Posidonia Oceanica (5) - Seagrass demise causing huge carbon release

According to research published this week in Nature Geoscience (DOI: 10.1038/ngeo1477) we could reach an extreme greenhouse situation much sooner than previously calculated as dying seagrass meadows release vast quantities of stored heat-trapping carbon.

An international team of researchers, collating old and new data from 946 seagrass meadows around the world, estimated that seagrasses capture 27.4 million tonnes of carbon each year, burying it in the substrate beneath their roots. Unlike forests that hold carbon for about 60 years then release it again, seagrass ecosystems have been capturing and storing carbon since the last ice age.

This means that up to 19.9 billion tonnes of carbon are currently stored within seagrass plants and the top metre of soil beneath them. If the seagrass dies, all of that could be released into the environment, says marine ecologist and study author James Fourqurean from Florida International University in Miami, US.

In the past 127 years, 29 per cent of seagrass has been destroyed world-wide, mostly by water pollution, dredging for new developments, and climate change. With seagrass meadows disappearing at an annual rate of about 7 per cent since the 1990s, 299 million tonnes of carbon are already being released each year.

Gary Kendrick, a co-investigator on the project from University of Western Australia at Crawley, Australia, says "These are scary numbers. It would put us very much into the extreme of greenhouse situations very very quickly."

This view is reinforced in another study published in Nature Climate Change (DOI: 10.1038/nclimate1533). Gabriel Jorda from the Mediterranean Institute for Advanced Studies in Esporles, Spain, reported that warming sea temperature is eradicating the Mediterranean seagrass Posidonia oceanica, which is likely to be extinct before 2050 if protection of this species is not increased. This is particularly concerning because Posidonia oceanica holds about 10 times as much carbon as other seagrass species.

Seagrass can hold up to 83,000 tonnes of carbon per square kilometer, more than twice the 30,000 tonnes of carbon per square kilometer a typical terrestrial forest can store. In addition to storing carbon, seagrasses filter out sediment before it gets into oceans, protect coastlines from floods and storms and serve as habitat for fish, crustaceans and other commercially important species.

Some of the study's authors are affiliated with the Blue Carbon Initiative, a global plan to mitigate climate change by conserving and restoring coastal marine ecosystems. The initiative is a collaboration between UNESCO, the International Union for Conservation of Nature and Conservation International.

Transplantation efforts have generally failed, but watershed management to improve water quality and habitat remediation are effective strategies in halting seagrass decline.