Habitat requirements for salt marshes
Salt marshes are found in the upper-tidal coastal zone between land and salt or brackish water in areas where strong wave action is absent and sediments are able to build up. The salt marshes are part of the environment silty shores. It’s important for the realisation of salt marshes that the right pre-conditions are present or can be realized. This Building Solution gives habitat requirements for salt marshes with mineral soils and low organic levels. It focuses on the necessary preconditions as salinity, inundation time, hydrodynamics, substrate, sedimentation and suspended solids.
In the figure below the light blue boxes show the advantages of using a salt marsh. In the black boxes some of the main necessities for creating a salt marsh are depicted.
This Building Solution contains a detailed overview of the habitat requirements for salt marshes. The Building Solution provides a guideline to check whether a certain location is suitable or can be made suitable for the establishment of salt marshes. Through an overview of conditions and a flow chart in the tab 'how to use', planners and designers can determine whether salt marshes are a promising option for incorporation into a design. Practical experience with the design (conditions) of salt marshes is gained in the projects of Harlingen and Delfzijl (The Netherlands). The lessons learned in these projects considering the habitat requirements are described under the tab ‘Practical applications’.
With this Building Solution , anyone having a basic level of knowledge and/or working experience in temperate coastal systems can make a first-order assessment of the suitability for salt marshes on a specific location. To fully assess the suitability for salt marshes one should have additional morphological and ecological expertise on salt marshes.
Natural salt marsh on the island of Texel, The Netherlands
The two main added values of a salt marsh to Building with Nature (BwN) type of projects are:
- Enabling the realisation of natural barriers for coastal defence due to wave reduction
- Enabling adaptation to sea level rise
Salt marshes provide several additional ecosystem services as improvement of the water quality, biodiversity, rest and foraging places for migrating birds, carbon storage, wave reduction, sediment management (beneficial use of dredged material), silt agriculture, limitation of bank erosion and a recreational value due to its attractive landscape.
- Salt marshes are attractive landscapes and have high nature values
- Decreases shore erosion by dissipating wave-energy.
- Water quality improvement through filtering nutrients and contaminants
- Can keep up with a (certain) rate of relative sea level rise by sediment trapping.
- Rest and foraging place for migrating birds
- Carbon storage
- For settling of seeds the soil needs to be relatively stable. High energy systems are not suitable for pioneer settling of salt marshes. However, the hydrodynamic energy needs to be sufficient to keep sediment suspended and to transport sediment to the upper part of the marsh.
- A seed bank in the very near vicinity of the pioneer salt marsh is necessary. Otherwise seeds or propagules need to be added articificially from similar watersystems elsewhere.
Habitat requirements for salt marshes
Salt marshes are ecosystems that are vegetated by halophytic plants and that are regularly flooded by the sea. The salt marshes discussed here are located in a temperate climate, at the upper part of the intertidal zone. They extend vertically from well below Mean High Tide up to the yearly highest water levels.
The tidal flow brings in fine-grained suspended sediment. Eco-engineering plants trap this sediment, resulting in a marsh surface that steadily grows upwards. This is called marsh accretion. Typical marsh accretion rates are in the order of a centimeter per year, depending on tidal range, soil stability by the local vegetation and the availability of suspended sediment. Marsh accretion rates need to be equal or preferably higher than current rates of enhanced sea-level rise to ensure a long-term sustainable salt marsh.
Several marsh types can be found in estuaries. Salt marshes, freshwater marshes and peat-based marshes.
- Salt marshes are inundated by saline water and have an average salinity greater than 0.5 g of solutes per kg of water (Odum 1988). Ample availability of sediments results in marsh accretion of these salt marshes.
- In areas with little sediment in the water, the soil consists of peat produced by the salt-marsh vegetation (Allen 2000).Here, marsh accretion is mainly promoted by the build-up of organic material from the local vegetation, resulting in peat-based marshes.
- Sometimes estuaries harbour comparable ecosystems but with a freshwater vegetation instead of salt-tolerant plants. In that case we deal with freshwater marshes.
Important habitat requirements handled in this Building Solution may differ for peat-based marshes and freshwater marshes. In this Building Solution focus is on salt marshes that are inundated by saline water. Furthermore, this Building Solution describes the habitat requirements for a salt marsh ecosystem, without a description of specific plant species. Depending on the geographic location of the marsh and salinity conditions, different species will establish/settle. Different developmental stages can be distinguished in time, with a succession from pioneer to climax salt marsh. For more information on plant species in temperate salt marshes, one is referred to http://www.waddensea-secretariat.org/monitoring-tmap/manual-guidelines and to table of species.
The Trilateral Monitoring and Assessment Programme (TMAP) is the common monitoring programme for the Wadden Sea. In this programme three main salt marsh zones are distinguished: the pioneer zone where plant growth starts at about 40 cm below mean high tide (MHT); the low marsh, inundated during mean spring tides (100-400 floods/year), and the middle/high marsh with less than 100 floods per year. In addition, the sandy green beach and the brackish marsh can be differentiated by a special type of vegetation. Adjacent to the salt marshes, fresh (anthropogenic) grassland occurs.
Salt marshes have high nature values and are protected under European and national laws. Due to their relatively high elevation within the tidal frame, they reduce wave height during storms. As a result, salt marshes play an important role in contributing to coastal protection.
The description of salt marsh habitat requirements is based on the 4-spheres approach: biosphere, hydrosphere, lithosphere and atmosphere. They all interact with each other and cannot be evaluated on their own. For each sphere, relevant salt marsh parameters are described. The limit values are provided if they are known and generally valid and applicable. These values are derived from literature. A complete overview of the four spheres and the parameters relevant to salt marshes is shown in the habitat requirement tree below.
Tip: Select a 'sphere' in the diagram below for more detailed information.
