Enhancing salt marsh development: habitat requirements
Salt marsh ecosystems create biodiverse transitional zones between land and water, which serve as natural barriers for coastal defence through increased friction that reduces wave heights. For the establishment, restoration or development of salt marshes it is important that suitable preconditions are present or can be realized. On this webpage, habitat requirements for salt marshes with mineral soils and low organic levels are given. Focus is on necessary preconditions including salinity, inundation time, hydrodynamics, substrate, sedimentation and suspended solids. Through this, planners and designers can estimate if salt marshes are a promising option to incorporate into a design.
Blue boxes: benefits of a salt marsh. Green boxes: main necessities for development of a salt marsh.
Salt marsh characteristics
Salt marshes are ecosystems that are vegetated by halophytic plants and that are regularly flooded by the sea. Salt marshes distinguish themselves from peat-based marshes and freshwater marshes in that they are inundated by saline water and have an average salinity greater than 0.5 g of solutes per kg of water (Odus, 1988) whereby marsh growth is the result of ample availability of sediments. The salt marshes discussed here are located in a temperate climate, at the upper part of the intertidal zone. No description is given of specific plant species. Depending on the geographic location of the marsh and salinity conditions, different species will establish/settle. 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.
Salt marshes extend vertically from well below Mean High Tide up to the yearly highest water levels.
The tidal flow brings in fine-grained suspended sediment. Plants trap this sediment, resulting in a marsh surface that steadily grows upwards. Typical marsh accretion rates are in the order of a centimetre 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.
Different developmental stages can be distinguished in time, with a succession from pioneer to climax salt marsh. In the Trilateral Monitoring and Assessment Programme (TMAP), a common monitoring programme for the Wadden Sea, 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);
- the middle/high marsh with less than 100 floods per year.
Salt marshes have high nature value and are protected under European and national laws.
Salt marsh benefits and their potential in coastal designs
Salt marshes can be interesting to incorporate in a (coastal) design as they:
- can be used for coastal protection as they reduce wave energy due to presence of foreland including additional friction provided by vegetation;
- are a sustainable form of coastal protection as salt marshes can keep up with (certain rates of) sea level rise by trapping sediment;
- can improve water quality as salt marsh vegetation traps sediment;
- have high natural value and can contribute to biodiversity
- form rest and foraging places for migrating birds
- store carbon
- form an attractive landscape / have recreational value;
- can be used for silt agriculture
Salt marsh habitat requirements - Biosphere
Seeds and propagules
Pioneer salt marsh vegetation require seeds or propagules to establish. If no salt marsh is present in the very near vicinity (adjacent to the proposed location), establishment will need to occur from seeds and propagules transported via the water. Most salt-marsh plants disperse by water-transported seeds or propagules that can travel quite large distances. Therefore, establishment of pioneer vegetation on a new salt marsh should occur relatively fast, unless there are no salt marshes located in the system. In the latter case it might be necessary to introduce seeds artificially.
Pioneer vegetation establishment
Salt marshes can develop at places where pioneer vegetation can establish, which is governed by several factors. Seed availability, environmental conditions allowing germination, and soil stability preventing seeds/seedlings from washing away, will together determine whether a pioneer vegetation can establish. Seeds of pioneer plant species can germinate under saline conditions, although less saline conditions will increase the proportion of seeds germinating (e.g. Huiskes et al. 1985). Sowing before rainfall is therefore advised but not necessary.
Pioneer vegetation stabilizes the soil and promotes fine-grained sediment to accumulate on the marsh. Sediment deposition (together with organic carbon accumulation from the local vegetation) results in: 1) an increase in marsh elevation, 2) a reduction in flooding frequency and 3) a reduction in high stress (waves/currents) environmental conditions. All these factors promote vegetation succession (Olff et al. 1997). Plants from later successional stages (e.g. several grass species) and increased plant diversity will further enhance the stability of the soil and increase the resilience of the marsh against strong current and waves (Ford et al. 2016).
