Science | December 2020

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

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. Shane Gross

Photographs by Shane Gross; Text by Katherine Harmon Courage

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.

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. Shane Gross

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.

an endangered green sea turtle feeds on seagrass — Egypt: Near the Red Sea resort town of Marsa Alam, an endangered green sea turtle feeds on Halophila stipulacea, a tropical seagrass that is also native to the Indian Ocean and Persian Gulf. When young, the turtles eat a variety of plants and animals, but they become strict herbivores in adulthood. Researchers have outfitted green sea turtles with radio-signal tags in order to track the animals to nesting and foraging grounds—and thus map thriving seagrass beds. Shane Gross

“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 immigrants. When the largest dinosaurs were in their heyday, these grasses drifted from dry land into the sea.

Indonesia: A tiny fish called a bilobed ghost goby, notable for translucent skin, lives—and hides—amid seagrass. Found in shallow waters from India to Indonesia’s Maluku Islands and north to Japan, the gobies grow to just over an inch long and feed on small crustaceans; despite their camouflage, the gobies, in turn, are prey to larger wrasses and juvenile groupers. Shane Gross

They have changed little since then. Like land grasses, they grow leaves, roots, rhizomes, veins and flowers. Their modest adaptations to the marine environment include aquatic pollination, neutrally buoyant seeds that can drift with the current before settling, and leaves that manage saltwater. These adaptations have led seagrasses to cover some 116,000 square miles of the world’s ocean floors, along every continent except Antarctica. Typically preferring depths of less than ten feet, most seagrasses are modest in height, but some can reach 35 feet long, such as the showy, ribbonlike Zostera caulescens, which grows off the coast of Japan.

Seagrasses have survived, not just as species, but often as individual clones, for thousands of years. Scientists studying Posidonia oceanica meadows in the Mediterranean Sea estimate that the largest clone, which stretches more than nine miles, has been around, sending out slow-growing rhizomes, for tens of thousands of years, and possibly as many as 200,000 years. It could be the oldest-known organism on Earth.

Cuba: An American crocodile in the Jardines de la Reina, a marine park protected since 1996 and regarded as a pristine Caribbean ecosystem. Seagrass beds, coral and mangrove islands are home to diverse species including reef sharks, Goliath groupers, rainbow parrotfish, long-spine sea urchins and hawksbill sea turtles. Shane Gross

Bahamas: A green sea turtle happens upon a researcher with the Centre for Ocean Research and Education, based on Eleuthera Island. The study is evaluating the health of seagrasses in local waters, where sharks hold grazing turtles in check, and in a Caribbean site where shark populations have been nearly wiped out. Shane Gross

Bahamas: A “scar” likely caused by a boat propeller. Scarring divides and isolates seagrass beds, increases erosion and makes coastal communities more vulnerable to storms. Researchers studying such gashes in Chesapeake Bay found that the beds can take 18 years to fully recover; sometimes, the scars never heal. Shane Gross

Throughout these millennia seagrasses have not only greened undersea landscapes but have also actively shaped them—“ecological engineers,” as researchers say. Roots hold seafloor sediment in place. Leaves help to trap floating sediment, improving water clarity. Seagrasses slow currents and help protect shorelines from storms. And they efficiently filter out polluting chemicals even as they cycle nutrients, oxygenate the water and pull carbon dioxide into the seafloor. The new U.N. report estimates that seagrasses may perform up to 18 percent of the ocean’s carbon sequestration, even though they cover only about 0.1 percent of the ocean floor.

Bahamas: Turtle grass (*Thalassia testudinum*) off Eleuthera. Shane Gross

Indonesia: Sun shines on a seagrass seascape. Shane Gross

Bahamas: Marine biologist Olivia Rhoades tends to her experiment off Eleuthera, where an action camera records which animals are feeding on seagrass. Shane Gross

Newfoundland: A tiny sea star uses seagrass as protection and food, eating the algae and other organisms that grow on a blade. Shane Gross

Bahamas: A major part of the queen conch diet comes from eating algae off seagrass. Queen conch is the official food of the Bahamas. Shane Gross

Indonesia: Seagrasses are flowering plants that returned to the sea after evolving on land. These are off Flores. Shane Gross

And they don’t do all this hard work silently. Carlos Duarte, a leading international seagrass expert at King Abdullah University of Science and Technology, on the banks of the Red Sea, in Saudi Arabia, describes a “scintillating sound when you lie in seagrass meadows,” which comes from the bursting of oxygen bubbles seagrasses produce and which sound, he says, “like little bells.” These faint peals may serve as clarion calls to some creatures that rely on seagrass meadows. For example, fish whose larvae, floating through the water column in search of a suitable place to land and mature, may depend on the sound for guidance.

Florida Keys: Bonnethead sharks, a species of hammerhead, live in shallow waters off both American coasts. In addition to a typical diet of crabs, clams, fish, squid and octopus, bonnetheads eat huge amounts of seagrass, and apparently not just by accident while gobbling prey. In fact, they digest about half of the green stuff—the only omnivorous shark species known to science. Shane Gross

Like many other ecosystems, seagrasses are also facing rapid decline. Approximately 7 percent of global seagrass coverage disappears each year, similar to the loss of coral reefs and tropical rainforests. This decline also threatens species that depend on seagrasses for food and habitat, including endangered manatees, green sea turtles, chinook salmons, and dugongs, and it serves as a warning of greater devastation to come.

