Effects of the invertebrate infauna on early saltmarsh plant colonisation of managed realignment areas in south-east England

Abstract

Saltmarsh erosion on the subsiding coastline of SE England is a major conservation problem and necessitates increased expenditure on maintaining sea walls. The preferred management solution is selective managed realignment (set-back), where sea walls are breached to allow new intertidal land to develop into saltmarsh. We tested the hypothesis that saltmarsh would not necessarily develop in low-lying realignment sites on subsiding coasts, because the accreting sediment would be colonised first by infaunal invertebrates, particularly the polychaete Nereis diversicolor, which would prevent development of vegetation through bioturbation and herbivory. The hypothesis was supported by data on the distributions of the plants and invertebrates in the experimental managed realignment sites at Tollesbury and Orplands, and from infauna exclusion experiments. Tollesbury mostly developed into mudflat colonised mainly by N. diversicolor and Hydrobia ulvae, with only a narrow zone of saltmarsh vegetation in the highest area where little sediment had accreted. In contrast, most of the Orplands site was relatively high and, apart from some low-lying basins, little sediment accretion occurred and the area was soon largely covered with saltmarsh vegetation. In experimental N. diversicolor exclusion areas, the abundance of microphytobenthos and filamentous algae and sediment accretion increased at both sites, but saltmarsh vegetation only developed at Orplands, probably because the seeds at Tollesbury were buried by rapidly accreting sediment. Further management of low-lying realignment areas is necessary to prevent invertebrate colonisation and help promote saltmarsh development. The experiments also indicate that marsh regeneration need not depend on realignment and should be possible on existing mudflats.

Crab bioturbation and herbivory reduce pre- and post-germination success of Sarcocornia perennis in bare patches of SW Atlantic salt marshes

“…In addition to herbivory, bioturbation can also have profound impacts during the course of successions that start in bare areas. In seagrass beds and marshes, bioturbation can prevent plant colonization of bare areas, acting mostly during the seedling phase (Dumbauld & Wyllie-Echeverría 2003, Paramor & Hughes 2005. Similarly, in our study system, Sarco-cornia perennis colonizes the mudflat and facilitates the establishment (through seedlings) of the competitively dominant Spartina densiflora by reducing the impact of crab herbivory (Alberti et al 2008).…”

“…Bioturbators can affect seedling densities by burying seeds (Paramor & Hughes 2005). To evaluate whether this could be occurring in our system, we conducted an experiment using glass beads at one Mar Chiquita marsh.…”

“…Bioturbation can also exert strong influences on the dynamic of bare areas. For example, burrowing crabs can promote erosion in some marshes , and polychaetes can dramatically reduce the colonization of tidal flats (Paramor & Hughes 2005). Recent evidence also shows the relevance of herbivory in marshes, being particularly important in the maintenance and expansion of bare areas (e.g.…”

“…salt marshes (Paramor & Hughes 2004, but see Morris et al 2004). And it has been suggested that they can delay the colonization of bare surfaces through seed burial (Paramor & Hughes 2005). In Canada, when grubbing for roots and rhizomes, the lesser snow geese Chen caerulescens caerulescens destroy all vegetation cover and heavily disturb the sediment, producing extensive bare grounds that remain unvegetated for several decades (Jefferies et al 2006).…”

The dispersal and colonisation behaviour of the marine polychaete Nereis diversicolor (O. F. Müller) in south-east England

“…It is an important ecosystem engineer (Widdows et al, 2009) as through its bioturbatory activities it will re-work and irrigate the sediment thus affecting chemical fluxes (pollutants, nutrients), affect microbial processes, and the stability of sediment surface (de Deckere et al, 2001;Kristensen, 2001;Kristensen & Mikkelsen, 2003). In the estuaries of SE England, it is often found high in the intertidal zone, at the saltmarsh-mudflat interface, where through bioturbation and herbivory it can increase sediment erosion, reduce sediment elevation, restrict the successional development of saltmarshes and increase the internal erosion of saltmarshes by facilitating creek expansion (Paramor & Hughes, 2004, 2005. Several aspects of the biology and ecology of Nereis are not fully understood including its dispersal behaviour (Abrantes et al, 1999).…”