Prairies of the Sea

A vast, mostly invisible ecosystem shapes life on Earth, from the food we eat to the air we breathe. And the more scientists learn, the more they say it’s in trouble

An ecosystem crucial to our terrestrial existence as well as the ocean’s vast web of life, seagrass is finally coming into focus. And just in time

Bright sunlight filters down through the clear Mediterranean waters off the coast of Spain, illuminating a lush meadow just below the surface. Blades of strikingly green grass undulate in the currents. Painted comber fish dart among clumps of leaves, and technicolor nudibranchs crawl over mounds. Porcelain crabs scuttle by tiny starfish clinging to the blades. A four-foot-tall fan mussel has planted itself on a rock outcropping. A sea turtle glides by.

This rich underwater landscape has been shaped by its humble covering, Posidonia oceanica. Commonly known as Neptune grass, it is one of about 70 species of seagrasses that have spread, over millions of years, across the globe’s coastal shallows, embracing and buffering continental shelves from Greenland to New Guinea. Seagrasses provide habitat for fish, sea horses, crustaceans and others; food for sea turtles, waterfowl and marine mammals; and nurseries for an astounding 20 percent of the largest fisheries on the planet.

“Seagrasses are the forgotten ecosystem,” Ronald Jumeau, a United Nations representative from the Republic of Seychelles, writes in a 2020 U.N. report. “Swaying gently beneath the surface of the ocean, seagrasses are too often out of sight and out of mind, overshadowed by colorful coral reefs and mighty mangroves.” But, he says, they “are among the most productive natural habitats on land or sea.”

Emmett Duffy, director of the Smithsonian’s Tennenbaum Marine Observatories Network, shares that view of seagrasses as underappreciated but essential: “They’re like the Serengeti grasslands of Africa—but hardly anybody knows about them.”

Yet this invisible ecosystem, once you do see it, has a primal if uncanny draw, at once alien and familiar, a remembered dream of a submerged meadow. This may be because, unlike seaweeds (which are algae, not plants) and corals, seagrasses are terrestrial

Spain: One of the oldest living organisms on Earth is a colony of Neptune grass in this vast meadow of the plant in the Mediterranean Sea. But warming ocean temperatures pose a threat to the species, Posidonia oceanica. Some scientists predict it may become extinct by midcentury.

Spain: A blade of seagrass serves as refuge, habitat or nourishment for other organisms, from microalgae to crustaceans and worms. Like land grasses, these marine plants flower, and they harness photosynthesis to produce chemical energy, yielding oxygen. Their leaves aren’t held up by rigid stems, though; they float.