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The assault on seagrasses comes in many forms. Fertilizer runoff fuels algae blooms, blocking the light needed for seagrasses to grow, as does excess topsoil runoff from coastal construction and development. Boat anchors and dredging uproot grasses and scar and fragment seagrass habitats. Overfishing large predators disrupts food chains, allowing mid-level predators to wipe out the worms and other small herbivores that usually clean algae off seagrasses. Rising sea temperatures threaten to outpace grasses’ ability to adapt or move, and exacerbate increasingly strong storms that can uproot entire meadows.

Indonesia: A woman and her son harvest sea urchins from seagrass beds. The spiny echinoderms, which feed using a unique jawlike structure called Aristotle’s lantern, have been known to overgraze seagrasses. In parts of Australia, restrictions on harvesting sea urchins have been lifted specifically to protect seagrass. Shane Gross

Egypt: A dugong near Marsa Alam. Known as sea cows for their avid grazing as well as their bulk, these cousins of the manatee can grow to more than 1,000 pounds while feeding almost exclusively on seagrass—up to 88 pounds a day. In part because of this dependence, global dugong populations are in rapid decline. Shane Gross

Seagrasses once thrived up and down the Eastern Seaboard of the United States. In some areas, such as the coastal waters off Virginia, meadows of Zostera marina, or eelgrass, were so abundant that, as recently as 100 years ago, local residents used clumps of the stuff that had washed ashore to insulate their homes. But in the 1930s seagrass meadows from North Carolina to Canada were practically eradicated, likely the result of a plague of slime mold disease combined with a devastating 1933 hurricane. Large swaths of coastal meadows had recovered by the 1960s, but important pockets remained barren.

A group of scientists, including Robert Orth, a marine ecologist at the Virginia Institute of Marine Science, noted that there was no reason the region’s waters couldn’t sustain seagrass meadows once again. So the researchers had a wild idea: Why not reseed the historic eelgrass beds? Beginning in 1999, Orth and others dispersed 74.5 million eelgrass seeds into 536 restoration plots covering an area of close to a square mile. Now in its 21st year, it is one of the largest and most successful seagrass restoration efforts on the planet.

Bahamas: A green sea turtle (*Chelonia mydas*) swims to the surface over a turtle grass meadow (*Thalassia testudinum*) off Eleuthera. Shane Gross

Newfoundland: A jellyfish floats into a sheltered eelgrass bay. Shane Gross

Newfoundland: American, or northern, lobster (*Homarus americana*) hiding in common eelgrass (*Zostera marina*). Shane Gross

Newfoundland: Juvenile Atlantic cod (*Gadus morhua*) use seagrass as a nursery off Newfoundland. Shane Gross

Indonesia: A lionfish hunts cardinal fish in seagrass, an important habitat for predator and prey, off Manado, Indonesia. Shane Gross

Soon new eelgrass meadows spread rapidly on their own; today, new growth covers nearly 13 square miles. Within a few years, new plots were hosting a diverse range of returning fish and marine invertebrates and were sequestering more and more carbon over time. “It’s a good-news story,” says Orth, who has been studying seagrasses for half a century. “If the plants aren’t challenged by water quality, they can spread naturally very quickly.”

Sites in Florida as well as Europe and Australia have also succeeded in reviving seagrass populations, even with passive restoration efforts such as reducing fertilizer and soil runoff.

New international efforts are also underway to create an up-to-date map of seagrass colonies all over the world—a baseline for assessing what we stand to lose. “Getting an accurate global map of seagrass distribution is really important for understanding the fisheries that depend on them as well as their contributions to carbon storage,” says Duffy, of the Smithsonian.

Egypt: In the complex web of life sustained by seagrasses, a reticulate whipray, also known as a honeycomb stingray, feeds on invertebrates and fish. The ray’s spots may help it elude some predators, but not humans. Consumers in Asia prize the exotically patterned hide for use in wallets, shoes, handbags and other goods. Shane Gross

Newfoundland: Atlantic cod, once abundant off the Atlantic coast of North America, were heavily overfished for decades, reducing stocks by 96 percent and causing the collapse of the commercial fishery. The fate of the bottom-dwelling species is intertwined with seagrass, which serves as a nursery for codling, providing cover. Shane Gross

Duffy and his colleagues are using drone imagery to study seagrasses along the North American Pacific Coast, where new outbreaks of slime mold disease, possibly fueled by warming ocean temperatures, threaten large seagrass meadows. Citizen scientists are pitching in, reporting seagrass locations with the smartphone app SeagrassSpotter. Duarte and others are even enlisting the help of radio signal-tagged creatures. “We are finding seagrass meadows by collaborating with sea turtles and tiger sharks,” Duarte says.

Indonesia: A school of juvenile striped eel catfish forage in a seagrass bed off the coast of Sulawesi. These ornamented catfish have fins that hide spines that deliver venom and can be fatal to the touch; fishermen cleaning nets in the Persian Gulf and Red Sea have reportedly been killed by exposure. Curiously, specimens that migrated to the Mediterranean Sea are thought to be less toxic. Shane Gross

Researchers are increasingly convinced of the value in working to expand seagrass beds, not just for the grasses’ own sake or for the marine creatures that depend on them, but for our own well-being. “If we invest in seagrasses, they can help us in lowering the global concentration of carbon dioxide,” says Jonathan Lefcheck, a research scientist at the Smithsonian’s Environmental Research Center. He notes that we are quick to recognize the importance of forests in keeping carbon out of the atmosphere. But a seagrass meadow can be just as effective as a temperate forest in sequestering carbon, sinking it into the sediment for decades or even centuries. “I’m pitching seagrasses as an ally in climate change,” he says. “They are an incredible ecosystem that continue to provide a wealth of benefits to humanity.