Salt marsh ecosystems are highly efficient in long-term sequestration of carbon that originates from both within and far beyond its own boundaries. They are therefore considered to be major blue carbon sinks with great potential for climate change mitigation strategies. As a consequence, scientific interest has gradually increased over the past decades and recent studies are now acknowledging promising carbon-climate feedbacks. However, many studies on sea level rise (SLR) induced carbon accumulation in tidal marshes are in most cases limited to a certain extent in their comparability and unambiguity due to various methodological and geographical-related constraints. The present study aims to address these recurring issues by studying two nearby minerogenic marshes on the back-barrier islands of Ameland and Schiermonnikoog in the Dutch part of the Wadden Sea. Both islands experience similar environmental conditions, yet also significantly different rates of SLR (9.7-11.7 mm yr-1 and 2 mm yr-1) due to severe land subsidence near Ameland. Consequently, the effects of accelerated SLR on local carbon sequestration processes along multiple levee-basin gradients for various rates of SLR could be examined. Sampling campaigns were organized during the summer and winter months of 2021 to collect water, sediment and above- and belowground biomass samples. Similar data from previous studies were reanalyzed. Thorough statistical analysis was carried out in Rstudio on various sedimentological, elemental (OC%, N%, C:N) and stable isotopic (δ13C) parameters and resulted in the following findings. Mean organic carbon accretion rates (OCAR) were quantified for Ameland (110.54 g m-2 yr-1) and Schiermonnikoog (60.12 g m-2 yr-1). Estimations of sediment organic carbon (SOC) stocks ranged between 35.96±7.52 and 73.35±15.99 Mg OC ha-1 on Ameland and between 20.83±3.44 and 24.94±4.55 Mg OC ha-1 on Schiermonnikoog. Furthermore, increases in relative SLR (RSLR) primarily affected sedimentological parameters, whereby sediment accretion, silt layer depth and sediment bulk density would experience two- to threefold increases. Mean OCAR and SOC stocks would double in size as a direct result. OCAR and SOC stocks were uniformly distributed in space across the marsh under low rates of RSLR, but displayed a growing imbalance between the levees and basins under accelerated RSLR. Levees were disproportionately affected and became hotspots of subsurface carbon sequestration. Moreover, negative and non-linear relationships between RSLR and OCAR/SOC along the small RSLR-gradient on Ameland indicated the beginning of marsh drowning at SLR rates between 10.7 mm yr-1 and 11.7 mm yr-1. Finally, the SOC stocks originated predominantly from old and recalcitrant marine allochthonous POC. Enhanced RSLR only intensified the marsh’s dependence on allochthonous sediment inputs in order to survive. |
