Habitat requirements for mangroves
This Building Solution focuses on the creation of tropical mangrove forests that dissipate wave energy in the intertidal zone and influence erosion and sedimentation. The development of a mangrove-based design requires integration of ecological and engineering knowledge. A first assessment can be made with limited site-specific knowledge, for instance making use of local and historical data. Subsequently, for each new case a detailed feasibility assessment has to be made, integrating generic and site-specific ecological and engineering knowledge with socio-economic aspects.
This Building Solution describes the habitat requirements for mangrove forests in muddy environments with a high sediment input. This guideline can be used to check whether a certain location is suitable or can be made suitable for the establishment of mangrove forests.
With this manual, anyone having a basic level of knowledge and/or working experience in tropical or subtropical coastal systems can make a first-order assessment. To fully assess the suitability for mangrove forests one should have additional ecological expertise on mangroves.
The added value of this Building Solution to Building with Nature (BwN) type projects is that it enables creating natural coastal barriers with forest-forming ecosystem engineers. Ecosystem engineers are certain key species that form structurally complex habitats. The implementation in BwN projects aims to combine the intended primary function, e.g., coastal protection and/or associated economic and social returns, with biodiversity enhancing habitat restoration/conservation.
Tip: In the figure below the light blue boxes show the advantages of implementing mangroves. In the black boxes some of the main necessities for creating a mangrove forest are depicted. Clicking on the image will give you more in depth information about mangroves.
Decreases shore erosion by dissipating wave-energy
Provides shelter for other species. Has a nursery function for juvenile marine animals. These animals are sometimes commercially interesting.
Providing wood and charcoal
Water quality improvement
Sustainable barrier which grows with (relative) sea level rise by trapping sediments
Mangroves only occur in tropical and subtropical environments
The inundation time must be between 7 and 13 hours a day
Needs space, a width of at least 150 m is necessary
Present (eco)system can be degraded
Maturation in protection function takes time. In Singapore it took 6 years after natural seedling before a mature mangrove forest was established (Lee et al., 1996). In the early stages of the mangrove forest it is sensitive and can only withstand low stress levels.
Habitat requirements mangroves
This Building Solution is developed for tropical mangrove coasts with relatively high sediment input, preferably clays and silts. For mangrove forests with a peaty substrate (> 30% organic material) ecosystem functions, accretion processes and relative sea-level rise impacts may differ and are not described herein. In addition, latitudinal limits (i.e. temperature zones) are not considered.
This Building Solution describes the habitat requirements for tropical mangrove forests based on the 4 spheres approach, because all these four spheres (Biosphere, Hydrosphere, Lithosphere and Atmosphere) interact with each other and cannot be evaluated on their own. The biosphere includes all living organisms, the hydrosphere relates to water in or near the earth, the lithosphere represents the solid materials, soils and rocks, and the atmosphere relates to weather and climate. For each of these spheres the relevant parameters are described, including the ranges within which the ecosystem can be established. These values are derived from literature. Based on these ranges one can estimate if a specific location can be made suitable for mangroves and under which conditions. A complete overview of the four spheres and the relevant parameters are shown in the flowchart in the 'How to Use' section.
Tip: for more detailed information on the spheres click the links below.
Determination flowchart for mangroves
How to Use
If an ecosystem engineer is considered to be included in a design for coastal protection or coastal rehabilitation, several questions need to be answered:
- Is it possible to create a suitable habitat for a specific ecosystem in the project area?
- What would be the envisaged (protection) services of this ecosystem?
- To what extent can the ecosystem contribute to the primary(protection) function of the design and how does this affect the design itself? For example, what dimensions of a mangrove forest are needed to reduce erosion or stabilize sediment? And what dimensions act as an efficient dissipator of wind and waves?
- 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 involved with including these ecosystem engineers in the design?
In this Building Solution, the focus lies on the habitat requirements for mangroves. The determination flowchart gives a first answer to the suitability of the project area as a habitat for mangroves. Other questions can be elaborated in subsequent or parallel steps.
