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

Abstract

Summary1. 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.

Suppressing antagonistic bioengineering feedbacks doubles restoration success

“…Surprisingly, however, these papers neglect the potential restoration benefits of the removal of negative interactions originating from ecosystem engineers, despite the fact that such interactions can play an important role in marine habitats (Rhoads and Young 1970, Philippart 1994, Hughes et al 2000, Hughes and Paramor 2004, Siebert and Branch 2006, Berkenbusch et al 2007, van Wesenbeeck et al 2007. In this study, we show that suppressing such negative ecosystem-engineering effects could be a useful approach in restoration and conservation efforts by giving at least a better start during the initial establishment phase.…”

“…For example, this has been shown for the negative effect of deposit feeders on filter feeders by making sediment more erosive, causing filter feeding to be hampered by more turbid water (i.e., trophic group amensalism; Rhoads and Young 1970). Several studies have also demonstrated that negative interactions between benthos and vegetation can be very strong (Philippart 1994, Hughes et al 2000, Hughes and Paramor 2004, Siebert and Branch 2006, Berkenbusch et al 2007, van Wesenbeeck et al 2007). For instance, ''biomechanical warfare'' between ecosystem-engineering salt marsh vegetation and infauna can cause a bimodal species distribution on the salt marsh-mudflat interface, as bioturbation prevented plant colonization in the invertebrate-dominated state and invertebrates were excluded by sediment compaction in the plantdominated state (van Wesenbeeck et al 2007).…”

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

“…The overall management implication of a coastal wetland, if validated to comply with multiple stable state theory, would be that the wetland is valuable and long-lived, but non-stationary, and once degraded very difficult to restore. In fact, high rates of failure in coastal wetland restorations already suggests such a case [46][47][48][49].…”

Is transplanting an effective means of preserving vegetation?

“…Other herbaceous wetlands subject to transplanting are declining coastal marshes (e.g., Bakker et al 1997, Hughes andParamor 2004). On the west coast of North America, plugs supporting five dominant marsh species were planted on constructed islands in an attempt to produce marshes like the natural ones that had been lost.…”