Deep-water seagrass is sequestering carbon, mitigating climate change.

Photosynthetic marine plants (especially seagrasses, tidal marshes and mangroves) “eat” carbon dioxide and release the oxygen component. Their residual biomass becomes part of the sea floor sediment, where their carbon remains for millennia. This sequestration occurs much more efficiently and lasts much longer than that of terrestrial forests.  Marine-sequestered carbon is called “blue” carbon.  

Most seagrass research has focussed on shallow-water species that are easier to access.  Deepwater (> 15m ) species are generally smaller and more sparsely populated. Their carbon sequestration has been assumed to be less significant – probably not much more than that found in bare sandy areas of the Reef lagoon.  

A research project based at the Australian Museum’s Lizard Island Research Station led by Paul York, Peter Macreadie and Michael Rasheed disproves this assumption. It shows levels of organic carbon (OC) in deep-water seagrass (Halophila species) are similar to those in shallow species (e.g. Halodule uninervis).  It also found deep seagrass sediments contain around 9 times more OC than surrounding bare areas.  See Blue Carbon stocks of Great Barrier Reef deep-water seagrasses (2018) 


Lizard Island blue carbon survey sites © TropWATER


a Shallow Halophila © TropWATER

b Mid Halophila © TropWATER

c Deep Halophila © TropWATER

d Halodule © TropWATER

e Thalassia © TropWATER


Other research estimates Halophila seagrass meadows in the Great Barrier Reef lagoon occupy a total area of 31,000 km2, roughly the size of Switzerland.  Paul York and his colleagues calculate that if OC stocks throughout this area prove similar to those found at Lizard Island, the total carbon resource stored by deep-water seagrass on the Reef would be approximately 27.4 million tonnes.

They recommend further research to validate their findings to a depth of 1 metre (in their Lizard Island study they surveyed only the top 30 cm of the lagoon floor) and to confirm that deep water OC counts are similar elsewhere in the Reef lagoon.  They also highlight the need for further study of deep-water seagrass on a global scale.  

Natural and man-made factors affecting greenhouse gases and climate change are vast and complex. As just one instance, seagrass meadows also precipitate calcium carbonate.  This releases carbon dioxide, offsetting benefits seagrass provides in long-term carbon sequestration. Further research is required to quantify such offsets.   

This work was supported by three research grants. Paul York and Peter Macreadie received a LIRRF Isobel Bennett Marine Biology Fellowship, funded by the Hermon Slade Raiatea Foundation. Peter Macreadie and Michael Rasheed were supported by an Australian Research Council Linkage Grant (LP160100492).  Peter was also supported by an Australia Research Council DECRA Fellowship (DE130101084). Their field research at Lizard Island is described in Seagrass & blue carbon and the embedded “Top Gun” video.