Seagrasses face extinction threat

Seagrasses around the world are disappearing, with some species now threatened with extinction.

The first global survey of individual seagrass species has found that 14% are at risk of going extinct.

More common species are also in decline, meaning both seagrass habitat and diversity is being lost.

Seagrasses provide food and habitat for a variety of ocean species including manatees, sea turtles and fish such as sea horses.

Seagrasses are flowering plants that grow on the ocean floor.

They form vast meadows that flower and seed underwater, having evolved from land-based plants that entered the water millions of years ago.

Seagrasses alone form important marine habitats.

They act as nurseries for young fish and shellfish, and are the primary food for large marine mammals such as manatees and dugongs, as well as reptiles such as some sea turtles.

They also contribute to the health of coral reefs and mangroves, salt marshes and oyster reefs.

It has been known for a while that seagrasses are declining in many parts of the world.

The reasons are many, seagrass expert Frederick Short told the BBC.

Professor Short, of the University of Hampshire in Durham, US is the director of SeagrassNet, an international seagrass monitoring program with 114 sites around the world.

For example, seagrasses are gone from the most developed coastlines due to pollution, said Professor Short.

Seagrasses are in decline in the developing world due to sedimentation, caused by runoff from impacted watersheds and deforestation, and being overloaded with nutrients flowing into the sea from sewage and agricultural runoff.

Seagrasses are also being directly damaged by the dredging of seafloors.

“But there has never been a review of individual species status,” said Professor Short.

So he and an international team of experts convened three workshops to gather all the knowledge about individual seagrasses, and used it to evaluate how at risk each species is. The workshops were hosted by Conservation International, the Global Marine Species Assessment programme and SeagrassNet.

The results are published in the journal Biological Conservation.

“I was surprised by the level of threat to many species of seagrass and to discover that seagrass biodiversity is under greater threat than I believed,” he said.

Of the 72 species, his team found that 15 seagrasses should be considered Endangered, Vulnerable or Near Threatened, under criteria laid down by the International Union for Conservation of Nature (IUCN) Red List.

Of those, ten face a significant risk of extinction.

Phyllospadix japonicus is an important habitat-forming grass along the rocky shorelines of China, North and South Korea and Japan. But it has gone from swathes of China’s coastline, due to seaweed aquaculture.

Zostera chilensis is known from only two locations on the coast of Chile, and seems to have already disappeared from one.

Of the 57 remaining species, 48 are considered of Least Concern, while sufficient data doesn’t exist to make a judgement on the others.

“Many widespread, common seagrass species which are not presently threatened are nonetheless in decline, so we have both an overall loss of habitat and a loss of biodiversity,” said Professor Short.

“Seagrasses are both direct food for important species and as they break down within the coastal ecosystem, they are part of a vast food web that provides food to many organisms within the coastal ocean, including many commercially and recreationally important species.

“Unfortunately, being submerged in the ocean they are rarely directly seen except by swimmers or snorkelers.”