Impact of Shoot Density on Corg Accumulation in Seagrass Sediments

Seagrass meadows are important global carbon sinks due to their ability to capture and bury large amounts of CO₂. Recent works have evidenced the potential of seagrass habitat restoration to enhance carbon sequestration in coastal zones, a practice that may be considered as a strategy for GHG emissions mitigation. Organic carbon in seagrass sediments, known as “blue carbon,” accumulates not only from in situ production but also from sedimentation of particulate carbon from the water column, as the seagrass canopies facilitate sedimentation and reduce resuspension and erosion. As a result, there is a net transfer of allochthonous carbon to the sediments of seagrass meadows that enhances their capacity for carbon sequestration. The trapping capacity of the canopy is not linearly correlated to the shoot density, since other factors such as leave bending interfere with the particle free sinking within the canopy. Here we assess the effect of Posidonia oceanica shoot density in particle sequestration and its implications in C_org accumulation. Sediment and carbon accumulation rates were determined by means of ²¹⁰Pb in meadows across a shoot density gradient ranging from 0 to 850 shoots m^-2. Organic carbon stocks accumulated in the last 100 years and burial rates ranged from 2.3 to 4.6 kg C_org m^-2 and from 17 ± 2 to 31 ± 6 g C_org m^-2 y^-1, respectively. Maxima accumulation occurred at meadows with a moderate projected surface area (LAI = 3, ca. 210 shoots m^-2). We use δ¹³C to evaluate whether this results from a higher input of allochthonous particles, as sedimentation rates were 0.063 ± 0.004 g·cm^-2·y^-1, about 20% higher than average sedimentation rates (0.053 ± 0.008 g cm^-2 y^-1). The accumulation of allochtonous carbon may be key to enhance the carbon sink capacity of seagrass meadows. Shoot density increases the magnitude of this process when density values are moderate in P. oceanica meadows, which may be of interest for restoration purposes.