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Blue carbon biosequestration

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Dr Peter Macreadie, Australian Research Council (ARC) DECRA Fellow at the School of Life Sciences at the University of Technology Sydney, explains how coastal ecosystems like seagrasses, saltmarshes and mangroves, help to capture and store carbon, thus reducing carbon emissions. Green carbon is carbon that is captured and stored by things on land that are green, such as trees. Blue carbon refers to the carbon that is captured and stored by the oceans. The term blue carbon was only coined in 2009 in a report that looked at carbon sequestration. The report highlighted the habitats responsible for this carbon capture process (seagrasses, saltmarshes, mangroves) and that much of the carbon captured comes from terrestrial sources, like run off, and the sea, where it is brought in by plankton.


One of the reasons that seagrasses, saltmarshes and mangroves are such great carbon sinks is that they create anoxic environments. Because there is not much oxygen beneath the ground in those habitats, microbes that could contribute to liberation of carbon dioxide are quite dormant. As a consequence, the carbon passes through that microbial gauntlet very efficiently, thus being sequestered under the surface continually over long periods of time. Anything that disrupts that anoxic environment, however, can switch on these microbes, which then can use the ancient stores of carbon for their metabolism, potentially releasing the captured carbon back into the atmosphere as carbon dioxide.

Seagrasses, saltmarshes and mangroves are disappearing at an alarming rate. NSW has lost half its seagrasses and worldwide, the disturbances of these habitats releases the equivalent of the carbon emissions of the United Kingdom and Canada combined, at a cost of billions of dollars, if carbon had a price. All of these habitats are at coastal interfaces, where human activity is high. These are the places people want to live, to moor their boats and to build jetties and are also the places where runoff enters the sea. 

Such habitats are often called the coalmine canaries of coastal ecosystems as they are one of the first systems to disappear when there are disturbances. They are very susceptible to stirred up sediments and decreased sunlight levels, so dredging is a major issue. The addition of nutrients is another. Run off into coastal shallows can lead to increases in algal blooms, which block the light source of the plants below the surface, affecting their photosynthesis and hence their carbon capture.
         Above: core from sea grass bed.                                   Below: processing cores at Fagans Bay

There are other issues as well, not least being the many artificial barriers humans have created to stop the effects of rising sea water. Seagrasses, the saltmarshes and the mangroves need time to adapt to any rising sea levels but have no space to adapt when barriers define where they can grow. Dr Macreadie advocates building green infrastructure rather than solid and concrete ones (grey infrastructure), preferring natural buffers against extreme weather events and sea level rise.

Grey infrastructure is cheap in the short term but green infrastructure has long term benefits. In the USA companies like Dow Chemicals, Unilever and Chevron have undertaken pilot projects for green infrastructure showing that it has economic benefits over grey infrastructure. Green infrastructure doesn’t depreciate and doesn’t need a lot of maintenance and also provides other benefits for marine ecosystems, improving fisheries and the like. A recent Nature Geosciences paper estimated that a sea grass meadow of 1km² captures and stores for the long term, up to 83,000 tons of carbon, equivalent to the annual emissions of 4,500 Australians or 92,000 Bhutanese or the annual carbon emissions of Vanuatu.

Dr Macreadie is currently based at the School of Life and Environmental Sciences at Deakin University in Victoria campaigning with companies that have a presence along its shoreline and with others that have a corporate or ethic responsibility to offset their carbon emissions. He is really pushing for the rehabilitation of coastal areas in Victoria and NSW, replacing grey infrastructure with green infrastructure. There has been some interest from commercial groups but it is still early days. Left: collecting sediment samples.

Bio sequestration is not new. It is the same process that gave us fossils fuels in the first place and it is recognised by both sides of federal politics. What is new is the knowledge that many parts of the food web can also affect carbon sequestration. Overfishing, for, instance, leads to an increase in bugs and animals in the sediments and since their activities effectivey punch holes in those sediments, such action can potentially increase the number of microbes that will release sequestered carbon if they become active.

Left: microbial samples
Sharks are another example of ecosystem links in food webs. Sharks regulate the populations of mega herbivores like dugongs and turtles (through fear and not predation) which has an effect on the amount of plant material that is being consumed by such mega herbivores, essentially maintaining a balance in those marine ecosystems.

In remote island locations, where top predators like sharks are lost, the increased populations of mega herbivores, (eg turtles) are forced to graze the below-surface plant material (something they would not normally do), eating the roots and rhizomes of the plants. This is a self destructive action since the plants cannot recover from such predation.

It seems that carbon biosequestration acts rather like a glue that helps binds coastal marine ecosystems together. Blue carbon needs blue gums.

For more information about Peter Macreadie's research projects:
visit his website
or watch some of his videos:

Seagrass is awesome: http://www.youtube.com/watch?v=JaJG-xQJrvY

Blue carbon research in Jervis Bay: http://www.youtube.com/watch?v=yBWsS72VDxE

Can they save the seagrass? http://www.youtube.com/watch?v=H_ynQbqZbRA



Dr Peter Macreadie was interviewed by Ruby Vincent for A Question of Balance. Images from Dr Macreadie. Summary text by Victor Barry, April 2014, re edited November 2015.

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