The determination flowchart is based on the habitat requirements described in the general introduction. It comprehends the possibility for (natural) establishment of mangroves and the requirements to enable coastal protection. The function as coastal protector requires the ecosystem to be sustainable and robust. The goal of the determination flowchart is to give an easy, accessible and low-cost indication in an early stage of a project on whether a suitable habitat for mangroves exists or can be created. The flowchart can also be employed to determine if restoring or improving existing mangroves forests is an option, as it helps finding the possible causes of mangrove degradation or development stagnation.
The potential ecosystem services of mangroves are:
- The coastal protection value, e.g. erosion control and storm and flood protection by dissipating wave energy
- Water quality improvement by acting as a nutrient filter, by reducing turbidity via sediment trapping, etc.
- Providing a sustainable barrier which grows with (relative) sea-level rise by trapping sediments
Other ecosystem services, such as provision, cultural and supporting services are not evaluated here.
Before employing the flowchart it is important to establish which (eco)system is present in the current situation and which function(s) it holds. Creating something new always comes at the cost of what presently exists. If a certain ecosystem is present, like a tidal flat, coral reef, seagrass meadow or salt marsh, it is necessary to understand that system in order to be able to prevent its degradation. Different communities may even enhance each other: a coral reef, for instance, can dissipate wave energy and create a sheltered area for seagrasses. The analysis of the current system should extend beyond the project site and consider adjacent systems. One reason is that coastal protection works usually have a large influence on the sediment budget and hence on adjacent coasts, another is that adjacent communities may produce seedlings for natural settling in the project area.
It is important to keep in mind that an existing (eco-)system may have other functions, such as cultural or supporting services, that may be lost by constructing something new. For example: creating a mangrove forest might be less attractive in a recreational area where people swim. If loosing the current functions is considered acceptable when a (new) mangrove forest providing the desired ecosystem services is created in return, the determination flowchart can be used.
The goal of the flowchart is to give the answer to the following question:
‘Does the intended project area have potential to (re-)establish a sustainable mangrove forest which provides the desired ecosystem services?’
The flowchart aims to indicate whether the important habitat requirements that allow establishment and sustainable presence of mangroves are naturally present or can be engineered. The result can only be a first-order indication, as the dynamics involved are too complex to comprise in a generic tool. When more precise information is needed in later project stages, local mangrove expertise should be called in.
Before employing the flowchart, several issues have to be considered. Firstly, mangroves only occur in tropical and subtropical environments. Secondly, the history of the proposed site can give a good indication on the suitability for creating a mangrove forest. If a mangrove forest was present in the recent history, and is now lost e.g. due to human activities, this can be an indication that the environmental conditions (salinity, wave exposure etc.) are suitable for creating or restoring a mangrove forest. The causes of disappearance can give insight in the habitat requirements which need to be adjusted or created. Finally, soil and water contamination will hamper mangrove establishment and growth. The possibility to remediate soil and water quality should be considered first, before going through the flowchart.
When going through the flowchart, note that comments are available to interpret the values provided. Keep in mind that every location is unique and has its characteristic dynamics, and that mangrove ecosystems are characterized by complex interactions. This makes defining exact thresholds a challenge, which may explain why many habitat requirements often remain unspecified.
Natural establishment vs. planting
This guideline can help to identify the main criteria determining the suitability of a site for natural recruitment of mangroves. Restoring the conditions which are favourable for natural establishment yields higher success rates of restoration projects than planting mangroves on any convenient mudflat. When mangroves establish naturally, natural competition and succession of species will result in a natural forest structure, density and species zonation. Associated fauna is an essential part of a sustainable ecosystem. For example pollinator species (e.g. bats) are essential for long term functioning of the forest, and so are burrowing crabs allowing soil oxygenation and flushing of excess salt from the soil. In case natural establishment is considered impossible by lack of propagule availability or dispersal, seeding or planting of mangrove trees can be considered. It is important to note that, in addition to the critical habitat requirements that need to be met, seedlings are more sensitive than mature trees and can withstand lower stress levels during the establishment phase (Balke et al, 2011). In the literature several guidelines are available which describe how mangroves should be restored. For more information, a set of literature on mangroves (and wetland) restoration is available and can also be found on the References section of this Building Solution.
