Major Seagrass Carbon Sinks Worldwide, Shark Bay, Western Australia

Ariane Arias-Ortiz1, Oscar Serrano2,3, Pere Masque1,2, Paul Lavery3, Carlos M Duarte4,5 and Gary A Kendrick2,6, (1)Universitat Autònoma de Barcelona, Física and Institut de Ciència i Tecnologia Ambientals, Barcelona, Spain, (2)The University of Western Australia Oceans Institute, University of Western Australia, Crawley, Australia, (3)School of Natural Sciences, Centre for Marine Ecosystems Research, Edith Cowan University, Crawley, Australia, (4)Institut Mediterrani d’Estudis Avançats IMEDEA (CSIC-UIB), Global Change Research, Esporles, Spain, (5)King Abdullah University of Science and Technology, Red Sea Research Center (RSRC), Thuwal, Saudi Arabia, (6)University of Western Australia, The School of Plant Biology, Crawley, 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 (Corg). We surveyed 30 seagrass meadows in Shark Bay accounting for species composition, seagrass contribution to the sedimentary Corg pool, and habitat variability. The sediment accumulation rates (SAR) and Corg 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 Corg stocks in each meadow. Corg stocks accumulated in the last century varied from 0.4 to 4.5 kg Corg m-2, with an average burial rate of 24 ± 11 g Corg m-2 y-1 (10 – 20 cm-thick deposits). Stocks in the top meter ranged from 4 to 30 kg Corg m-2, which is equivalent to a long-term carbon burial rate averaging 8 ± 5 g Corg 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 Corg in the top meter, which would represent about 1% of the Corg stored in seagrass meadows worldwide. Spatial and temporal variability in Corg storage results from various factors, including biological (e.g. net primary production), chemical (e.g. recalcitrance of Corg 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 Corg. Moreover, sediments with highest Corg stocks were characterized by high δ13C, suggesting that the plant itself plays a key role in Corg 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 Corg.