Regime shifts are common phenomena in ecological systems that can have pervasive societal implications, due to the loss of ecosystem service (ES) provision. Saltmarshes are globally-distributed grasslands living at the sea-land boundary, dominated by hydrodynamic forces. Increased inundation levels and disturbances may induce saltmarsh collapse into alternative stable states, where all vegetation is lost. The extent to which saltmarsh-linked ES, such as carbon (C) sequestration, are lost following collapse remains an open question. Saltmarshes are among the most powerful C-sinks on the planet. Understanding the relationship between marsh stability and C-sequestration is necessary, given increasing pressures linked to climate change. Hence our main questions: (i) are saltmarsh C-stocks resilient to ecosystem transitions, and (ii) are saltmarsh C-fluxes in dynamic areas faster/slower than in stable ones? To answer these questions we took a paleoecological approach, and extracted 1.5-m long sediment cores from three sites in Wales (UK). In each site, three cores were extracted respectively from ‘dynamic’ and ‘stable’ marsh areas. These areas had been previously identified using a GIS containing historical aerial images. Once extracted, the sediment cores were split open, analysed under an X-ray fluorescence scanner to assess elemental signatures at each cm and subsampled to assess C-stocks and fluxes.
Results/Conclusions
Our results strongly support that sediment cores obtained from stable areas displayed a continuous C-rich saltmarsh layer that occupied most of the 1.5-m-long core, whereas cores from dynamic areas presented several C-rich saltmarsh layers punctuated with bare sand layers in between. Overall, C-fluxes (in terms of g C year-1) were faster in marsh layers from dynamic cores than in the only marsh layer present in cores from stable areas. However, total carbon sequestration was still higher in stable saltmarsh areas, due to the low amount of C stored in the unvegetated layers from dynamic areas. Thus, our results show that while recovery of C-sequestration ES from an unvegetated state is fast, due to the higher C-accretion rates of newly established saltmarshes, stable saltmarshes are more valuable C-sinks due to their long-term stability. These results highlight the importance of taking into account ecosystem stability and persistence when evaluating ES. In the case of saltmarshes, stability has been linked to the position of the marsh in the tidal frame and the geomorphology of the coast. Our work, points out the relevant saltmarsh features that should be borne in mind when scaling up C-sequestration in saltmarshes.