How to use
If an ecosystem engineer is considered to be included in a design for coastal protection or coastal rehabilitation, several questions need answering:
- Is it possible to create a suitable habitat for a specific ecosystem in the project area?
- What would be the envisaged services provided by this ecosystem?
- To what extent can the ecosystem contribute to the primary function of the design and how does this affect the design itself? For example, what dimensions of a shellfish reef are needed to reduce erosion or stabilize sediment? And what dimensions to act as an efficient wave reflector?
- What effects do the ecosystem engineers in this ecosystem have on the existing physical, ecological and socio-economical system?
- What are the costs, uncertainties and risks ensuing from including these ecosystem engineers in the design?
In this Building Solution, the focus lies on the habitat requirements for shellfish. The determination flowchart gives a first answer to the suitability of the project area as a habitat for shellfish. Other questions can be elaborated in subsequent or parallel steps.
Determination flowchart for salt marshes
Determination flow chart
This flow chart can be used to determine the suitability of a site for marsh development. At the end of the flow chart we assume establishment of pioneer vegetation (Spartina anglica and/or Salicornia spp.) with the proper environmental conditions for a sustainable marsh to develop. The set-up of the flow chart and the habitat requirements for salt marshes are discussed below. The colours in the flow chart correspond to the colours as used for the spheres under the section on 'habitat requirements'.
Use of the flow chart is further demonstrated in the two case studies presented under 'Practical Applications'.
Two practical examples are presented, Mud Motor Wadden Sea Harlingen, and Salt Marsh development, Marconi, Delfzijl. Both Delfzijl and Harlingen are harbour cities situated in the north of the Netherlands.
Harlingen (left) borders the Wadden Sea area. The Wadden Sea is a coastal sea protected from the North Sea by barrier islands. It is home to the largest unbroken system of intertidal sand and mud flats in the world. Since 2009 it serves as UNESCO world heritage site. (http://whc.unesco.org/en/list/1314/).
Delfzijl (right) is situated along the Ems-Dollard estuary. This estuary connects the Ems river to the Wadden Sea. The Ems-Dollard estuary is one of the last two open estuaries in the Netherlands.
Both Wadden Sea and Ems-Dollard estuary are of significant ecological importance for estuarine and intertidal flora and fauna.
Click on the area maps for a project description.
- Allen JRL (2000) Morphodynamics of holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe. Quaternary Science Reviews 19: 1155-1231
- Bakker JP, Bunje J, Dijkema K, Frikke J, Hecker N, Kers B, Körber P, Kohlus J & Stock M (2004). Chapter 7: Salt marshes. In: Essink K, Dettman C, Farke H, Laursen K, Lüerβen G, Marencic H & Wiersinga W (Eds.). The Wadden Sea Quality Report 2004. Wadden Sea Ecosystems No. 19-2005. Trilateral monitoring and assessment Group, Commen Wadden Sea Secretariat, Wilhelmshaven, Germany. http://www.waddensea-secretariat.org/sites/default/files/downloads/08-saltmarshes-10-09-21_0.pdf
- Cahoon DR (2006) A review of major storm impacts on coastal wetland elevations. Estuaries and Coasts 29: 889-898.
- Ford H, Garbutt A, Ladd C, Malarkey J, Skov MW (2016) Soil stabilization linked to plant diversity and environmental context in coastal wetlands. Journal of vegetation Science 27: 259-268.
- Odum WE (1988) Comparative ecology of tidal freshwater and salt marshes. Annual Review of Ecology and Systematics 19: 147-176.
- Olff H, De Leeuw J, Bakker JP, Platerink RJ, Van Wijnen HJ, De Munck W (1997) Vegetation succession and herbivory in a salt marsh: changes induced by sea-level rise and silt deposition along an elevational gradient. Journal of Ecology 85: 799-814.
- Huiskes AHL, Stienstra AW, Koutstaal BP, Markusse MM, Van Soelen J (1985) Germination ecology of Salicornia dolichosachya and Salicornia brachystachya. Acta Botanica Neerlandica 34: 369-380.
- Kirwan ML, Guntensperger GR, D’Alpaos A, Morris JT, Mudd SM & Temmerman S (2010) Limits on the adaptability of coastal marshes to rising sea level. Geophysical Research Letters 37, L23401.
- Temmerman S, Govers G, Meire P & Wartel S (2003) Long-term tidal marsh growth under changing tidal conditions and suspended sediment concentrations, Scheldt estuary, Belgium. Marine Geology 193: 151-169
- Temmerman S, Govers G, Meire P, Wartel S (2004) Simulating the long-term development of levee-basin topogrpahy on tidal marshes. Geomorphology 63:39-55.
- Temmerman S, Bouma TJ, Van de Koppel J, Van der Wal DD, De Vries MB, Herman PMJ (2007) Vegetation causes channel erosion in a tidal landscape. Geology 35: 631-634
- Van Duin WE, Dijkema KS (2012) Randvoorwaarden voor kwelderontwikkeling in de Waddenzee en aanzet voor een kwelderkansenkaart. Rapport C076/12, IMARES, Wageningen University & Research. http://edepot.wur.nl/220052
- Van Loon-Steensma JM, De Groot AV, Van Duin WE, Van Wesenbeeck BK, Smale AJ (2012) Zoekkaart kwelders en waterveiligheid waddengebied. Alterra Rapport 2391, Alterra, Wageningen University & Research. http://edepot.wur.nl/244770
Natural salt marsh on the island of Texel, The Netherlands (Alma de Groot, Wageningen Marine Research)
Overview of main preconditions (green) and the ecosystem services (grey). Adjusted from 'Deltares - Borsje'
Saltmarsh development, Marconi Delfzijl (NL) (under construction)