Variability in the Carbon Storage of Seagrass Habitats and Its Implications for Global Estimates of Blue Carbon Ecosystem Service

The recent focus on carbon trading has intensified interest in ‘Blue Carbon’–carbon sequestered by coastal vegetated ecosystems, particularly seagrasses. Most information on seagrass carbon storage is derived from studies of a single species, Posidonia oceanica, from the Mediterranean Sea. We surveyed 17 Australian seagrass habitats to assess the variability in their sedimentary organic carbon (C(org)) stocks. The habitats encompassed 10 species, in mono-specific or mixed meadows, depositional to exposed habitats and temperate to tropical habitats. There was an 18-fold difference in the C(org) stock (1.09–20.14 mg C(org) cm(−3) for a temperate Posidonia sinuosa and a temperate, estuarine P. australis meadow, respectively). Integrated over the top 25 cm of sediment, this equated to an areal stock of 262–4833 g C(org) m(−2). For some species, there was an effect of water depth on the C(org) stocks, with greater stocks in deeper sites; no differences were found among sub-tidal and inter-tidal habitats. The estimated carbon storage in Australian seagrass ecosystems, taking into account inter-habitat variability, was 155 Mt. At a 2014–15 fixed carbon price of A$25.40 t(−1) and an estimated market price of $35 t(−1) in 2020, the C(org) stock in the top 25 cm of seagrass habitats has a potential value of $AUD 3.9–5.4 bill. The estimates of annual C(org) accumulation by Australian seagrasses ranged from 0.093 to 6.15 Mt, with a most probable estimate of 0.93 Mt y(−1) (10.1 t. km(−2) y(−1)). These estimates, while large, were one-third of those that would be calculated if inter-habitat variability in carbon stocks were not taken into account. We conclude that there is an urgent need for more information on the variability in seagrass carbon stock and accumulation rates, and the factors driving this variability, in order to improve global estimates of seagrass Blue Carbon storage.