The receding tide at the mouth of the Yellow River in Dongying, Shandong province, one late-September morning revealed a vast sea grass bed.

Dozens of workers walked back and forth along the shore in the city's Kenli district, gathering Japanese eelgrass seeds to be sown next spring. The seeds, about the size of sesame seeds, were not fully mature, and the workers put them in mesh bags and soaked them in water to allow the eelgrass to continue growing.

"Sea grass seeds are too light and will be easily washed away by the waves when they are fully mature, so collecting them around the time of the Mid-Autumn Festival is the best strategy," said Zhou Yi, a researcher at the Chinese Academy of Sciences' Institute of Oceanology.

Zhou, who has been studying sea grass beds for more than 20 years, visits the special "grassland" in the Yellow River estuary every other month. "This sea grass bed is about 30 hectares in size, and most of it has been restored in the past three or four years," he said.

The Yellow River estuary was once the largest Japanese eelgrass bed in China, and it still appears endless and lush like grassland on land when the tide is low. In 2019, Typhoon Lekima destroyed about 667 hectares of sea grass beds, leaving large stretches of bare beach.

Zhou Licheng, director of the Shandong Yellow River Delta National Nature Reserve's scientific research center, said sea grass beds play a similar role to forests on land, supporting small fish and crabs that are eaten by a variety of birds. The spread of smooth cord grass, an invasive species native to the United States' Atlantic coast, and frequent typhoons have badly damaged sea grass beds in the Yellow River estuary, Zhou Licheng said, creating breaches in the local intertidal ecosystem.

Smooth cord grass has been identified by the International Union for Conservation of Nature as one of the world's 100 most dangerous invasive species and was one of the earliest invasive species officially recognized as a threat by Chinese authorities.

In order to restore the ecosystem, the reserve has cooperated with the Institute of Oceanology and the Ocean University of China, both in Qingdao, Shandong, and other institutes since last year to promote the repair of sea grass beds. Growing sea grass underwater is quite different from growing grass on land.

"We have tried direct planting, direct seeding and other methods, but due to the influence of submarine sediment, ocean currents and other factors, the plants and seeds sown can be easily washed away or eaten by marine organisms," Zhou Yi said, adding that after many attempts, the team finally developed a combination of transplanting seedlings and planting "rolled clay balls".

He said the rolled-clay-ball method involved wrapping the seeds in soil, adding necessary nutrients and then using machinery to sow the balls on beaches.

Research has shown that one-third of the world's sea grass beds have been lost due to a combination of human activities and global climate change over the years.

On Nov 28, Dongying's city government announced at a news conference that more than 99 percent of its troublesome smooth cord grass had been eradicated, with 100 hectares of local sea grass beds restored.

Zhou Yi said: "We will explore new ways while observing their results. If this restoration method is proved effective by long-term practice, we will promote it in more places."

Xinhua