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 are lacking on the importance of deep-water (greater than 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 (GBR) lagoon. We found deep-water seagrass ( Halophila species) contained similar levels of OC to shallow-water species (e.g. Halodule uninervis) (0.64 ± 0.08% and 0.9 ± 0.1 mg C cm^-3, 0.87 ± 0.19% and 1.3 ± 0.3 mg C cm^-3, respectively), despite being much sparser and smaller in stature. Deep-water seagrass sediments contained significantly higher levels (approx. ninefold) 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 samples varied among depths and habitats (-10.9 and -17.0), reflecting contributions from autochthonous and allochthonous sources. If the OC stocks reported in this study are similar to deep-water Halophila meadows elsewhere within the GBR lagoon (total area 31 000 km²), then OC bound within this system is roughly estimated at 27.4 million tonnes.