Blue Carbon stocks of Great Barrier Reef deep-water seagrasses

Shallow-water seagrasses capture and store globally-significant quantities of organic carbon (OC), often referred to as ‘Blue Carbon’; however, data is lacking on the importance of deep-water (>15 m) seagrasses as Blue Carbon sinks. We compared OC stocks from deep-, mid- and shallow-water seagrasses at Lizard Island within the Great Barrier Reef Lagoon. We found deep-water seagrasses (Halophila species) contained similar levels of OC as shallow-water species (e.g Halodule uninervis) (0.64 ± 0.08% and 0.9 ± 0.1 mg C cm3, 0.87 ± 0.19% and 1.3 ± 0.3 mg C cm3, respectively), despite being much sparser and smaller in stature. Deep-water seagrasses sediments contained significantly higher levels (~9-fold) of OC than surrounding bare areas. Inorganic carbon (CaCO3) levels were relatively high in deep-water seagrass sediments (8.2 ± 0.4%), and if precipitated from epiphytes within the meadow, could offset the potential CO2-sink capacity of these meadows. The δ13C signatures of sediment sampl...