Major Seagrass Carbon Sinks Worldwide, Shark Bay, Western Australia

Seagrasses are marine foundation species that provide valuable ecosystem services including the stabilization of sediment, carbon dioxide sequestration, and habitat for diverse fauna and flora. Shark Bay, Western Australia, registered as a World Heritage Property, has the largest reported assemblage of seagrass meadows worldwide, thus has an important role in producing, sequestering and storing organic carbon (C_org). We surveyed 30 seagrass meadows in Shark Bay accounting for species composition, seagrass contribution to the sedimentary C_org pool, and habitat variability. The sediment accumulation rates (SAR) and C_org accumulation over short and long terms were determined by means of ²¹⁰Pb and ¹⁴C dating. Sediment grain size was used to characterize sedimentary environments and δ¹³C analyses to determine the sources of sedimentary C_org stocks in each meadow. C_org stocks accumulated in the last century varied from 0.4 to 4.5 kg C_org m^-2, with an average burial rate of 24 ± 11 g C_org m^-2 y^-1 (10 – 20 cm-thick deposits). Stocks in the top meter ranged from 4 to 30 kg C_org m^-2, which is equivalent to a long-term carbon burial rate averaging 8 ± 5 g C_org m^-2 y^-1 (over the last millennia). With an area of 4,000 km², seagrass meadows in Shark Bay store the vast amount of 45 ± 23 Tg C_org in the top meter, which would represent about 1% of the C_org stored in seagrass meadows worldwide. Spatial and temporal variability in C_org storage results from various factors, including biological (e.g. net primary production), chemical (e.g. recalcitrance of C_org stocks) and geological (e.g. hydrodynamic energy and sediment accumulation rates). Higher SAR and smaller sediment size appeared to contribute to a higher accumulation and preservation of C_org. Moreover, sediments with highest C_org stocks were characterized by high δ¹³C, suggesting that the plant itself plays a key role in C_org storage. These findings combined with sediment chronologies help us to understand the formation of seagrass deposits, and their potential to change the depositional environment and produce large stable reservoirs of C_org.