sargasso Sea, part of the vast whirlpool known as the North Atlantic gyre, often has been described as an oceanic desert—and it would appear to be, if it weren’t for the floating mats of sargassum.

The seaweed may seem unremarkable at first glance—just bunches of drifting plant matter—but as Lapointe has helped illuminate through his work, sargassum is the basis of a complex ecosystem that nurtures a stunning array of marine life. It serves as a mobile shelter and a movable feast.

For 36 years Lapointe, a biologist with Florida Atlantic University’s Harbor Branch Oceanographic Institute in Fort Pierce, has combed the Sargasso Sea, observing sargassum by satellite and experiencing it firsthand in scuba gear. He wanted to figure out where the weed comes from, how it moves, what it sustains, and what sustains it—and to unravel the complex relationship sargassum has with other forms of marine life, from seahorses to great white sharks. Only by learning about this vital resource, he says, can we protect it from potential threats, such as ocean acidification and pollution.

When it needs protecting, that is.

During the past few years, sargassum has been making the news not as life-giving manna but as a scourge, mounds of it fouling beaches in the Caribbean and Mexico. No one’s talking about protecting sargassum anymore, Lapointe says. “It’s more like, how do we get it to go away?”

The sailors aboard Christopher Columbus’s Santa María were of like mind. The weed in some places was “so thick that it actually held back the ships,” reads a September 20, 1492, entry in the ship’s log. Early explorers noted that the air bladders keeping the seaweed afloat reminded them of a grape they called sargazo.

Sargassum originates in nutrient-rich zones close to the coast of the Americas, particularly in the Gulf of Mexico. Currents carry it around the Florida Peninsula, where it’s taken up by the northward-flowing Gulf Stream and eventually ends up in the Sargasso Sea.

Oceanographer Sylvia Earle, who helped initiate an effort to make the Sargasso Sea the first high seas marine protected area, likens sargassum to a golden rainforest. It’s an apt metaphor, because the weed forms a kind of canopy at the ocean surface. Sargassum brings to my mind a floating reef or even a marine grassland—a Serengeti of the sea.

Its tangled tresses support an astonishing diversity of organisms that hide in and feed off the weed—the larvae and juveniles, according to one study, of 122 different species of fish, as well as hatchling sea turtles, nudibranchs, seahorses, crabs, shrimps, and snails. The seaweed in turn is nourished by the excrement of these organisms.

Larger creatures such as fish and turtles find plenty to eat amid the sargassum, and they attract bigger predators—triggerfish, tripletails, filefish, mahi-mahis, and jacks, on up the chain of life to sharks, tuna, wahoos, and billfish. Tropic birds, shearwaters, petrels, terns, boobies, and other birds of the open ocean roost and forage on sargassum mats. [. . .]

The Sargasso Sea has long been associated with mystery. Eighteenth-century sailors referred to this part of the Atlantic as the horse latitudes because, the story goes, ships would get becalmed there and have to dump their horses overboard as freshwater supplies dwindled. And the Sargasso overlaps with the mythic region known as the Bermuda Triangle, where ships and planes are said to have disappeared without a trace. Whether or not you buy into the legends, when you’re out on the Sargasso Sea, you can’t help but be touched by moments of the sublime. [. . .]

For full article, see https://www.nationalgeographic.com/magazine/2019/06/sargasso-sea-north-atlantic-gyre-supports-ocean-life

[Map: Riley Champine, National Geographic. Source: Brian E. Lapointe, Florida Atlantic University.]

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