Saltmarsh erosion and restoration in south-east England: squeezing the evidence requires realignment.
Abstract
Saltmarshes in south-east England have been eroding rapidly since 1960. Recently, Hughes & Paramor (2004) and Morris et al. (2004) have presented contrasting views on the extent to which physical and biological processes might contribute to the erosion. There are three contentious issues: (i) saltmarsh erosion is the result of coastal squeeze, where sea walls prevent a landward migration of a saltmarsh in response to sea level rise; (ii) saltmarsh erosion is linked to bioturbation and herbivory of seedlings by the ragworm Nereis diversicolor; (iii) new saltmarshes will not develop on managed realignment sites where existing sea walls have been removed because of the effects of ragworms. In this paper, we provide a literature review of physical and biological processes relevant to the above three issues, and discuss the relative importance of these processes at different spatial and temporal scales. Our synthesis shows that, at a regional scale, the combination of strong winds, high tides and increased wave height appears to be responsible for the increased rate of marsh erosion and creek dissection recorded in the 1970s. There is also some laboratory evidence that bioturbation and herbivory from populations of Nereis can lead to sediment instability and loss of pioneer plant species, such as Salicornia spp. However, the field evidence is more equivocal and has been conducted at small spatial scales. At a large number of different managed realignment sites there is strong evidence that even if bioturbation and herbivory by Nereis have occurred, overall the effects have been insufficient to restrict plant succession of exposed sediment. Synthesis and applications. There is an urgent need for long-term field studies that integrate and quantify physical and biological processes and the related feedbacks at different spatial and temporal scales. Until this is completed, terms such as coastal squeeze will remain contentious and management decisions will invite criticism.