Salt marsh habitat requirements - Hydrosphere
Salt marshes are vegetated habitats bordering saline or brackish water bodies under tidal influence. Salt marshes grow in the upper part of the intertidal zone. Regular flooding is essential. Within a salt marsh, various zones can be distinguished based on flooding frequency. The pioneer zone is flooded twice daily, the low marsh zone is inundated during mean spring tides (100-400 floods each year), and the middle/high marsh zone is flooded less than 100 times per year (Bakker et al. 2004).
Sheltered hydrodynamic conditions
Shelter from strong currents (cross-shore and longshore) and tidal waves is essential to prevent erosion and allow establishment of pioneer vegetation to stabilize the area. In contrast, the hydrodynamic energy should be high enough to transport sediment to the upper part of the marsh. A current velocity in the order of 1.2 m/s may be considered as an upper threshold for natural marsh development (Van Loon-Steensma et al. 2012). However, this is highly location specific, depending on direction of the currents, grain size of the initial substrate and age of the system. Older marshes can withstand higher hydrodynamic energy than younger marshes.
Suspended sediment concentration
Enough sediment in the system is essential for salt marshes to develop towards a long-term sustainable marsh (Temmerman et al. 2003). Based on a theoretical model a mean suspended sediment concentration (SSC) of ~20 mg/L is given as the minimum concentration necessary for marshes to keep up with conservative projections of sea-level rise (Kirwan et al. 2010).
Salt marshes have an average salinity between 18.0 and 35.0 ppt; a salinity below 0.5 ppt is considered as tidal freshwater marsh (Odum 1988). Within a marsh, there is a gradient of salinity inversely related with increasing marsh elevation. This is represented by zonation with different plant species.
Salt marsh habitat requirements - Lithosphere
Marshes increase in elevation and thereby counteract soil subsidence. Typical marsh accretion rates are in the order of 10 cm/yr. Very strong rates of soil subsidence (higher than the local marsh accretion rate) will cause marshes to drown in the future. Especially when taking an increase in the rate of sea level rise.
At low tide, the marsh surface needs to be above water for vegetation to establish. The marsh needs to be flooded regularly to allow sediment to settle on the marsh surface. A pioneer zone is generally located at ~20 cm below Mean High Tide (max. 40 cm below MHT), whereas a low marsh is located at MHT (Van Duin & Dijkema 2012).
The slope will reduce the hydrodynamic energy of the incoming water and therefore reduce local erosion rates. Marsh slopes can range between 1:50 and 1:500. A 1:100 slope would be advised for a new marsh (Van Duin & Dijkema 2012).
Salt marsh substrates can vary from pure sand to clay and peat. Marsh development generally starts on an initial bare sandy plane (Olff et al. 1997). Presence of fine-grained sediment (silt and clay) in the substrate will increase soil stability (Ford et al. 2016), it will enhance the chances of pioneer vegetation to establish and reduce erosion during storm events (Houwing 2000).
During marsh development a natural drainage system generally forms consisting of a network of creeks. The drainage system forms due to feedbacks between vegetation, water flow and marsh morphology (Temmerman et al. 2007). From an engineering perspective, digging drainage channels is possible and stimulating drainage could increase the changes of pioneer vegetation to establish. However, this should not be necessary as channels should form naturally. It is advised to closely monitor the autonomous development of the drainage system.
Marsh accretion rate
The net marsh accretion is affected by sediment deposition, organic carbon accumulation from dead local plant material, erosion and soil compaction. The net marsh accretion rate needs to be equal and preferably higher than the enhanced sea-level rise together with any soil subsidence present.
After initial pioneer vegetation establishes and a salt marsh starts to develop, feedbacks between the vegetation, sediment deposition and local erosion will determine the marsh morphology. These feedbacks are a key feature of marshes. Patches of vegetation redirecting the water flow will cause a drainage system to form (Temmerman et al. 2007). Additionally, large heavy particles (sand) will travel a smaller distance from the sediment source (marsh edge or creek) than small lighter particles (silt) (Roner et al. 2016). The larger grains settle near the creeks, creating creek banks alternating with lower elevated depressions (Temmerman et al. 2004). This will positively affect plant diversity.
