Seagrass loss linked to greenhouse gas emissions

An international team of researchers has found that the disappearance of seagrass meadows could be contributing to the release of carbon dioxide that has been stored for centuries under the sea.

The team of six researchers from Spain and Australia studied the impact of disappearing seagrass meadows, Posidonia australis, at Oyster Harbour in Albany, Western Australia, where long-term restoration of seagrass has been highly successful.

These unusual marine flowering plants are called ‘seagrasses’ because in many species the leaves are long and narrow and often grow in large “meadows that look like grassland”. But they also account for more than 10% of the ocean’s total carbon storage and, per hectare, they hold twice as much carbon dioxide as rainforests.

Every year, seagrasses sequester about 27 million tonnes of CO2 but global warming is expected to cause some species to become extinct, resulting in the release of vast quantities of CO2 back into the atmosphere.

In the latest study, the team of scientists used sediment-dating techniques to quantify the accumulation of carbon in repopulated areas and calculate the erosion of carbon in areas that were not revegetated.

Writing in a paper published in the Journal of Ecology, the researchers say the results suggest that restoring seagrass meadows would prevent the erosion of important deposits of organic carbon.

Meadows act as carbon sinks

Lead author Professor Núria Marbà, from the Mediterranean Institute for Advanced Studies in Spain, said the loss of underwater seagrass meadows posed two problems: “These areas can no longer capture and store atmospheric carbon dioxide, and they can become a source of this gas by eroding and freeing decades-old, and even centuries-old carbon stored in the meadow,” Marbà said.

“Seagrass meadows act as carbon sinks on a global scale and the conservation and restoration of heavily impacted areas can help mitigate man-made emissions.”

The researchers assessed whether revegetation of underwater meadows was effective in restoring their capacity to act as carbon sinks in relation to the time needed to achieve this over decades.

Marbà said revegetation of meadows prevented the erosion of these organic carbon deposits that had accumulated over hundreds of years. The study results indicated that the loss of this ecosystem must have also represented an important loss in its capacity to sequester and store carbon in the sediments of underwater meadows.

Oyster Harbour is colonised by a meadow of Posidonia australis that was mostly lost from the 1960s to the end of the 1980s. After 1994, the meadow recovered, in part because of revegetation efforts under the direction of co-author Geoff Bastyan, an honorary research fellow at the University of Western Australia’s school of plant biology.

Bastyan was named a “Southseas Oceans Hero” last year, an award given annually to a champion from the community whose work addresses solutions to degradation and loss of ocean resources. The seagrass restoration project at Oyster Harbour was carried out until 2006 and has become the most successful seagrass restoration project in the world.

Another co-author of the study, Professor Pere Masqué from the Autonomous University of Barcelona, Spain, said the potential areas available worldwide to carry out seagrass revegetation projects were enormous.

“These can help reconstruct carbon sinks, as well as preserve older deposits,” Masqué said.

The results of the study will help dispel doubts that were hindering the development of ‘blue carbon’ strategies in underwater meadows. ‘Blue carbon’ is the term given to carbon captured by marine and coastal ecosystems in the form of biomass and sediments.