Influence of water depth on the carbon sequestration capacity of seagrasses

The actual estimates of carbon stocks beneath seagrass meadows worldwide are derived from few data, resulting in a tendency to generalize global carbon stocks from a very limited 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 (C-org) stocks and accretion rates. This study showed a fourfold decrease in C-org stocks from 2-4m to 6-8m depth P. sinuosa meadows (averaging 7.0 and 1.8kgm(-2), respectively; top meter of sediment) and a fourteenfold to sixteenfold decrease from shallow (2m) to deep (32m) P. oceanica meadows (200 and 19kgm(-2) average, respectively; top 2.7m of sediment). The average C-org accretion rates in shallow P. sinuosa meadows were higher (10.5gm(-2) yr(-1)) than in deeper meadows (2.1gm(-2) yr(-1)). The reduction of sedimentary C-org stocks and accretion rates along depth-related gradients of light reduction suggests that irradiance, controlling plant productivity, meadow density, and sediment accretion rates, is a key environmental factor affecting C-org storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows at Balearic Islands (Spain), containing the highest areal C-org stocks of all seagrasses (estimated in up to 691-770kgm(-2) in 8-13m thick deposits). Seagrass communities are experiencing worldwide decline, and reduced irradiance (following e.g., 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.