The impact of sediment grain size on the carbon storage potential of intertidal seagrass meadows in the northern Wadden Sea

The Wadden Sea comprises the largest tidal flats of the world and extensive intertidal seagrass meadows occur in its northern part. However, their carbon storage potential is largely unknown. Belowground burial of carbon was assessed from different locations in 4 seagrass meadows. All meadows are large (76 – 441 ha), with a similar seagrass cover density (60 – 80 %), species composition (strongly dominated by Zostera noltei) and comparable age (> 90 years). The major difference is the sediment: 3 meadows have established on sandy tidal flats and 1 on mud flats. Sediment cores were taken down to 45 - 65 cm depth and organic matter was measured along vertical profiles. In the sandy sediments, the permanent carbon storage was really low, with an organic carbon content below 0.4 % (average 0.22 – 0.38 %). Input to carbon storage originates mainly from internal biomass production and the dominant species Zostera noltei is relatively small (leaf length about 15 cm) and sheds its leaves in winter, which are carried away by the current. However, in muddy sediment the carbon content was also low but with 1.1 % about 3 times higher. Muddy sediments have a low hydraulic connectivity and the sediment is waterlogged also during air-exposure at low tide. Waterlogged conditions make the sediment more anoxic, generating more reduced conditions which slows down decomposition. Furthermore, muddy sediment particles have a larger surface-to-volume ratio, allowing more organic material to adhere to the particles. Also, the sandy sediments occur in higher energy areas, where resuspension is more common, facilitating export of organic matter and lower burial rates.