Abstract
The current estimates of carbon stocks beneath seagrass meadows worldwide are based on limited data, leading to a tendency to generalise global carbon stocks from a very small number of seagrass habitats. We surveyed Posidonia oceanica and Posidonia sinuosa meadows along depth-induced gradients of light availability to assess the variability in their sedimentary organic carbon (Corg) stocks and accretion rates. This study revealed a fourfold decrease in Corg stocks from 2–4 m to 6–8 m depth in P. sinuosa meadows (averaging 7.0 and 1.8 kg m−2, respectively; top metre of sediment) and a fourteen- to sixteenfold decrease from shallow (2 m) to deep (32 m) P. oceanica meadows (200 and 19 kg m−2 average, respectively; top 2.7 m of sediment). The average Corg accretion rates in shallow P. sinuosa meadows were higher (10.5 g m−2 yr−1) than in deeper meadows (2.1 g m−2 yr−1). The reduction of sedimentary Corg stocks and accretion rates along depth-related gradients of light reduction suggests that irradiance, which controls plant productivity, meadow density, and sediment accretion rates, is a key environmental factor affecting the Corg storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows in the Balearic Islands (Spain), containing the highest areal Corg stocks of all seagrasses (estimated at up to 691–770 kg m−2 in 8–13 m thick deposits). Seagrass communities are experiencing a worldwide decline, and reduced irradiance (following, for example, eutrophication or sediment regime alterations) will lead to photoacclimation responses (i.e., reduced plant productivity and shoot density), which may impact the carbon sequestration capacity of seagrasses.
