Saltmarshes on the Fringe

SALTMARSHES ON THE FRINGE – Restoring Degraded Shorelines for Conservation and Coastal Defence

Context

Saltmarsh habitats are recognised as having a rich and unique biodiversity and are increasingly valued for delivering many ecosystem services to society. However, in the Eden Estuary, on the east coast of Scotland, this once-common shoreline habitat has suffered considerable loss from ad hoc rubbish tips (Figure 1) and hard sea defences created during past industrial activities. In some cases, these now-ageing structures have become unstable and failed, especially during storm and high wave energy conditions (Figure 2).

Figure 1. A rubble and inert waste shoreline created in former times along the shore of the Eden Estuary.

Figure 2. A breach created in a stone embankment by the combined effect of an onshore storm and equinoctial spring tides in March 2010. (Image courtesy of RMM Crawford.)

Protecting the valuable land assets around the estuary from coastal flooding and erosion is an expensive and ongoing issue that rising sea levels can only exacerbate. Furthermore, over half of the Eden Estuary’s saltmarsh has a poor ecological status and should the die-back continue (Figure 3), shoreline exposure and instability will become even greater.

Figure 3. The die-back and fragmentation of the saltmarsh buffer on the shores of the Eden Estuary.

The practice of saltmarsh creation as a way to buffer the shoreline from wave attack and erosion is presently re-emerging in ecological restoration science, because of the need to find more natural and cost-effective solutions to coastal flooding and erosion. Directly planting saltmarsh transplants into wave-impacted and degraded shores is a simple method that has been used in other parts of the world, with varying degrees of success. In the Eden Estuary, this technique was successfully demonstrated in the 1940s with the Common Cordgrass (Spartina anglica), until an awareness of its invasive nature led to an eradication policy during the 1980s, which has since developed into the current management strategy of local containment.

Native, ‘swamp’ saltmarsh communities (e.g. Bolboschoenus and Juncus) however, are prevalent in pockets around the estuary, and during the last decade some of these stands have expanded (Figure 4). This appears to coincide with increasing rainfall and freshwater input to estuaries on Scotland’s east coast, and given climate change predictions, the range of this ecotype might spread further. Hitherto, Sea Club Rush (Bolboschoenus maritimus) has not been considered for use in erosion control on estuarine shorelines (in terms of sedimentation and wave dissipation) nor tested in its efficiency as an estuarine restoration species (in terms of adaptability or ecosystem benefits).

Figure 4. Swamp saltmarsh (Sea Club Rush) expanding over degraded saltmarsh (formerly Saltmarsh Grass and Red Fescue mixed grass community).

The loss of regionally and nationally important saltmarsh habitat and the impact of climate change and rising sea levels on the increasingly susceptible shore of the Eden Estuary has therefore provided the opportunity to improve the knowledge base of fringe saltmarsh restoration and the ecosystem benefits it can deliver.

 

The Restoration Project

The saltmarsh restoration project is a university-led initiative that aims to recreate a fringe of this valuable habitat around the shores of the locally and internationally important Eden Estuary. Field planting trials were established between 1999 and 2003 during a PhD study and the most successful findings from the research were put into practice in a phased planting strategy between 2010 and 2013 (Figures 5 & 6).Figure 5. Sea Club Rush trial plot in July 2015, originally planted in 2003.

Figure 6. Sea Club Rush transplants in June 2011, planted in February 2011.

During this attempt, more than 1,000 m2 of ‘new’ saltmarsh habitat was created along longer stretches of more wave-exposed and eroded shorelines. The establishment and sediment accretion rates have been relatively high, but several issues were highlighted, the most immediate being: 1) The washout rate of the transplants can be significant during a seasonal storm if it occurs immediately post-planting, 2) The transplants are vulnerable to wave-stress during the first two to three years of establishment, and 3) Relatively small ‘donor’ saltmarshes can sustain only a limited amount of harvesting for transplant purposes.

The current project (2014 – 2016) therefore has three objectives:

  • To encompass another two to three kilometres of restoration sites along the Eden’s shore
  • To develop propagation techniques to increase the transplant yield of native saltmarsh species for use in restoration projects
  • To test simple techniques to reduce wave stress and the wash out rate in new saltmarsh, using field trials at the restoration sites

The results from this project will have many benefits, such as improving the ecological status of the saltmarsh habitat in the Eden Estuary, providing a working example of adaptive coastal management and ultimately producing saltmarsh restoration guidelines to help Scotland comply with European and UK legislation on climate change adaptation and habitat conservation.

This work has been funded and supported by: