On the loss of saltmarshes in south-east England and methods for their restoration

Abstract

1. The saltmarshes of south-east England have been eroding rapidly for about the last 50 years, at a continuing rate of about 40 ha year-1, with deleterious consequences for conservation and coastal flood defence. The possible reasons for this erosion and suitability of methods of saltmarsh restoration are discussed. 2. The prevailing hypothesis that the saltmarsh erosion is due to coastal squeeze, where sea walls prevent a landward migration of saltmarsh in response to sea level rise, is rejected because: (i) as the sea level rises saltmarshes accrete vertically as well, at least at the same rate, and may even extend seaward; (ii) in recent decades the rate of rise in sea level has been no higher than in the past when the saltmarshes developed; (iii) the pattern of vegetation loss, mostly of pioneer zone species, is opposite to that predicted by coastal squeeze, where the upper marsh plants should disappear first. 3. Alternative explanations and hypotheses are proposed that relate the recent saltmarsh erosion to changes to the intertidal biota, an increase in abundance of the infaunal polychaete Nereis diversicolor, and a decrease in abundance of intertidal seagrasses. Bioturbation and herbivory by Nereis cause the loss of pioneer zone plants, increase sediment instability and exacerbate the erosion of saltmarsh creeks. The erosion of the seaward edge of some marshes may also be due to increased wave action, and increased tidal current speeds in estuaries, following the loss of intertidal seagrasses since the 1930s through wasting disease. 4. Synthesis and applications. The current strategy for saltmarsh creation is based on managed realignment, where some sea walls are breached to provide new intertidal habitat. The conclusion that the causes of saltmarsh loss are not related to sea level rise calls into question this dependence on management realignment as the most appropriate means of saltmarsh creation, not least because many realignment areas are unlikely to develop vegetation. Other methods should be considered for creating new marshes and for reducing/reversing marsh erosion. These include, alone or in combinations, exclusion of the infauna, use of dredged material for strategic intertidal recharge, and transplantation of intertidal seagrasses.