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Stemming the rising tide: The protective role of saltmarshes

Final Report Summary - STORM (Stemming the rising tide: The protective role of saltmarshes)

Sea level rise and the associated flooding of coastal regions are predicted to lead to severe impacts in many European countries. It is recognised that coastal habitats will play a vital role in mitigating the effects of sea level rise, through increasing sediment accretion rates and thus surface elevation. In Europe, saltmarshes are the primary vegetated coastal habitat and are widely distributed along the European coastline. Both geological evidence from the Holocene, when sea levels rose quickly and significantly and models of contemporary sea level rise suggest that saltmarshes are able to keep pace with sea level rise when sediment supply is sufficient, thus protecting inland habitats from inundation. Saltmarshes have also been shown to be very effective at attenuating wave energy during storm surges. Vertical accretion rates in vegetated coastal habitats are the result of complex interactions between geomorphological (e.g. geological subsidence, sedimentation rates) and biological processes (e.g. root growth and organic matter accumulation and thus require a multidisciplinary research approach. Salt marsh growth and physiology can be an important driver of change in vertical accretion but the lack of data on biological processes, especially belowground, has been identified as a confounding factor for salt marsh models. This project is academically innovative, as it will simultaneously determine the role of biological processes (both above and below ground) and geomorphological processes in the overall changes to vertical accretion under different global change scenarios and incorporate those into existing salt marsh evolution and surface elevation models that predict the vulnerability of coastal areas to sea level rise.

Over the past 24 months we have achieved most of our project goals.

1) We have successfully constructed and executed an elevated CO2 experiment, which measured how elevated CO2 and eutrophication would impact vertical accretion rates in saltmarshes. We found a significant, positive, influence of elevated CO2 on vertical accretion in vegetated coastal habitats and, using boosted regression modelling, identified a number of factors contributing to this, namely decomposition rates, root growth and plant water use (Reef et al. 2016). We have also identified a key role for the grass Puccinellia maritima in encouraging vertical accretion. Puccinelllia maritima is a climate sensitive and grazing sensitive species and our results can be used to guide management policy regarding the conservation of saltmarshes in the face of sea level rise. The mesocosm study identified a significant change in canopy morphology with eutrophication and elevated CO2 and the impact of such changes on surface accretion was addressed in experiment 3.

2) Field campaigns planned at both Tillingham (Essex) and Donna Nook (Lincolnshire) saltmarshes have been completed over the two-year period. Our fieldwork allowed us to measure the biological components (root growth and decomposition) of vertical accretion for both the winter and summer season (manuscript #1 in advanced stages of preparation). Our study suggest a significant, previously overlooked gain to marsh elevation by biological contributions. In this component of the study we find that there is a spatio-temporal variation in the contribution of belowground biological processes to surface accretion on saltmarshes, ranging from a vertical gain of 1.6 mm/y in the upper marsh in the summer to 0.3 mm/y in the lower marsh in the winter. Using a UAV survey, we are able to identify the marsh zones and apply the correct biological contribution to an entire foreshore for the summer and the winter season.

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