In tandem with extreme weather events such as storm surge, sea-level rise is predicted to increase greatly the potential for coastal erosion and damage (1). Although erosion is a major issue for supra-littoral ecosystems such as sand dunes, the problem is not confined there. Changes in sediment transport concomitant with shifts in established deposition and erosion patterns will likely alter sub-littoral and inter-tidal habitats, exacerbating the ecological impact of climate change on near-shore marine ecosystems (2). In addition to possible negative effects on biodiversity, the loss or degradation of the wave attenuation service provided by kelp and sea grass beds, biogenic reefs, and salt marshes could further impact upon coastal integrity (3 ,4). Integrating the conservation and restoration of intertidal ecosystems in coastal defence approaches is now recognised as a sustainable, cost-effective and ecologically sound alternative to conventional coastal engineering (3 ,4). Application of such innovative approaches does however require in depth knowledge on the establishment, resilience, and decay of these ecosystems (3).

 

In this project, we focus specifically on salt marshes, as they have the highest coastal defence value (3). The coastal defensive capability of salt marshes stems from two characteristics; first their ability to temper wave energy over a very small area (as high as 92% attenuation over only 310m (5)); second they can accumulate sediments rapidly and so keep pace with sea-level rise (6). Nonetheless, sea level rise will impact upon marsh morphology, increasing erosion on the seaward slope but facilitating enhanced sediment deposition in the upper marsh (3, 7). Where the marsh is restricted by anthropogenic development (‘coastal squeeze’), retreat may not be an option (4). In such situations active management of the lower marsh may be an important way of mitigating sea-level rise, but to do this, we need to combine an understanding of population biology, hydrodynamics, and sedimentology to attempt to maintain or extend the sea-ward extent of the present-day marsh.

 

Spartina spp. are recognised as the keystone pioneer plant in most European salt marshes. Increased sedimentation stimulates their growth because inundation time is reduced as elevation increases; i.e. there is a positive feedback between Spartina establishment, sediment accretion, and plant growth (5, 8). In addition Spartina tussocks divert water flow leading to creek formation, so aiding marsh drainage (9). Recent work by the applicants on the EU-funded THESEUS project suggest that both the extent to which Spartina-dominated marshes can cope with sea-level rise (and thus contribute to coastal defence), is negatively related to the local tidal amplitude (3,10). That is, areas with small tidal amplitudes are most prone to drowning by sea-level rise and less suitable for coastal defence, whereas areas with larger tidal amplitudes may be more sensitive to lateral erosion rather than drowning. Accordingly salt marshes subject to differing tidal amplitudes require different methods of active management.

 

We hypothesize that in areas with large-tidal amplitudes, active management should aim at creating new pioneer zones for Spartina; fronting laterally eroding marshes as well as facilitating the lateral expansion of existing pioneer zones. This would require novel technologies to create the required windows of opportunity (e.g., by bed-level stabilisation and positive facilitative interactions), and to exclude negative interactions from bioturbating benthos (9, 10). In contrast, in areas with small-tidal amplitudes, active management should aim at identifying methods to enhance functional diversity so that more species with different flooding tolerance will coexist. For example, it would be valuable to introduce more pioneer species higher up in the marsh where existing vegetation is at risk of drowning. Ideally, the latter would be achieved by stochastic disturbances that create bare-patches for colonisation, if dispersal of low-marsh species is sufficient at the high marsh. With these management options in mind this PhD project has the following aims, to be studied at sites with contrasting tidal amplitudes:

 

1) Experimentally study how (1a) new Spartina establishment and (1b) expansion of existing Spartina can be best achieved: creating windows of opportunity vs. creating positive facilitative interactions vs. excluding negative interactions.

2) Experimentally determine the importance of functional species diversity for marsh resilience against drowning-risk from sea-level rise.

3) Asses the possibility to use different management techniques to enhance functional plant diversity within a (especially high) marsh: comparing diversity effects of both grazing-induced and stochastically anthropogenic induced disturbance-recovery dynamics

4) Integrate findings of 1-3 into a model to elucidate and predict the extent that mitigation measures at a European scale can be optimised to local tidal amplitude.

 

Unlike much research linking climate change to ecophysiological response, we will deliver practical solutions for the active management of coastal systems. This will be made possible by conducting experiments in a range of sites varying in tidal amplitude via collaboration between Plymouth, NIOZ, and Bologna, allowing us to make generalisations about best management practice across Europe. In addition to field-work in rias in Devon, and larger salt marsh systems in the Scheldt and Po estuaries, will the project will benefit from access to flume facilities at NIOZ. This research is at the forefront of marine climate change science and while ambitious and novel in scope, the expertise and proven (collaborative) track record of the supervisory team ensure that these inter-linking objectives can be delivered within a 3 year project lifespan.

 

References

 

Inter-tidal habitats are at the forefront of coastal defence, a role that is set to increase as climate change and economic pressures compel ecosystem-based coastal defence to become a cornerstone of European environmental policy. With limited budgets, an extensive coastline and a number of negative consequences associated to traditional hard-engineering, defence based on hard-engineering alone cannot solve the problem. So-called ‘soft-defences’ such as sand dunes, mangroves and salt marshes are likely to form an important part of an integrated defence strategy. It is essential therefore that ecologists gain a better understanding of the processes that govern salt marsh establishment and persistence at the critical inter-face with the open sea. 

 

This is the first time that both salt-marsh management and salt-marsh resilience has been related to variation in tidal amplitude, local hydrodynamics, and bioturbation within a consistent study across Europe. Consequently we expect the studentship to generate at least 4 high-profile publications, reflecting the 4 clearly defined aims of the PhD project: 1 - optimising Spartina establishment on tidal flats; 2 - Effect of salt marsh diversity on resilience against drowning; 3 – Facilitation of plant species diversity in the upper marsh; 4 – Pan-European model for salt marsh management in a climate-change world. The project will provide proof of concept data for subsequent (Pan European) application to examine how the interactive effects of microhabitat, sea-level rise, and tidal amplitude influence marsh establishment and persistence. 

 

The studentship offers training in salt marsh ecology, ecophysiology, flow dynamics, sedimentation, modelling (UoP and NIOZ), and in the design and analysis of ecological experiments (Bologna). In undertaking this training, the student will develop new technologies in the use of water flumes to model Spartina response to altered tidal prisms and field techniques to determine impacts on sediment stabilisation and bioturbation. In addition to manuscript preparation, the student will present key findings at international conferences (e.g. the British Ecological Society annual meeting) and also disseminate results more via the local and international media (through the UoP press office). The student will also be expected to set up and maintain a project web site and continue Hanley’s existing interaction with the UK Environment Agency (the EA have primary responsibility for coastal protection in the UK and the student will be required to discuss experiments and report findings to local EA contacts on a regular basis).