From Weed to Wonder

Once dismissed as an underwater nuisance, scientists are beginning to see seagrass meadows as vital to marine and coastal health.

As the outboard engine quiets to a rumble, Bo Lusk takes a hand off the wheel of his small motorboat. He points ahead toward a distinct boundary in the few feet of green-brown water where we drift. “See that line?” asks Lusk, a coastal scientist at The Nature Conservancy. “That’s grass.”

Soon the boat glides over ribbons of Zostera marina, a seagrass that undulates in the shallow water like spinach-infused linguini. From our vantage, this protected lagoon off the Virginia coast looks like a monoculture of rippling green. But the submerged meadow beneath us is actually crammed with fauna, from juvenile sand bar sharks (Carcharhinus plumbeus) and seahorses that wrap their tails around Lusk’s proffered little finger to micro-invertebrates visible only as a fuzzy coating on each strand of grass.

There hasn’t been this much seagrass here since the beginning of the 1930s, when the region hosted the country’s largest bay scallop fishery. The seagrass wasn’t popular: It seemed designed to clog that era’s boat motors. But then small dark spots from a wasting disease caused by a slime mold began appearing on seagrass leaves. Within a few years, the disease rotted through 90 percent of Zostera marina meadows on both sides of the Atlantic, leaving only small patches of seagrass.

In some locations, these remnant patches eventually spread outward. But the coast of Virginia remained almost entirely bereft because of a 1933 hurricane, its distance from surviving meadows, and perhaps some overzealous dredging by scallop fishermen. An ecosystem that had once offered animals food, shelter, and a living mesh of sediment-anchoring roots was reduced to bare sand and mud. With the vanished grass, bemoaned a field guide to duck hunting published in the ‘40s, “went the wildfowl, the cream of salt-water fishing, most of the clams and crabs, and all of the scallops.”

The coastal bays stayed bare until the late 1990s, when conservationists discovered a patch of grass that had drifted in from a surviving Zostera marina population in the Chincoteague Bay off the coast of southern Maryland. Robert “JJ” Orth, a researcher at the Virginia Institute of Marine Science (VIMS), started collecting more seeds from nearby Chesapeake Bay. Then Orth and his colleagues boated through these bays, eyes flicking back and forth between the water and their GPS systems. At planned coordinates, they flung handfuls of tiny, oblong seeds overboard into neatly demarcated plots.

Almost immediately, the bays started to sprout darker rectangles of grass. Imagery from aerial surveys even showed squiggles of seagrass growing along the boats’ exit paths, where researchers had tossed leftover seeds into the water. Seagrass restoration here almost seemed simple, a rarity in the world of marine conservation, and in ecosystem restoration in general. For one grant proposal to NOAA, the National Oceanic and Atmospheric Administration, scientists seeded Zostera marina so that it grew into 50 meter-tall letters spelling out “NOAA” on the seafloor. “That was the cover photo of our grant application,” Lusk says, laughing. They won their funding, and within a few years the space between the letters had grown in.

Starting from zero seagrass cover, Virginia-based conservationists have seeded more than 600 acres to date, and those plants have begun spreading seeds of their own. Lusk estimates that seagrass meadows have regrown across about 10,000 acres of Virginia’s coastal bays, transforming everything from food webs to water quality to the ecosystem’s ability to stow away planet-warming carbon.

But as Lusk pulls on a dive mask and snorkel and slips off his boat into a newly-restored seagrass meadow, his focus is much more narrow. He points out the tiny snails and crustaceans on each blade of grass. “This is a huge, huge place, especially if you’re something like this big,” he says, pinching two fingers together to the size of a grain of rice.

The invertebrates clinging to each green blade are completely dependent on seagrass species like Zostera marina. And over the past two decades or so, scientists have begun to suspect that underwater meadows provide similarly vital support to broader communities of animals—and of people.

Today, 72 known species of seagrasses grow in shallow waters from the tropics to the Arctic. Their meadows pop up from the Mediterranean to the Yucatán to Western Australia and beyond, possibly blanketing, according to one conservative estimate, a total area greater in size than the United Kingdom.