A final remark on natural establishment versus planting: natural establishment will start with germination of pioneer species. They will be succeeded and supplemented by middle zone species. After one year pioneer species can already be one meter high and function as a wave energy dissipator. From that moment on, the moment the mangrove forest starts to function as a coastal protection component. The effectiveness of the coastal protection function depends on the density of the forest, its age and the species in it. In Singapore, maturation of the forest after natural seedling establishment takes about 6 years (Lee et al., 1996).
On the right a couple of cases are presented providing some application experiences that can be used as inspiration for future designs.
This section describes the application of the Building Solution to a practical case. The study area for this application is East Coast Park (ECP), a recreational coastal park in Singapore. This site is selected for the practical application to show the use of the determination flowchart. The additional value of mangroves for a recreational area is not considered, but is probably low.
East Coast Park has been built on reclaimed land, which is subject to coastal retreat since reclamation in the 1970s. Numerous hard structures, mainly headland breakwaters, were constructed to combat these erosion problems.
There is an opportunity for a Building with Nature (BwN) approach to mitigate erosion at ECP and enhance biodiversity in the area. Currently, ecosystems like coral reefs, seagrass meadows or mangrove forests are limited or absent at ECP. With a BwN design, a suitable habitat can be created for such an ecosystem. An advantage of a BwN design is that the natural systems component is able to adapt to relative sea level rise.
Currently mangrove forests are not present at ECP. From the top figure on the right it can be derived that no mangroves were present at the old ECP coastline (before reclamation). At that time, the coast was fringed by intertidal sand and mud flats. Presently, beach plants such as sea hibiscus occur at the eastern side of East Coast Park. Sea hibiscus signifies the high water mark and the boundary between salt water penetration and the freshwater system, i.e. the back of the mangrove zone. Propagules of mangrove species frequently strand on the beaches near the high water line, due to the close proximity of other mangrove forests. These propagules are however not establishing because the habitat requirements are presently not met.
Fill in determination flowchart
Following the flowchart the aim is to find the reason for the absence of mangroves at ECP and to have an indication of whether mangroves can be considered as an ecosystem engineer for a BwN design there. At this moment no detailed design for ECP has been created yet. This provides the opportunity to include the BwN-concept in an early stage of the design phase in such a way that an optimal habitat for mangroves is created. Later in the design process, the possibilities for adaptation of the design will decrease as more aspects have become final and cannot be changed anymore. Yet, the possibility to incorporate mangroves in the final design can be considered in a later stage (using the flowchart).
For each parameter in the determination flowchart, information at the specific project location is needed. The following sources of information can be considered:
- Current state of the ecosystem at the site (occurrence and health of species present)
- Available literature
- Computational modelling
- Data collection (measurements/field work)
The above is hierarchic, based on the amount of effort it usually takes to obtain information from that particular source. Note, however, that this effort can differ significantly from site to site, due to local knowledge, specific circumstances, availability of measuring equipment, etc.
In the following section, the flowchart is discussed from top to bottom to give a description of the reasoning behind the habitat requirements.
Pollution - There is no indication that at present contamination is a problem at ECP, but economical activities, such as industries and shipping/harbours, and run-off from the densely populated area might cause pollution. High Nitrogen load might be an issue, as several drains discharge at the ECP coastline. High nutrient loads pose a problem for matured mangrove trees and are therefore not considered a limiting factor for the germination phase. If high nutrient load were a problem, at least small mangrove trees should be present at ECP. As they are not, pollution is probably not a limiting factor for mangrove establishment.
Inundation - The average tidal range in the area is 2-3m. In combination with the typical erosional beach profiles at ECP, this results in an intertidal zone of approximately 100 m width. The area offers a variety of inundation times and a shallow subaqueos area as the seafloor is sloping downwards. Hence, inundation is not a limiting factor.
Morphology - Mangrove forests can only establish on a wide convex mudflat which gradually dampens wave energy and provides for a wide intertidal flat. Since the coastal profile at ECP is steep, as a result of the land reclamation, the present bathymetry is one of the main reasons for the current absence of mangroves.
