Sinking feelings
However, much less recognised is the amount of organic carbon stored in our oceans. So called 'blue carbon' initiatives try to change this. It includes the Blue Carbon research initiative by the GRID-ARENDAL centre, which supports the United Nations Environment Programme (see also our 2011 dossier 'Ocean Views').
In 2009, the project published a report according to which around 51% of atmospheric carbon captured by living organisms is taken up at sea. Of this amount an estimated 71% is fixed by coastal ecosystems, which include mangroves, salt marshes, seaweed and the often extensive beds or meadows of seagrasses.
However, seagrasses are now among the world's most threatened ecosystems. It is estimated that factors such as dredging and filling activities and the degradation of water quality through poor land-use practices have led to the loss of around 30% of seagrasses that existed at the beginning of the 20th century. Not only does this decrease the potential uptake of carbon from the atmosphere, but it also releases organic carbon stably stored in soils under seagrass meadows back into the ocean-atmosphere CO2 pool.
...Money still makes the work go round
Compared to terrestrial ecosystems few studies have addressed carbon sequestration and cycling in coastal ecosystems and this is especially the case with seagrasses and seaweeds.
This was highlighted by a Nature Geoscience article published by researchers including from the University of Western Australia's Ocean Institute in May 2012.
The study analysed compiled published and unpublished measurements of carbon stored in seagrass meadows around the world. This led to a conservative estimate that seagrass ecosystems store between 4.2 and 8.4 petagram carbon. Per unit area the carbon storage capacity of seagrass is similar to that of forests and stably accumulates over millennia.
Seagrass meadows, of which the largest areal stocks are found in the Mediterranean, may thus be far more important carbon sinks than previously realised, the researchers write.
However, they estimate that at present rates of seagrass loss up to 299 terragram carbon per year could be transformed from its organic storage form into inorganic molecules, such as CO2.
This has not only ecological but also economic implications. Thus, there could be a significant monetary value of the carbon fixed in seagrass meadows if it is accounted for in a carbon offset scheme.
In 2011, the Ocean Institute reported preliminary research suggesting that the extensive seagrass meadows in WA’s Shark Bay could store around $350 million tonnes of carbon equivalents. The researchers calculated that a carbon price of $23 per tonne of emitted carbon this would equate to $8 billion (see also our ARDR dossier 'Ocean Views' from 2011).
Complicating the issue is that so far studies on the carbon sink characteristics of seagrass are mainly based on a single species, Posidonia oceanica, which is commonly found in the Mediterranean Sea. Yet it is likely that the variability among seagrass species and their range of habitats affects their carbon storage potential.
In September 2013, a study led by Edith Cowman University researchers was published in the journal PlosOne in September 2013 which investigated 17 Australian seagrass habitats encompassing 10 different species. This equates to around 1/3 of all Australian seagrass species. Based on an analysis of the top 24 cm of sediment, the study shows an 18-fold variation in the carbon storage capacity of different habitats. Taking this into consideration, the researchers estimated that the carbon stored in the seagrass sediments may be only around one-third of that calculated based on datasets from the Mediterranean Sea.
Nevertheless, based on this calculation the Australian seagrass ecosystem, which covers around 92,500 square kilometres of Australia’s coastline, could store a total of around 155 million tonne of carbon. Assuming a 2014-15 fixed carbon price of $25.40 and a market price of $35 per tonne of carbon by 2020 the researchers calculated a total value of our seagrass meadows of between $3.9 and $5.4 billion.
The authors estimated that per year Australian seagrass meadows take up around 1 million tonne or around 0.6% of the country's total annual CO2 emissions. According to the authors, this emphasises the value of this ecosystem for the nation.
All they need is light...
One factor that poses a threat to seagrass meadows is the rise in sea levels as in deeper waters seagrass may be deprived of the light it needs to stay alive.
However, research from the University of Queensland's Global Change Institute and the ARC Centre of Excellence for Environmental Decisions suggests that early intervention could be effective in minimising the impact.
In August 2013, the researchers published a study in the journal Global Change Biology, in which they used Australia's Moreton Bay as a laboratory to investigate seagrass meadows under the scenario of rising sea levels.
Moreton Bay is listed as a an area of international importance under a convention on wetlands known as the Ramsar Convention.
The researchers used a habitat distribution model to predict the interactive effects of known factors stressing seagrasses - sea level rise, changes in water clarity and land use. They estimated that under a scenario of water levels rising by 1.1 metres, as may occur by the end of the century, seagrass cover will decline by 17%.
However, they found that the potential loss could be reduced to around 5% through a coastal retreat strategy designed to improve the light conditions at the deep edge of seagrass meadows. This would include the removal of impervious surfaces, such as roads and houses, from newly inundated regions.