Pioneer salt marsh restoration for coastal protection - Eastern Scheldt, NL
Location: Eastern Scheldt, The Netherlands
Date: 2012 - present
Involved parties: IMARES, NIOZ (former NIOO-CEME), Ecoshape.
Technical Readiness Level: 5 ( technology validated in relevant environment )
Topics: Ecology, Spartina anglica, cord grass, saltmarshes, coastal protection, measures against erosion, biodiversity.
|Building with Nature design||Traditional design|
Saltmarshes grow in the highest zone of beaches and mud flats. They can protect shorelines by stabilizing sediments and reducing currents and wave action. In this project Spartina anglica plants fixed in coconut mats are applied at three locations in the Eastern Scheldt, in order to investigate the potential of this method to re-establish pioneer saltmarsh. Under the right conditions (e.g. sufficient sediment input), the Spartina plants should grow out and form larger tussocks and subsequently meadows, thus adding to coastal protection, biodiversity and ecosystem functioning.
In the Eastern Scheldt (The Netherlands), a continuous net erosion of the intertidal flats takes place. Salt marshes are declining and experience erosion, whereas pioneer saltmarsh has almost disappeared. This is a consequence of the morphological disequilibrium caused by the construction of the storm surge barrier and compartmentalization dams in the 1980s. In this pilot Ecoshape investigates a new method to re-establish Spartina anglica (cord grass) in an attempt to protect higher intertidal areas against erosion and to promote the development of pioneer saltmarsh. When certain conditions are met (e.g. sufficient sediment input), the plants may grow in time into healthy salt marshes and add to the biodiversity and ecological functioning of the area.
Salt marshes can protect coastal zones from currents and waves by stabilizing sediments and reducing wave action. They grow in the highest zone of beaches and mud flats. The structures associated with the plants also induce deposition of sediment and organic material from the water column. This material is consolidated between the plants and is a source of food for fauna associated to these habitats. Under certain circumstances, salt marshes are able to maintain the coastline relative to sea level rise by accreting sediment at a level comparable to or even higher than sea level rise, providing a further reduction in vulnerability to hazards and climate change. Besides this dynamic interaction with the physical environment, salt marshes provide economic benefits and contribute to a healthier ecosystem functioning.
The project objective is to create/restore pioneer salt marsh with cord grass (Spartina anglica) in the high intertidal zone in the Eastern Scheldt, thus contributing to coastal protection, biodiversity and ecosystem functioning. The restoration of Spartina marsh is one of the objectives for the Eastern Scheldt in the light of the European Water Framework Directive. Spartina marsh is also a protected habitat within Natura2000.
Salt marshes are characterized by a succession in age and elevation from the sea towards the land (e.g. Van Wijnen and Bakker, 2001). Spartina is one of the pioneer species of salt marshes. Single-plant establishment in the pioneer zone requires characteristics such as rapid anchoring through rapid root emergence and development. The associated sediment binding and dewatering increases the shear strength of the soil around the seedling (Friess et al., 2012, and references therein), which protects the juvenile plant from dislodgement by wave- and current-induced drag forces.
To facilitate the establishment of Spartina in the high intertidal, plants are grown on coconut mats in which the roots can anchor. These mats can be placed on the high mudflat and will protect the Spartina plants against dislodgement during the initial period of rooting. Under the right conditions, the plants can form tussocks and later a continuous meadow. The coconut fibre of which the mats are made will degrade over time and the salt marsh remains. By taking this approach instead of building dams to protect marshes from cliff erosion, existing salt marshes are protected from wave action and erosion without being restricted in their future extension towards the sea.
The current pilot project provides the Building with Nature program with:
- Critical thresholds for the survival of transplanted cord grass.
- Scientifically supported design rules and norms.
- Insight into the effectiveness of this technique.
Within the Building with Nature program, the concept of using ecosystem engineers for coastal protection and flood mitigation is being studied. Several pilot experiments are ongoing, among which the use of shellfish reefs and mangroves. Yet, shellfish need a relatively high inundation time and can therefore only be used in the lower intertidal. Vegetation like mangroves and willow forests occur high in the intertidal. Mangroves only occur in tropical ecosystems. In temperate regions coastal vegetation is mainly saltmarsh. Saltmarshes grow in the high intertidal zone of sheltered beaches and mudflats.
The use of ecosystem engineers or vegetation instead of the more traditionally used hard infrastructure for coastal protection, has the additional value of providing a number of other ecosystem services that may be utilized. Examples are food and raw material production, nutrient cycling, biologically mediated habitat, gas and climate regulation and disturbance prevention (Beaumont, Austen et al., 2007). Saltmarshes constitute ecologically valuable habitats and have a function in stabilizing the seabed and reducing wave action on the shoreline (e.g. Mangi, Davis et al. 2011, Shepard et al. 2011). The plants increase deposition rates by trapping sediment and organic matter. In the last century, saltmarshes development was actively stimulated in the Netherlands, for coastal protection and shore stabilization and to accelerate reclamation (e.g. through the artificial introduction of Spartina anglica).
