Analysis of succession and the relation with hydrogeochemistry in a former tidal wetland in the Grevelingen, The Netherlands over a period of 43 years
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Deltas and estuaries are widely recognized as areas with important high natural values that are under threat by human activities such as coastal defence. The Grevelingen estuary in the South West of The Netherlands was closed off from the North sea in 1971 to protect the area from disasters similar to the storm flood in 1953. The closing of the Grevelingen dam removed the tidal influence from coastal wetlands in the Grevelingen like the ‘Slikken van Flakkee’ leading to the development of a unique natural area. In the northern part of the area, natural succession has led to the emergence of a diverse landscape containing forests and different types of grasslands. The southern part of the area which has been influenced by nature management in the form of extensive grazing and regular mowing has led to a more uniform landscape containing Red list species with high natural values. The aim of this research is to determine which hydrogeochemical processes occur at the ‘Slikken van Flakkee’ area and what their influence is on the different vegetation types. Soil samples were collected from nine representative locations in the field and analysed in the laboratory. Vegetation data gathered since 1972 were analysed to determine the major trends in vegetation succession. We found evidence for desalination, pyrite oxidation and cation exchange. Desalination was the only hydrogeochemical process that has a significant effect on species composition. The observed difference between the vegetation types at the northern and the southern part of the area is due to the different management strategies. A modelling study is recommended to gain insight into the past and future development of the hydrogeochemical conditions at the ‘Slikken van Flakkee’ to investigate whether the current management choices suffice to maintain the high valued Red list species in the area.