Soil conditions - Mangroves flourish in muddy environments (clay and silty soil). For the reclamation, coarse sandy material was placed on top of the original clayey/sandy deposits. The historic deposits are still exposed further offshore. The absence of mud in the intertidal zone, supports the conclusion that the habitat is unsuitable for mangroves. In general, when the coast is not muddy, it is an indication that the hydrodynamic energy might be too high for mangroves. Conclusion: soil conditions at ECP are not suitable for mangroves.
At this point, following the determination flowchart, it leads to the grey text balloon: ‘This site is not suitable for sustainable mangroves’. The large scale morphology (coastal profile and soil conditions) prevents mangroves to grow at ECP, despite propagule supply.
For demonstration purposes, the remaining steps in the flowchart will be followed.
Redox - No measurements or literature of the redox parameter are available. To determine this parameter, field measurements are required. The main purpose of measuring the redox potential is to ensure that anoxic conditions will not limit mangrove rooting.
Space - The coastal profile at ECP is steep and the intertidal area has a width of approximately 100 m. Hence, space for a sustainable mangrove forest, with a pioneer zone, middle zone and back zone, is insufficient.
Salinity - Mangroves need freshwater input. This can be provided by runoff or precipitation. At ECP, no large rivers debouch. The area seasonally receives large amounts of precipitation, directly, via surface runoff and through drains. The geometry of the coastline supports flushing of the freshwater by the tide. Hence, retention times of freshwater, in the area that would potentially support mangroves, are negligible. Conclusion: salinity at ECP might be too high for mangroves.
Hydrodynamic energy - A wide convex mudflat can only be maintained when hydrodynamic energy is low. Mature mangrove trees can withstand reasonable amounts of hydrodynamic energy, but the muddy subsoil will erode easily. Mangrove roots do not penetrate deep into the soil (~0.5 m), a few decimeters of erosion can result in uprooting of the tree. The wave climate at ECP is moderate, with waves rarely exceeding 1 m in wave height. This will not limit mangrove establishment at ECP.
Sedimentation - Mangroves mainly occur in tropical intertidal zones. They need enough sedimentation in order to keep up with sea-level rise and subsidence. In general, sedimentation may not be too fast, otherwise it will smother the breathing roots. Since erosion occurs at ECP, continuous sedimentation will probably not limit mangrove establishment.
Connectivity - Proximity to other mangrove forests will supply the propagules and diaspores needed to support a sustainable mangrove ecosystem. Fauna can also migrate easily to the new mangrove spot when distances are not too large. Small crabs and lobsters will cause bioturbation of the soil and prevent anoxic conditions. This all contributes to a healthy and sustainable ecosystem. Proximity to other mangrove forests will increase the chance of success of including mangroves in a BwN design. Elsewhere in Singapore mangrove forests occur. Depending on currents and distance, propagules may arrive at ECP, but this needs to be checked by testing if propagules are found at ECP in the period that mangroves elsewhere in Singapore produce them.
This initial assessment shows that the large scale morphology of ECP in its present state is not suitable for mangrove development. Historical data show that the area has not supported mangroves before, which is a first indication that establishment will be difficult. The area lacks a wide convex mudflat, which is essential for wave damping and space requirements of a sustainable forest. Currently, wave energy is dissipated in a narrow zone. In addition the sediment is sandy, whereas mangroves prefer a muddy soil type and a sheltered location with sufficient tidal through flow. A convex mudflat indicates an accreting coast and a gradual wave attenuation. The salinity is possibly too high for establishment, due to the damming of the Gaylang rivers (reduced outflow) and the short retention time.
Although ECP is not a suitable environment for a sustainable coastal mangrove forest, this does not preclude other ecosystems like seagrass meadows and coral reefs as parts of a BwN development. Coral reefs can be a more attractive alternative (because of recreation values as well).
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- 'Sea hibiscus present at ECP (Singapore)' and 'Waves arriving at the ECP beach (Singapore)'. Photos by Daniel Martens.
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