Ecological Community Changes and Ecosystem Functioning During Structured Wetland Development
Saltmarshes provide important ecosystem services, including protection from tidal inundation, grazing for livestock, resources for birds and breeding grounds for fish. After a long history of saltmarsh reclamation there is now emphasis on restoration of existing or destroyed saltmarshes to provide dynamic coastal defences providing ecosystem services. The engineering technique of Managed Realignment (MR) is a popular method of restoration where existing coastal defences are deliberately breached. The process of change associated with MR, including the influences of starting state, construction, and saltmarsh community structure changes require further study to provide guidelines for optimal restoration practices. Here, the ecological changes during the early stages of saltmarsh restoration via MR at Steart Marshes in Somerset, UK are studied. Common Standards Methods were used to monitor vegetation and invertebrate community composition changes from pre-breaching (September 2014) through 4 years of tidal regime post-breaching. Soil and sediment characteristics, and soil accretion were studied at the same time. All data were collected from permanent sample plots. Changes were compared with a managed brackish site (Otterhampton Marsh) with a similar starting state, and a mature SSSI saltmarsh in Bridgwater Bay was treated as a ‘target state’. Soil characteristics included soil moisture, pH, electrical conductivity, soil organic carbon, nitrate-nitrogen, ammonium-nitrogen and phosphate-phosphorus. Changes of these were compared to the target state and the managed brackish site. The National Vegetation Classification (NVC) scheme was partly used to assess vegetation communities, and multivariate methods including cluster analysis and ordination were used to assess changes of flora, invertebrates, and soil characteristics. Saltmarsh vegetation colonised a former pasture location more rapidly than a former arable location and was slowest in the managed brackish location. Colonisation by invertebrates and soil characteristics followed a similar pattern. The most abundant invertebrate species were identified to species and quantified, and their abundance on Steart Marsh was compared to the mature SSSI saltmarsh and brackish site. The three most abundant species overall were Orchestia gammarellus, Campiglossa plantaginis, and the invasive planthopper, Prokelisia marginata. C. plantaginis was positively correlated with the abundance of colonising Aster tripolium, and P. marginata had a positive relationship with colonising Spartina anglica. It is likely that soil compaction, partly caused by pre-breach engineering, influenced differences in soil characteristics between study sites and in plant and invertebrate colonisation, principally due to the prevalence of standing water associated with anaerobic conditions. Compaction and land height (AOD) influenced plant colonisation across all sites and soil moisture and electrical conductivity influenced colonisation by plants and invertebrates on Steart Marsh and Otterhampton Marsh after the breach. Soil carbon storage was greatest in the former pasture site. Vegetation, invertebrates and soil characterstics in the former pasture site on Steart Marsh started to resemble that of the SSSI saltmarsh three years after tidal inundation commenced, and overall the results indicate that restoration is influenced by former site use and engineering practices involved in Managed Restoration.
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