UNESCO's International Day for Biological Diversity on May 22 focussed strongly on conserving our marine diversity. One of the important marine ecosystems are the seagrass meadows around the coasts of the world. A new global scientific research study just released has shown that seagrass meadows store significantly more carbon than any land based forest. They are very important as carbon sinks. But they are also suffering a major decline due to pollution from agricultural and mining development and chemical runoff, coastal development changing water turbidity upsetting photosynthesis in seagrass, and increasing sea surface temperatures affecting seagrass growth due to global warming.
The new global study of seagrass meadow ecosystems has found that coastal seagrass beds store much more carbon than can be stored in even the most carbon dense forests, such as the temperate native forests of Victoria. Seagrass meadows can store up to 83,000 metric tons of carbon per square kilometre, mostly in the soils below them. In comparison, a typical land forest stores around 30,000 metric tons per square kilometre mostly as wood. It is the first global study to analyze the carbon storage capacity in seagrasses.
Seagrass meadows support a high level of biodiversity with many fish species using the seagrass during their juvenile period, as well as providing homes for crabs, sea urchin, seahorses shrimp and prawns. Sea tutles, dugongs and manatees depend on seagrass meadows as a primary food source.
Results were gathered for the study from 3640 observations of 946 distinct seagrass meadows across the globe, including from east and west coasts of Australia, the Caribean, and Mediterranean Sea. Seagrass meadows store ninety per cent of their carbon in the soil which can accumulate over centuries. Although seagrass meadows occupy less than 0.2 per cent of the world's oceans, they are responsible for more than 10 per cent of all carbon annually buried in the ocean. They rival the carbon storage capacity in the extensive peat deposits of mangroves. Some of the seagrass sites in the Mediterranean store carbon in deposits many meters deep.
"Seagrasses only take up a small percentage of global coastal area, but this assessment shows that they're a dynamic ecosystem for carbon transformation," said James Fourqurean, the lead author of the paper and a scientist at Florida International University. "Seagrasses have the unique ability to continue to store carbon in their roots and soil in coastal seas," said Fourqurean. "We found places where seagrass beds have been storing carbon for thousands of years."
The global study - Seagrass ecosystems as a globally significant carbon stock (abstract) - was published online in Nature Geoscience, with contributions from scientists at the Spanish High Council for Scientific Investigation, the Oceans Institute at the University of Western Australia, Bangor University in the United Kingdom, the University of Southern Denmark, the Hellenic Center for Marine Research in Greece, Aarhus University in Denmark and the University of Virginia.
Professors Gary Kendrick and Carlos Duarte from the University of Western Australia Professors contributed to the study, which was led by Dr James Fourqurean, a professor of biology at Florida International University.
"These results show that seagrass meadows are key sites for carbon storage and probably are far more important as carbon dioxide sinks than we realised," Professor Kendrick said.
But seagrasses are also among the world's most threatened ecosystems, and industrialisation and development of our coastal areas has seen a marked decline in seagrass meadows and their capacity to store carbon. A study published in December 2011 found that Coastal ecosystems suffer 100 fold decrease in capacity to store carbon mitigating climate change.
Around 29 per cent of all historic seagrass meadows have been destroyed, mainly due to dredging and degradation of water quality. We are currently losing a further 1.5 per cent of seagrass meadows each year. The study estimated that emissions from destruction of seagrass meadows can potentially emit up to 25 per cent as much carbon as deforestation on land.
According to the Australian CRC Reef Research Centre, Australia has more seagrass species than any other continent. Of some 60 seagrass species found worldwide, 30 species are found in Australia and 15 species are found in Queensland. Port development for the expansion and export of coal along the Queensland coast, along with the ten fold increase in shipping through the World Heritage Listed Great Barrier Reef Marine Park, is impacting the coastal marine environment. Marine Park Authority chairman Russell Reichelt recommended a port rethink in a special ABC Four Corners report in November 2011 on Health of the Reef at the crossroads.
But elevated sea surface temperatures resulting from global warming is also impacting the viability of some species of seagrass. A study published in Nature Climate change on 20 May 2012 - Mediterranean seagrass vulnerable to regional climate warming (Full article) - explicitly warns that the seagrass variety Posidonia oceanica may be headed for functional extinction due to global warming heating up seawater temperatures. The full abstract reads:
The Mediterranean Sea, one of the regions warming fastest under climate change, harbours lush seagrass (Posidonia oceanica) meadows that form the basis for a key ecosystem in the region. Recent field results have shown that increased maximum annual seawater temperature in the Mediterranean has already led to increased seagrass mortality. Here we project the trajectory of P. oceanica meadows under the warming expected in the western Mediterranean through the twenty-first century to conclude that warming will lead to the functional extinction of P. oceanica meadows by the middle of this century (year 2049±10) even under a relatively mild greenhouse-gas emissions scenario. Efforts to alleviate local stresses adding to the loss of P. oceanica meadows will have a limited effect in conserving the meadows under climate change. Efforts to mitigate climate change are urgently needed to preserve this key ecosystem.
But there is a ray of hope if we can reduce agricultural and mining chemical runoff, and water turbidity from shipping and coastal development. Seagrass meadows can be restored in some environments which can restore the ecosystem diversity and balances and allow significant carbon sink uptake capacity.
"The good news is if seagrass meadows are restored they can effectively and rapidly re-establish lost carbon sinks and stores as well providing a range of other valuable ecosystem benefits, including water quality protection, and as an important biodiversity habitat," Professor Kendrick said.
Other ecosystem benefits of restoring and conserving seagrass meadows include filtering sediment from the oceans; protecting coastlines against floods and storms; and serving as habitats for fish and other marine life. With rising sea levels seagrass will provide some level of protection from storm surges.
Significantly, UNESCO celebrated today - May 22nd, 2012 - as International Day for Biological Diversity with a focus on conserving our marine diversity. Preserving and restoring Seagrass meadows should definitely be a priority both for it's biological diversity and to help us mitigate climate change through storage of excess carbon we have pumped into the atmosphere.
- James W. Fourqurean et al, Nature Geoscience, 20 May 2012, Seagrass ecosystems as a globally significant carbon stock (abstract) doi:10.1038/ngeo1477
- Media Release, University of Western Australia, 21 May 2012 - New global study: seagrasses can store as much carbon as forests
- Media Release, National Science Foundation, 21 May 2012 - Seagrasses Can Store as Much Carbon as Forests
- Gabriel Jordà, Núria Marbà, & Carlos M. Duarte, Nature Climate change, 20 May 2012 - Mediterranean seagrass vulnerable to regional climate warming (Full article)
- Image of Ribbon seagrass (Cymodocea rotundata) in the middle of the seagrass lagoon of Chek Jawa, Singapore. Photo: Ria Tan (2010) www.wildsingapore.com Creative Commons CC BY-NC-ND 2.0