In recent decades, salt marshes have declined considerably in the Netherlands, especially in the South-western Delta, as a consequence of land reclamation and hydro morphological changes. In the South-western Delta mainly mature marshes are left, whereas new pioneer marsh is found only locally. Physical constraints most likely hamper plant establishment (e.g. Friess et al. 2012). This project will take the first steps towards utilizing cord grass (Spartina anglica) as an ecosystem engineer in the Dutch Eastern Scheldt. Cord grass is a pioneer salt marsh plant that grows at the mudflat/marsh transition zone. By planting cord grass in front of existing mature salt marshes or dikes it will add to coastal protection. If the plants catch on and form a healthy meadow, they may lead to extension of the existing marsh over time. This project focuses on designing and testing a new method, based on growing Spartina plants grown in coconut fibre mats before putting them in place, in order to protect them from dislodgement. This gives the plants a better chance to settle, survive and thrive. It may help to protect the coast and preserve ecologically valuable tidal wetlands.
Planning and design phase
In 2012 Building with Nature tested whether Spartina plants pre-grown in coconut fibre mats can be installed in front of existing salt marshes and/or artificial salt marsh defense structures. Before planning and design could lead to actual construction, the project had to be legally approved. Because the Dutch Eastern Scheldt is a national park under Natura 2000, local authorities had to verify whether the installation of the cord grass mats would not jeopardize the local conservation goals of the area.
To see whether the pre-grown cord grass mats had an advantage over individually planted Spartina plants, cord grass mats and loose plants were compared at various sites in the Eastern Scheldt. Three locations were chosen: Rattekaai, Sint Annaland and Dortsman (see figure on the right). The locations differ in hydrodynamic stress (from sheltered to exposed). At Rattekaai and Sint Annaland, the patches were installed directly in front of the salt marsh and in front of the dams designed to protect them. At Dortsman similar patches were installed, but only in front of the marsh edge as no dams are present there. Additionally, patches were installed at two distances from the “cliff” edge (cliff edge distinguishes a man-made hard structure e.g. dam, while the marsh edge is the natural edge of the marsh), 5 and 40 m offshore, respectively (see figures on the right). This was done because survival of cord grass close to a cliff (dam or marsh edge) may be difficult due to extra wave action induced by wave reflection from the cliff. The different distances from the cliff also represent the middle and high intertidal area, which enables comparing the combined effects of the cliff and the inundation time.
Three replicas were applied at each saltmarsh and at each distance from the edge. In total about 300 m2 of mats were installed, divided over the three locations: 120 m2 at Rattekaai and Sint Annaland each and 60 m2 at Dortsman. An individual mat of 5 by 1 m contained 10-14 plans per m2 on average. The mats were cut up in 4 by 1 m mats and 1 by 1 m mats. The experimental set-up consisted of two 4 by 1 mats placed next to each other and several 1 by 1 m mats next to that. Different configurations were tested. Similar numbers and densities of loose cord grass plants were installed at 10 m distance from the mats, in order to have no interference and yet as similar conditions as possible. After installation, the survival and growth of the Spartina plants, as well as their effect on the sedimentation/erosion of the tidal flat was monitored. As the experiments are on-going, only preliminary results can be discussed.
There were two potential threats: the cord grass used in this experiment originated from the Swansea salt marsh in Wales and the mats were also pre-grown in Wales. The former might lead to a take-over of the existing Spartina population, the latter involved the possibility of hitch-hiking other organisms to invade the Eastern Scheldt. Both threats were thought to be non-significant, because: 1) all cord grass Spartina anglica in western Europe is genetically closely related to its source population in Poole Harbour; 2) the mats were pre-grown in freshwater basins, so there was no chance for other organisms to survive the transfer to the salt water of the Eastern Scheldt. Experts were consulted and they supported the pilot experiment and did not foresee any impediment. For these reasons, local authorities granted legal permission for the experiment.
The concept of ecosystem engineering is adopted when investigating the potential use of cord grass (Spartina anglica) in the higher intertidal zone for consolidation and stabilization of tidal flats, and for the creation/restoration of pioneer salt marsh zones. One of the problems is that (re-)establishment of coastal vegetation on bare mudflats is often excluded by the harsh environmental conditions (i.e. wave- and current-induced hydrodynamic forces) which hamper settlement and survival. Therefore, the project aims at developing a suitable substrate in which Spartina plants can establish before being put in place, in order to make them less prone to wash-out by the hydrodynamic forces. Whether the Spartina plants can grow out into larger tussocks and subsequently into healthy meadows depends on additional factors such as sufficient sediment input.
The project fits within a larger concept of using a cascade of natural stabilizing measures from the low intertidal up to the dike. As such the project is an extension of the BwN oyster reefs projects in the lower intertidal zone.
