Biological and Physical Processes That Affect Saltmarsh Erosion and Saltmarsh Restoration: Development of Hypotheses
Saltmarsh habitats are under increasing threat, particularly from sea-level rise (SLR) associated with global warming and increased wave action associated with climate change. The saltmarshes of southeastern England, which developed for several centuries
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8.1 Introduction Saltmarsh habitats are under increasing threat, particularly from sea-level rise (SLR) associated with global warming and increased wave action associated with climate change. The saltmarshes of southeastern England, which developed for several centuries under conditions of local SLR, are now disappearing rapidly. These marshes offer the potential for study of the processes that determine loss of vegetation, and of the processes necessary for the managed amelioration of these losses, as some experimental managed realignment schemes are in progress. Burd (1992) reported that, in the 15 years prior to 1988, the total losses of saltmarsh vegetation in the estuaries of southeastern England, from the River Orwell in Suffolk to the Swale in Kent, varied from 23 % in the Blackwater to 44 % in the Stour (Fig. 8.1). Most of the losses were from the pioneer zone where up to 74% was lost during the same period. Loss of these marshes is causing concern because of the reduction in protection offered by the vegetation to the sea walls that surround most of this coastline, and because of the conservation importance of these habitats. These estuaries contain about 28,000 ha of mudflats and 8500 ha of vegetated saltmarshes, of which about 85 % is internationally important, largely because of their use by migrating and overwintering birds. Enclosure of saltmarshes behind sea walls, primarily for agricultural use, has occurred in these estuaries for several centuries (e.g. Burd 1995). While these losses are attributable, the recent erosion is less easy to explain. Some factors that may contribute to saltmarsh loss include; removal of sediment by dredging, which may increase exposure to wave action; distant sea walls and groynes, which may limit erosion and transport of recharging sediment; and adjacent sea walls, which may accelerate local erosion by altering tidal flow and by reflecting wave energy back across the fronting saltmarsh. The loss of most of the intertidal eelgrass Zostera marina because of the wasting phenomenon from the 1930s, may have increased sediment erosion in front of
Ecological Studies, Vol. 151 K. Reise (ed.) Ecological Comparisons of Sedimentary Shores © Springer-Verlag Berlin Heidelberg 2001
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R.G. Hughes Total Orwell Stour Colne Blackwater Crouch
40 44 12 23 26
Pioneer Zone
74 60 53 74 25
Fig.8.1. The proportion of the total saltmarsh area and pioneer zone area lost between 1973 and 1988 from some of the estuaries of SE England. (Data from Burd 1992)
the marshes and left them more exposed to wave action. Beardall et aI. (1988) reported that the loss of Zostera from the River Stour led to the loss of 15 million m 3 of sediment and increased its tidal volume by 30 %. Notwithstanding the possible impacts of the factors listed above, the generally accepted explanation for the erosion of these saltmarshes is coastal squeeze (Davidson et aI. 1991; Burd 1992). It is generally considered that the upper limits of the distribution of saltmarsh plant species are determined by interspecific c
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