Compared to flashier coastal ecosystems—technicolor coral reefs, gnarled mangrove forests— it took many fishers, officials, and even some scientists longer to see seagrasses as more than nuisance plants. That left those who conducted research on seagrass working in a figurative, if not often literal, backwater. Some of the most authoritative early studies on seagrasses were published in a journal called Weed Science, says Jessie Jarvis, an ecologist at the University of North Carolina Wilmington and the current president of the World Seagrass Association, a network of researchers and coastal managers.

“They are flowering plants that live in saltwater. It’s crazy to me!”
—Jessie Jarvis, University of North Carolina Wilmington

By the 1980s, biologists had started to get more interested. Aside from a superficial resemblance, seagrasses aren’t really “grasses” at all. Scientists found that they instead hail from lily-like land plants. Some 100 million years ago, during the heyday of the dinosaurs, these seagrass ancestors headed back into the seas of their own primordial past.

“They are flowering plants that live in saltwater,” Jarvis says. “It’s crazy to me!”

This marine way of life brought new challenges, and with them, new innovations. To get oxygen to underwater roots buried in otherwise toxic, sulfur-rich sediments, for example, seagrasses evolved to absorb more light than almost any other plants in the world, restricting them to shallow, crystal-clear waters. And while seagrasses mostly rely on water currents to spread their pollen, underwater pollinators have stepped up, too.

Scientists working in Mexico in 2016 found, for example, that small crustaceans and marine worms visit the tropical seagrass species Thalassia testudinum at night, carrying a slimy mucus packed with pollen between its male and female flowers. Studies also show that dugongs, manatees, birds, fish, and turtles help disperse seagrass seeds over large distances by swallowing them whole and pooping them out later—just like many birds and mammals do above the waves.

Aside from sexual reproduction, some seagrasses also spread by extending new roots horizontally just under the seafloor, forming perfect clones. Scientists estimate one such field in the Mediterranean of a species called Neptune grass (Posidonia oceanica) has likely lived for more than 100,000 years, making it one of the world’s longest-known living organisms. Another cloned meadow, in Australia, stretches for around 70 square miles (180 square kilometers) of genetically-identical stalks of Poseidon’s ribbon weed (Posidonia australis), giving it the claim of one of the world’s most widespread known clones.

In turn, an array of marine fauna have evolved to take advantage of the mermaid-hair topology of seagrass meadows. “There’s plenty that the plants do themselves, but then their added oomph comes from supporting these other organisms,” Jarvis says.

Atlantic cod (Gadus morhua) use seagrass as a nursery in Scandinavia and Newfoundland, while bonnethead sharks (Sphyrna tiburo) with surprisingly omnivorous palates munch on it around the Florida Keys. In Virginia’s restored seagrass beds, researchers have spotted loggerhead sea turtles (Caretta caretta) alongside gamefish species like tarpon (Megalops atlanticus), red drum (Sciaenops ocellatus), and speckled trout (Cynoscion nebulosus)—all drawn into the swaying meadows to hide, hunt, or both.

Overall, scientists estimate that one-fifth of the world’s major fisheries rely on seagrass. So do the subsistence communities and industries that depend on such fisheries. And a recent, parallel thread of research has shown seagrass meadows engineer their own environments, too.

Their root systems lock sediments in place, preventing waves and storms from stirring them into the water column, and buffering human communities from shoreline erosion. Seagrasses’ waving stalks, meanwhile, help slow incoming water currents, which allows fine sediments to settle to the bottom.

All told, the water that flows through a seagrass meadow is clearer, with less suspended sediment. And it’s cleaner: Seagrass meadows scrub away bacterial pathogens that target coral reefs, fish, and humans. It’s also lower in the greenhouse gas carbon dioxide. In the last decade, a flurry of studies have shown that seagrass meadows trap carbon in their roots and in the sediment beneath them. One U.S. state has even acted on that information. In Virginia, where studies estimate that the restored seagrass beds have sucked up more than 3,000 metric tons of carbon, a 2020 measure allows the state government to begin selling seagrass-based credits on a carbon market that Lusk and The Nature Conservancy are working to establish by the end of 2023.

