Hesketh Marsh: a comparative study of managed realignment and natural saltmarsh.
Supervisors: Claire Evans Phillip Warwick, Amani Backer (NOC, Soton)
The saltmarsh ecosystem is a dynamic coastal habitat that provides a multitude of invaluable ecosystem services crucial for both human well-being and environmental health. Saltmarshes play a pivotal role in flood regulation, natural buffer against storm surges and tidal inundations. Their vegetation and topography attenuate wave energy, reducing the impacts of extreme weather events on coastal communities. Saltmarshes enhance water quality by filtering pollutants and nutrients, contributing to improved marine biodiversity. However, this ecosystem is facing a rapid decline due to a number of the anthropogenic threats and climate change. Managed realignment is considered as one approach to restore the ecosystems which involves controlled breaching of embankments to create or restore intertidal habitats, allowing natural processes to flourish. This research investigates differences in sediment structure and geochemical properties between managed realignment and natural marsh areas within the Hesketh Out Marsh Managed Realignment Site, in the United Kingdom. Three sites were studied using consistent methodologies, with sediment cores taken as transects across each site. One site represented an unaltered natural marsh, while the other two sites underwent reclamation through embankment construction in the 1980s, which were subsequently breached as part of managed realignment efforts. Hesketh Out Marsh West (HOMW) embankment breach occurred after 27 years of agricultural use, and Hesketh Out Marsh East (HOME) breach followed 37 years of similar use. Non-destructive core scanner analysis was employed to assess sediment structure and early geochemical evolution post-realignment. Overall, the findings show both realigned sites, exhibited higher sediment densities within their cores, indicative of prolonged drainage. Enhanced lamination was observed in the natural marsh site, suggesting regular saltmarsh flooding and storm events. The allochthonous sediment content introduced following embankment breach in the managed realignment sites was evident, impacting the ecosystem. Although the ITRAX Scanner revealed less developed redistribution of significant redox-sensitive elements after realignment, microbial sulfide reduction processes led to surface sediment sulfide production. Saltmarsh ecosystems in the Ribble Estuary acted as sinks for trace metals, indicating a reduction in heavy metal pollution from historical mining and industrial sources. Radiometric dating highlighted the dynamic nature of the Ribble Estuary saltmarsh ecosystem, exhibiting spatial heterogeneity in sediment deposition rates. Both realigned sites displayed complex 137Cs activity profiles, likely reflecting the perturbation caused by past agricultural usage. In conclusion, this study sheds light on sedimentary and geochemical changes resulting from managed realignment practices and their implications for saltmarsh ecosystems. The findings contribute to understanding ecosystem responses to realignment interventions and underscore the dynamic nature of coastal saltmarsh environments.