Seagrass blue carbon stocks in the German Baltic Sea

A total of 169 sediment cores (30 cm length; 5.5 cm inner diameter) were sampled in seagrass meadows (n = 110 cores) and nearby unvegetated sediments (n = 59 cores). Nine cores (with some exceptions) were collected at each site, from three sublocations: (1) in the high-density part of the meadow ("dense seagrass" hereafter), (2) low density or fringe of the meadow ("sparse seagrass" hereafter), and (3) adjacent unvegetated sediments at least 5 m from seagrass ("unvegetated"). Dense and sparse seagrass sublocations are collectively referred to as "seagrass-vegetated" sediments or sublocations. Sediment cores taken within the seagrass meadow were sampled at least 10 m apart from each other. Several biophysical parameters were collected alongside each core, including seawater depth, sediment grain size, current velocity at the seafloor, and seagrass complexity. The measure of "seagrass complexity" was defined as the product of seagrass canopy height and shoot density, to obtain the sum of leaf heights within a unit of area (in m/m2). Cores were collected between 1 and 5 m seawater depth manually via self-contained underwater breathing apparatus (SCUBA) divers pounding impact-resistant PVC tubes into the sediment with a rubber mallet. Sediment total Corg was determined using an Elemental Analyzer (EURO EA Elemental Analyzer).