Pre-grown cord grass mats were provided by Nautilus–Ecociviel . Installation of the mats at the three test locations was done by NIOZ, IMARES and Nautilus–Ecociviel.
The company Nautilus-Ecociviel grew Spartina plants in mats of coconut fibre and grew them for 1.5 years in freshwater basins, as no salt water basins were available (see figure on the right). Nautilus-Ecociviel has experience with the growing of freshwater and brackish water littoral plants in coconut fiber mats (Aqua-Flora Filter Mats type FM15). Plant density was about 10-14 per m2.
Mats were rolled up for transportation. Because the mats were heavy and the installation sites far from the road, transportation of the mats to the sites was difficult. Only one marsh could be reached by car. A special boat was used to bring the mats to the right place during high tide. The actual installation of the mats was simple. During low tide the mats are rolled out on the mudflat and fixed with biodegradable pins. Edges of the mats were dug into the bed to prevent dislodging. Single plants were planted according to the design scheme.
Operation and Maintenance phase
During the operation and maintenance phase of the Building with Nature pilots, the mats and cord grass and their effects on the adjacent intertidal flats were monitored.
Effectiveness of cord grass planting
The growth of Spartina on coconut fibre mats is technically possible. At the time of deployment, however, the condition of the plants was not optimal and a few weeks later sub optimal growth and even some mortality was observed. There are several possible explanations for this. First, after being pre-grown in a freshwater environment, the Spartina plants suffered from salt stress. A short acclimatization period in brackish water basins did not help. Second, the Spartina plants experienced heat stress during the installation period, because of the very hot summer weather at the time. This may have influenced the survival of the plants. Third, after a relatively long pre-growth period in the fresh water, the root system of the Spartina plants was probably too far developed. This, combined with the poor condition of the plants, hampered the formation of new roots after installation on site. Mats with younger plants grown in salt water are installed earlier in the growing season are likely to have a better chance to succeed. This needs further testing.
In October 2012 mats were still present, as well as the plants inside the mats. Several of the loose plants have disappeared. At some sites the mats were covered with a layer of sediment as a result of local hydrodynamics, whereas at other sites the mats are still clearly visible. After the winter the survival will be monitored again.
Although the salt marsh did not develop in the way it was planned in this project, lessons can be learned from it.
- Pre-growing Spartina plants in coconut fibre mats is technically possible. The growth is best done under environmental conditions similar to those where the mats will be deployed. In the current pilot, cord grass was grown in freshwater basins, due to logistic constraints. This is probably one of the reasons for the lesser growth performance after installation in the field.
- All mats were installed in very hot weather. There was significant heat stress during transport and before the mats were inundated by the sea. This may also have contributed to the mortality observed after installation in the field. Installation of the mats would therefore better be planned earlier in the growing season (e.g. in June).
- After installation, cord grass rooting formed a problem. This problem was possibly due to the overall condition of the plants. After 1.5 year in the fresh-water basins, all plants contained a highly dense and mature root system. Using mats with younger plants with a less developed root system would probably increase rooting after installation. Also clipping of the roots might help.
- New pilots are needed to see if cord grass mats can be used as a means of sustainable salt marsh restoration in estuarine systems such as the Eastern Scheldt and Western Scheldt.
- As sediment availability in the Eastern Scheldt is very low (suspended sediment concentrations in the channels on average below 20 mg/l), the outgrow of the plants in larger tussocks is expected to be slow. In more turbid estuaries with a higher sediment availability, such as the Western Scheldt, outgrow is expected to be faster, especially at less exposed sites.
- Beaumont, N. J., M. C. Austen, et al. (2007). Identification, definition and quantification of goods and services provided by marine biodiversity: Implications for the ecosystem approach. Marine Pollution Bulletin 54(3): 253-265.
- Friess, D.A., K.W. Krauss, E.M. Horstman, T. Balke, T.J. Bouma, D. Galli, E.L. Webb (2012). Are all intertidal wetlands naturally created equal? Bottlenecks, thresholds and knowledge gaps to mangrove and saltmarsh ecosystems. Biological Reviews 87: 346-366.
- Kirwan, M. and S. Temmerman (2009). Coastal marsh response to historical and future sea-level acceleration. Quaternary Science Reviews 28: 1801-1808.
- Mangi, S. C., C. E. Davis, et al. (2011). Valuing the regulatory services provided by marine ecosystems. Environmetrics 22(5): 686-698.
- Shepard, C.C., C.M. Crain and M.W. Beck (2011) The protective role of coastal marshes: a systematic review and meta-analysis. PLoS ONE 6(11): e27374.
- Van Wijnen, H. J. and J. P. Bakker (2001). Long-term surface elevation change in saltmarshes: A prediction of marsh response to future sea-level rise. Estuarine, Coastal and Shelf Science 52(3): 381-390.
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