In the pioneer stage a limited nutrient availability limits the primary production. The local nutrient availability will increase as nutrient-rich sediment is depositing on the marsh surface (Olff et al. 1997). This stimulates primary production, resulting in vegetation succession and increase in plant diversity.
Salt marsh habitat requirements - Atmosphere
Salt marshes occur in temperate zones. In tropical zones mangrove systems can be regarded as salt marshes.
In early development, rain will increase the germination rate of marsh plants. In older marshes the impact of rain becomes increasingly important. In time, marsh zones nearest to the marsh edge get inundated more frequently and sediment with the largest particles settles here. This results in a faster marsh accretion rate near the marsh edge. A limited flooding frequency, poor drainage and high precipitation could cause the high marsh to become more brackish. Long-term standing water could cause plant die-off and local erosion to occur. This could reduce the resilience of a marsh to cope with extreme events such as storms.
Storms can have a large impact on the marsh accretion and erosion rates (Cahoon 2006). A marsh surface stabilized by local vegetation and with a high plant diversity will increase the resilience of a marsh to withstands strong waves and prevent erosion (Ford et al. 2016).
Sea level rise
Marshes can increase in elevation and thereby keep pace with the increase in sea level, making them very valuable ecosystems for coastal defence. The critical threshold value for an increase in sea level seems to be 1 cm/yr (Kirwan et al. 2010). Beyond this threshold, intertidal flats start to disappear, which may result in erosion of the pioneer zone (Bakker et al. 2005). Stone dams could be necessary to protect the marsh edge and prevent erosion at this point. When estimating the ability of marshes to keep up with enhanced sea-level rise, presence of soil subsidence should be taken in account as well.
How to use
When salt marshes (or comparable ecosystem engineering solutions with for example mangroves or shellfish banks) are 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 this 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 salt marsh are needed to reduce erosion, stabilize sediment or dissipate wave energy?
- 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?
Here, we focus on the first question, the other questions can be elaborated in subsequent or parallel steps.
Determination flow chart
To determine the suitability of the project area for salt marshes it should be checked whether the habitat requirements are present or can be created. The determination flowchart gives a first answer to the suitability of the project area as a habitat for salt marshes. 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. For information on the thresholds, see the text on salt marsh habitat requirements above.
Determination flowchart for salt marshes (for information on thresholds, see text on salt marsh habitat requirements above).
The following projects are two practical examples:
- Salt marsh development Marconi Delfzijl;
- Mud Motor in Port of Harlingen.
Both Delfzijl and Harlingen are harbour cities situated in the north of the Netherlands.
Salt marsh development Marconi, Delfzijl (NL)
At the far northeast coast of the Netherlands, salt marshes are developed with sediment from the surroundings. At this test site EcoShape acquired knowledge about the successful development of natural salt marshes. The aim is to investigate the best way to restore salt marshes by reusing sediment, while developing nature that contributes to the water quality, ecology, coastal defences and the attractiveness of the coast.
Click here to view full-sized infographic (with additional info)
Mud Motor in Port of Harlingen, Harlingen-Koehoal (NL)
The Mud Motor project looks at the potential for enhancing salt marsh development by making optimal use of the sediment transportation capacity of ambient flows.
Approximately 1.3 million m³ of mainly fine sediment is dredged annually from the harbour basins in the Port of Harlingen (NL) to maintain navigability. The dredged sediment is deposited in a nearby area and from there, a lot of dredged sediment flow back into the port relatively quickly. Therefore, an innovative approach to sediment management was proposed: deposit the dredged sediment further north of Harlingen and let natural processes spread the sediment to nearby salt marshes.
This way, the Mud Motor is expected to generate three beneficial effects:
- less recirculation towards the harbour, and therefore less maintenance dredging;
- promotion of the growth and stability of salt marshes, improving the Wadden Sea ecosystem;
- stabilisation of the foreshore of the dikes, and therefore less maintenance work on the dike.
- 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)
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