No abstract

Summary1. The saltmarshes of south-east England are eroding rapidly. Field and laboratory experiments were used to test the hypotheses that: (i) at the mudflat-saltmarsh boundary there are two alternative states, one dominated by pioneer zone vegetation that excludes burrowing infauna, and the other dominated by infaunal invertebrates that exclude vegetation; and (ii) the major cause of the loss of saltmarshes in south-east England is internal creek erosion, which is exacerbated by bioturbation and herbivory by the infaunal polychaete Nereis diversicolor . 2.In laboratory experiments Nereis ate the seeds and seedlings of Salicornia spp., and Salicornia deterred burrowing by Nereis . In field experiments, at Tollesbury in Essex, UK, exclusion of Nereis from the sediment surface increased the density of Salicornia , but only when a source of seeds was close by. In the Tollesbury saltmarsh the plants and Nereis had mutually exclusive distributions within a vertical zone of overlap. The recently vegetated area of the managed realignment site at Tollesbury contained no Nereis , but Nereis colonized areas where Salicornia had been removed. These observations and data support the first hypothesis. 3. Much of the loss of the Tollesbury saltmarsh is by lateral erosion of the internal creeks. Physical factors alone cannot be responsible for this erosion because experimental exclusion of Nereis led to sediment accretion. These results support the second hypothesis. Creek erosion may create a positive feedback where creek enlargement leads to faster tidal currents and further erosion until creeks have widened to their new equilibrium morphology. 4. Synthesis and applications . We conclude that the infauna are a major cause of reduction in area of saltmarsh vegetation and this has implications for the management of saltmarsh restoration. These results call into question the assumption that saltmarsh erosion in south-east England is due to sea level rise and coastal squeeze, and demands reexamination of the role of management realignment in the regeneration of saltmarshes. Reducing the rates of saltmarsh creek erosion, by exclusion of the infauna, and/or by reducing current velocities in the saltmarsh creeks, would reduce the need to replace eroded marshes by managed realignment, and would reduce future erosion of existing sea walls by wave action.

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.

Studies on the distribution of epifaunal species on their host have concentrated on those of plants, particularly laminarian algae (Fowell, 1939; Ebling, Kitching, Purchon& Bassindale, 1948; Sloane, Ebling, Kitching & Lilly, 1957; Scarratt, 1961, Ph.D. Thesis, University College of Wales, Aberystwyth; Norton, 1971), Sargassum algae (Kato et al., 1961; Nishihira, 1971; Ryland, 1974) and eel grasses (Nishihira, 1968; Nagle, 1968). Recently, however, the distributions of the animals living on ascidians Microcosmus sp. (Monniot, 1965) and the bryozoan Flustra foliacea (L.) (Stebbing, 1971) have been investigated.

Saltmarshes are areas of vegetation subject to tidal inundation and are important to birds for several reasons. Saltmarshes are areas of high primary productivity and their greatest significance for coastal birds is probably as the base of estuarine food webs, because saltmarshes export considerable amounts of organic carbon to adjacent habitats, particularly to the invertebrates of mudflats. In addition, saltmarshes are of direct importance to birds by providing sites for feeding, nesting and roosting. Climate change can affect saltmarshes in a number of ways, including through sea‐level rise. When sea‐level rises the marsh vegetation moves upward and inland but sea walls that prevent this are said to lead to coastal squeeze and loss of marsh area. However, evidence from southeast England, and elsewhere, indicates that sea‐level rise does not necessarily lead to loss of marsh area because marshes accrete vertically and maintain their elevation with respect to sea‐level where the supply of sediment is sufficient. Organogenic marshes and those in areas where sediment may be more limiting (e.g. some west coast areas) may be more susceptible to coastal squeeze, as may other marshes, if some extreme predictions of accelerated rates of sea‐level rise are realized.