Limited Blue Carbon Potential of Intertidal Seagrass Meadows in the Wadden Sea – A Case Study in a Tidal Basin

Abstract. Seagrass meadows are widely recognized as a key blue carbon ecosystem. However, their sequestration potential varies remarkably between species and regions. In the world’s largest tidal flat system, the Wadden Sea, the highly dynamic hydro-morphological environment affects carbon cycling, making a quantitative analysis of the sequestration potential of seagrass meadows extremely challenging. To fill this gap, we applied a coupled hydro-eco-morphodynamic model which resolves dynamic changes of seagrass to estimate carbon uptake and sequestration rates in a tidal basin of the Wadden Sea, the Sylt-Rømø Bay, as a case study. We further applied particle tracking to assess the fate of shed seagrass leaves.

Our results indicate that the seagrass meadows in the bay have a carbon uptake of 895 ± 465 tC yr⁻¹ (or 40.8 ± 7.7 gC m⁻² yr⁻¹). Approximately 24 % (9.8 ± 1.6 gC m⁻² yr⁻¹) of the annually captured carbon is stored in the meadow sediments, with a major part originating from the below-ground biomass. Approximately 28 % of the seagrass biomass is exported from the meadows as flotsam, and about a fifth of that (~6 % of the total biomass) is deposited in salt marshes.  The remaining seagrass biomass (~48 %) are either deposited as POC outside of the seagrass meadows, remineralized or grazed.

Overall, we estimate a sequestration rate of 10.1 ± 1.5 gC m⁻² yr⁻¹ (including both autochthonous and allochthonous carbon) within the seagrass meadows, which is lower than previous estimates for both the seagrass in the region and global averages. Our findings also show that the continuity of seagrass meadows is crucial for the mid-term (decennial), and likely also for the long-term (centennial or longer) sequestration of organic carbon in the sediment.