As scientists learn more about seagrasses, initiatives to protect and restore them are cropping up around the rest of the world, too. But replicating the success of Virginia’s restoration effort hasn’t been straightforward.

After the slime-mold pandemic, the biggest factor limiting the return of seagrass in the Virginia coastal bays turned out to be the availability of seeds. Orth solved this by developing a system to harvest and store Zostera marina seeds. But Virginia also has unusually clean coastal water, thanks to limited development around the state’s bays. Many other places aren’t so lucky.

Even though seagrass meadows can clean water, there’s only so much incoming pollution they can handle. Worsening water quality is the biggest cause of modern seagrass declines, says Jarvis: Algal blooms from nutrient runoff or eroded sediment can quickly choke and kill a meadow. Indeed, seagrass isn’t faring well globally; studies estimate that up to 7 percent of the world’s seagrass disappears each year. The opportunistic slime-mold pathogen that swept through the Atlantic in the 1930s is still around too. A particularly virulent strain could emerge, or meadows weakened by climate change might give it new openings to spread.

Another basic obstacle to seagrass conservation is that scientists don’t yet know where to find it all. In 2018, a research team announced that they had put satellite trackers on green sea turtles in the Indian Ocean, and then—like a PI tailing a suspect’s car—followed the turtles back to a vast, barely-researched meadow thriving in up to 95 feet (29 meters) of water, deeper than most known shallow-water seagrass ecosystems.

This past November, scientists announced that using the same tactic on tiger sharks in the Bahamas—this time equipped with not just trackers, but miniature video cameras that returned POV adventure shots of the sharks cruising through seagrass—had helped clarify the full extent of another meadow that may stretch across 35,500 square miles (92,000 square kilometers), an area about the size of Maine.

Documenting these meadows is vital for protecting them. So is learning to look past earlier misconceptions of seagrass to see it as essential, even beautiful. Marine photographer Shane Gross hopes that his work documenting these submerged meadows does just that.

Gross has captured images of juvenile cod hiding in seagrass to avoid predators in the North Atlantic; a banded sea krait (Laticauda colubrina) snuffling into the sediment as it hunted prey in Indonesia; and a reclusive dugong (Dugong dugon) searching for a seagrass snack in Egypt. He has also taken mesmerizing aerial shots of the restored meadows off the Virginia coast. And like Lusk, Gross often found his attention turned to smaller quarry, for whom each blade of grass might be a towering redwood.

Whether zoomed in at the scale of a tiny filter-feeder or admired from a birds-eye-view, thriving seagrass ecosystems are a reminder of the latent potential of once-ruined habitat, and our own power to shift how society values, conserves, and restores the natural world.

In Virginia, the story of seagrass’s journey from weed to absence to wonder is within human memory. Lusk remembers his grandmother, who had been a teenager living nearby in the 1930s, passing on a story: how there had once been grass in the ocean-side bays, and how when it vanished the men who worked the scallop-shucking houses were out of work. Now her grandson scans for returning bay scallops as he snorkels in the restored meadows. Lusk finds a live scallop, wraps it gingerly in living seagrass, and carries it back to the boat to see if it will thrive in his seagrass tanks.

He also examines another scallop shell out of the water, but it lolls open, dead, and he laughs as instead of a mollusk, two tiny fish that had been sheltering inside pop out one after the other, splashing back into the meadow below.

Shane Gross

Shane Gross is a Canadian marine conservation photojournalist. His childhood passion for sharks has grown into a love for everything that lives underwater. Telling long-form narratives about the ocean and human's impact on her, both positive and negative, are what drive him.

Joshua Sokol

Lauren Owens Lambert

Joshua Sokol is a freelance science writer based in Raleigh, North Carolina. His stories
have appeared in Science, The New York Times, Quanta, and other outlets.



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