Since the time of the dinosaurs, four groups of flowering plants known as angiosperms colonised the oceans, with the oldest to be found in Australian and Mediterranean waters, dating at well over 100,000 years old and making it the oldest living organism found today.

Known as ‘seagrass’, they are the only flowering plants that can live underwater and are found in meadows along the shore of every continent except Antarctica. Seagrass meadows filter sediment and other nutrients from the water and are constantly building and securing sediment, which buffers coasts from erosion, storms, and flooding. Carbon accumulates in seagrasses over time and is stored almost entirely in the soils, which have been measured up to four meters deep.

Although seagrasses account for less than 0.2% of the world’s oceans, they sequester approximately 10% of the carbon buried in ocean sediment annually (27.4Tg of carbon per year). Per hectare, seagrasses can store up to twice as much carbon than terrestrial forests. The global seagrass ecosystem organic carbon pool could be as high as 19.9 billion metric tons.

Seagrasses are among the world’s most threatened ecosystems, with annual global loss of around 1.5% and accelerating in recent decades. Globally, about 29% of Earth’s seagrass ecosystems have been lost. Major threats to seagrasses include degradation of water quality due to poor land use, such as deforestation and dredging.

Researchers from the University of Western Australia’s Ocean’s Institute analysed the DNA of the seagrass at 40 sites across 3,500 kilometres of the Mediterranean Sea, from Spain to Cyprus.

Seagrasses are integral to coastal ecosystems. But despite flourishing for so long, tests show they have waned across the world over the past 20 years and are now declining at an estimated rate of 5% annually.

Having thrived for millennia, their current decline suggests they may no longer be able to adapt to the unprecedented rate of global climate change. Ocean acidification and recent anthropogenic pressure on coastal areas resulting in changes in water quality, eutrophication, and nutrient load were threatening the future of seagrasses, the team wrote in their paper.

Professor Eric Wolanski, from the Australian Center for Tropical Freshwater Research at James Cook University, has spent several years studying seagrass off the coast of the Philippines. He said the meadows there had been reduced to just 5% of their original size, a fact that is entirely due to human impact in his opinion.

HOPE FOR THE SURVIVAL OF SEAGRASS

Replanting seagrass is rarely successful as the development of root systems is a lengthy process, and roots do not easily take hold. An example of poor low seagrass restoration is the seagrass meadows in the Great Barrier Reef that were wiped out by Cyclone Yasi last year. They are reappearing much slower than hoped as the quality of the water doesn’t provide the right conditions for seagrass to regrow optimally.

Some seagrass rehabilitation projects have nevertheless been successful, growing in nurseries and replanting. The example of Sam Rees from Seagrass Ocean Rescue, who featured in the Earthshot Prize ‘Repairing Our Planet’ tells a tale of success in Pembrokeshire where 7.5 million seeds were planted into 536 individual restoration plots, resulting in 3.5 km of vegetated bottom from virtually no coverage previously.

Citizen science has always been useful, with a straightforward monitoring process that utilizes everyone (divers, holiday makers, fisheries) to use technology to map where they spot seagrass.

Perhaps one of the greatest hopes for the survival of this species lies in global monitoring schemes of existing seagrass meadow. The use of technology to monitor ecosystems is rapidly increasing, with the use of drones or autonomous vessels to map areas of seagrass. This technology allows cutting edge acoustic sensors to provide a new and comprehensive means of mapping seagrass beds.

These systems centre upon the use of low-impact, fully electric, uncrewed data acquisition platforms and non-invasive survey techniques, and involve developing and training new machine-learning algorithms to classify submerged aquatic vegetation. The solutions monitor both seagrass coverage and canopy height, with the sensors being trained to provide a rapid and robust coverage and biomass assessment that can inform ongoing monitoring programmes.

Although in their infancy, environmental monitoring schemes are increasing in popularity and funding, allowing scale up of protection for this much needed species.