What keeps the 'underwater prairies' thriving?

A sea turtle, swimming above seagrass. Michelle Roux 

Seagrass support a range of marine life. Photo by Ben Jonson

In the shallow waters of Palk Bay, hues of emerald gleam as sunlight streams through the ocean’s tides. A closer gaze into the water reveals an enchanting world—a bustling town of fish, sea slugs, turtles, crabs, sea cucumbers, shrimps, squids, and seahorses, perhaps even dolphins and dugongs navigating through the blades of green grass. That’s the view the planet’s third most-valued ecosystem—the seagrass meadow—has to offer.

“Seagrass beds are paradise underwater,” says marine researcher and conservationist Vedharajan Balaji, who studies seagrass and mangrove ecosystems in Palk Bay, a shallow water body between the southeast coast of India and Sri Lanka. He is also the founder of the Organisation for Marine Conservation, Awareness and Research (OMCAR) Foundation.

Seagrasses are marine flowering plants belonging to the grass family, and are different from seaweeds. Seagrass meadows are present in both tropical and temperate seas.

They thrive in shallow, tropical, waveless sea conditions that receive ample sunlight. Like other plants, they are ‘primary producers’ that convert sunlight into organic food through photosynthesis.

Scientists believe seagrass evolved from their terrestrial ancestors and recolonised the world’s oceans about 100 million years ago. Today, there are 72 species of seagrass of which 10 are on the verge of extinction, and three are endangered. In India, they are found in the Gulf of Mannar, Palk Bay, Andaman and Nicobar Islands, Lakshadweep, Gulf of Kutch and Chilika Lake in Odisha.

Ecosystem engineers

Although not as colourful and glamorous as the headline-grabbing coral reefs, these "prairies of the sea" rank next only to wetlands and estuaries for the benefits they provide. Seagrasses act as “lungs of the sea”, providing oxygen for marine life. Their extensive roots absorb nutrients, stabilise the sea bed, slow down the flow of water and reduce erosion of coastlines during storms. The roots double up colanders, trapping sand, dirt and silt particles, improving water quality and preventing the deposition of sediments on nearby coral reefs.

Like forests, seagrass meadows trap atmospheric carbon dioxide and store it as ‘blue carbon’ in the sea bed. Scientists estimate that the world’s seagrass meadows can capture up to 83 million metric tonnes of carbon each year. As we face the effects of human-induced climate change, seagrass meadows could be our ally in reducing greenhouse gas emissions and ocean acidification that impacts coral growth.

Seagrass meadows are nurseries for various fish and invertebrates, including octopuses, squids, cuttlefish, snails, bivalves, sponges, shrimps, worms and sea urchins—all vital to the marine food chain. An acre of seagrass supports around 40,000 fish and 50 million invertebrates.

They also support dugongs, iconic marine herbivores that devour 40 to 50 kg of seagrass each day. As ‘farmers of seagrass’, dugongs control the growth of seagrass meadows and disperse their seeds. Green turtles also feed on seagrass.

Thousands of local fishing communities rely on these fish species for livelihood. In Palk Bay, for example, squid catchers on wooden paddleboats paddle above the seagrass beds to catch squids and cuttlefish using hooks. “Without seagrass, all those artisanal fishers will face difficult situations,” says Balaji.

“Seagrasses are ecosystem engineers as they decide the trophic level (i.e. position in the food web) and productivity of that ecosystem,” says Kaladharan Edavilakathil, a marine biologist and Principal Scientist at the Central Marine Fisheries Research Institute.

Conservation

Globally, the world is losing seven percent of seagrass meadows each year. Fisheries, pollution, global warming, diseases and storms threaten them worldwide. “In Palk Bay, drag nets run by motorboat fishers and trawlers, wastewater from shrimp farms, and sewage water are threatening the seagrass ecosystems,” says Balaji. In the Gulf of Kutch, pollution and deposition of sediments and silt cause seagrass destruction.

Degrading seagrass habitats is a cause of concern for locally-endangered dugongs as they lose their feeding grounds. Hence, the CAMPA-Dugong Recovery Program, launched in 2016, focuses on monitoring and surveying seagrass ecosystems. “Dugong conservation is nothing but seagrass conservation. We can’t separate a species from its habitat,” says K Sivakumar, a senior scientist at the Wildlife Institute of India, who heads the recovery programme.

Various seagrass restoration efforts are underway in the Gulf of Mannar and Palk Bay to revive damaged seagrass meadows. The Suganthi Devadason Marine Research Institute (SDMRI) has successfully launched many pilot-scale efforts to rehabilitate Cymodocea serrulata and Syringodium isoetifolium species of seagrass, which are food sources for dugongs.

Balaji’s OMCAR has developed a low cost, eco-friendly seagrass restoration method using coconut ropes and bamboo frames. Seagrass sprigs from natural seagrass meadows are tied to these frames, which are then fixed on the seafloor. In about two months, these sprigs take root in the sea bed.

The good news, however, is that seagrasses are incredibly resilient.

“If we let seagrass beds be without disturbing them, they will restore themselves from damages caused by human activities and natural disasters,” says Balaji. Bringing more seagrass habitats within the Marine Protected Areas of India and promoting seagrass-friendly fishing activities are other ways. “This is the easiest option for us to safeguard the future livelihoods of fishermen in India as well as the long-term survival of dugongs,” opines Sivakumar.

With so much to offer, these mitigators of climate change deserve some recognition for all that they do. The international seagrass research and conservation community is calling on the United Nations to declare a World Seagrass Day to recognise the importance of seagrass meadows to the health and well-being of the planet.