Seagrass doesn’t get as much attention as more familiar ecosystems, like coral reefs or rainforests. But it’s hard to overstate its importance when it comes to slowing global climate change, protecting endangered species, and conserving coastlines.
These flowering plants grow submerged in shallow marine waters all over the world. They form huge underwater meadows that provide myriad benefits, including:
Food. Endangered animals as varied as dugongs, sea horses, and sea turtles all graze on seagrass. Many more animals feed on small organisms such as larvae and eggs left on seagrass leaves or, like dolphins and herons, stalk seagrass beds for prey. Without seagrass, these animals disappear.
Shelter. Many species of fish, crustaceans, and other animals shelter in seagrass beds for at least some part of their life cycle.
Storm and erosion mitigation. The extensive root systems of seagrasses stabilize the soft ocean bottom. This inhibits erosion and slows down waves, reducing damage to coastlines from big storms.
Better water quality. Seagrasses trap fine particles, which increases water clarity. They also filter nutrients in runoff from land, protecting coral reefs from contaminants.
Economic support. Because many economically important fish species (including salmon, snapper, and grouper) spend at least part of their life cycle in seagrass communities, seagrasses are vital to fishing industries. One estimate is that seagrass meadows support the production of a fifth of the world’s biggest fisheries. Their loss threatens food security for millions of people.
Reduced acidification of seawater. As the oceans warm, they become more acidic, threatening coral reefs and other marine ecosystems. A six-year study in California found that seagrass meadows reduce water acidity by up to 30 percent.
Carbon sequestration. An acre of seagrass can store about three times as much carbon as an acre of rainforest can. Keeping all of that carbon dioxide, a key greenhouse gas, out of the atmosphere is a huge win in the climate emergency fight.
When Seagrass Vanishes
Seagrass meadows and other coastal ecosystems “are among the most threatened habitats in the world,” according to a 2021 UN study. One study estimated that a football-field-sized area of seagrass disappears every half-hour—torn out by dredging, choked by polluted runoff, or killed by disease. That adds up to a loss of about seven percent of the world’s seagrass every year. It’s estimated that 92% of the seagrass has disappeared from the United Kingdom’s coast in the last century.
When seagrass is destroyed, there are rapid, damaging effects throughout the marine ecosystem. For example, in Puget Sound in the United States, herring spawn in seagrass. If the seagrass disappears, the herring disappear, too. Then the salmon that ate the herring have nothing to eat. Next, the orcas that preyed on the salmon are left without their primary food source.
The loss of seagrass also has dire consequences for the climate emergency. Clearing an acre of seagrass can release three times as much carbon dioxide into the atmosphere as clearing an acre of rainforest does.
Saving Seagrass
As scientists have sounded the alarm about disappearing seagrass, conservation groups have taken up the challenge. As with any environmental challenge, there is more than one good way to approach the problem.
Regulating coastal development and pollution
Managing this marine resource begins with managing the land, because land use practices have a big effect on seagrass. Runoff from farms introduces chemicals into the water that can damage or kill seagrass. Fertilizer runoff typically contains lots of nitrogen, which promotes rapid blooms of algae. The blooms deplete oxygen in the water and block sunlight, killing the seagrass. Erosion deposits sediment in coastal waters, making it impossible for seagrass to get enough sunlight.
Controlling what enters the water from the land can have a big effect. Boston, for example, used to dump untreated wastewater into its harbor, with an unsurprising result: one of the dirtiest harbors in the country. But after it installed a treatment facility in 2001, wastewater was pumped much further out into deeper water, and nitrogen levels in the harbor fell by 80%. By 2016, eelgrass (a type of seagrass) had increased by 50%. Similarly, in Florida, upgrades to sewage treatment plants and power plants cut nitrogen emissions by two-thirds. As a result, water quality improved, and the area covered by seagrass doubled in size.
In other regions, however, decreases in nitrogen runoff have not brought big improvements in seagrass cover. Rapid climate change, which is affecting not only the ocean’s temperature but also its salinity, acidity, and oxygen levels, is complicating management and recovery efforts.
Preventing damage from boats
Boat crews often damage seagrass by dropping anchor on it. They’re not doing it intentionally— sometimes, in relatively deep water, seagrass simply isn’t visible from the surface. In the western Mediterranean, studies suggest that beds of the endemic seagrass Posidonia have declined by 34% over the last 50 years. This catastrophe has multiple causes, but trawling and anchoring are the most destructive of the human-caused factors.
Better information for the boat captains could lessen the damage. In Spain, a nonprofit organization has developed a phone app that shows all the seagrass beds around the Balearic Islands. The app, which has been downloaded thousands of times, shows boat crews exactly (within one meter) where the seagrass is. This lets them move away and drop anchor where they won’t damage seagrass. The mapping data come from lateral-scan sonar, drones, and human divers. Seacology funded the completion of mapping around Formentera Island, one of the Balearic Islands.
Restoring Lost Meadows
When seagrass has disappeared, it sometimes comes back by itself, once an underlying problem such as pollution is mitigated. But in many places, active replanting measures are needed.
First, it may be necessary to fix whatever problem caused the seagrass die-off in the first place. If excessive runoff, sediment, or pollution killed the seagrass, for example, there is no use in trying to restore the area without addressing the water quality issues. Seagrasses need clear water that sunlight can penetrate. Other factors that can affect replanting include oxygen level, temperature, and salinity.
There has been some success with transplanting mature plants from healthy meadows or seedlings grown in laboratories. It’s a very labor-intensive process, which means that it’s also expensive. Each plant must be anchored to the bottom, so it doesn’t float away. Many methods have been tried, including tying plants to wire frames and putting them in peat pots. The best method may depend on the particular characteristics of the location.
A potentially more cost-effective method is to collect seeds from healthy plants and broadcast them in the area to be restored. Working with seeds, though, brings its own challenges. Seeds may be eaten by animals, or float away from shore and sink in water that’s too deep for them to grow in. Experimenters in Virginia bays have had good success with seeds. In 1999, the bays had no seagrass; after years of reseeding, there are now 4,700 acres (1,900 hectares) of eelgrass meadow. Seacology is funding a reseeding program in the Puget Sound of Washington state.
The good news is that when a small patch of seagrass is successfully started, it starts a virtuous circle of good environmental effects. The first seagrass improves water quality, which encourages more seagrass!