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The majority of present-day coastal infrastructure can be redesigned to ensure not only coastal protection but also to create suitable habitats for certain ecosystems. This can be beneficial, as many natural ecosystems can contribute significantly to coastal protection. Moreover, most ecosystems, as opposed to traditional hard structures, are able to adapt to (relative) sea-level rise.
‘Ecosystem engineers’, i.e. species that influence their own habitat, form complex structures in the subtidal and intertidal zone and can provide sustainable shoreline or shoal edge protection. Next to coral as described here, other ecosystem engineers are mangroves, salt marshes, seagrass and shellfish. To successfully include ecosystem engineers in coastal protection, certain requirements have to be met for establishment and sustainable growth. Examples are requirements on hydrodynamic conditions, water quality, soil characteristics, light availability, etc., but also biological preconditions (e.g. connectivity to other or similar ecosystems). Parts of these requirements can be engineered or fostered through human intervention, while others cannot. This building block describes the habitat requirements for coral reefs.
General Building Block Description
There is increasing effort to identify ecologically-friendly alternatives to shoreline protection. The ideal is the replacement of man-made hard structures (the ‘hard approach’) with more natural systems (‘soft approaches’). 'Soft approaches' provide ecosystem services such as wave attenuation and sediment trapping. Most frequently and successfully employed are hybrid approaches that incorporate built structures in combination with ecosystem engineers such as vegetation, or with habitat-enhancing alterations. Completely soft-engineering strategies, which often include habitat restoration or rehabilitation, require the incorporation of engineering principles to make sure that the primary function of the design will be fulfilled.
The establishment and persistence of coral communities are wholly dependent on a range of highly-linked variables that in total define the environmental conditions of coral taxa, see Figure 1 (Harricott and Banks, 2002). While some of these may be manipulated, e.g. provision of hard substrata and correct elevation to reduce light limitation, many are ultimately dependent on large-scale processes and cannot be controlled. This Building Block is intended to provide ecological guidance on habitat requirements that are specifically needed to support coral communities rather than coral reefs. A coral reef is a complex ecosystem which can take a long time to develop and contains a large number of species. In this Building Block, the focus is on coral communities, as they are less complex but still have the ability to function as an ecosystem engineer. The Building Block is intended to help with a first assessment of feasibility for a coral community on a specific location by using available local data.
The following pages contain topics or projects related to corals:
- Artificial Reefs
- Species Response Curves
- Reduce impact of dredging on corals (Dominican Republic)
- Enhance the coastal values of coastal protection measures (Singapore)
- Tropical shelf seas and shores (usually this is the environment most suitable for corals)
With this manual, anyone having a basic level of knowledge and/or working experience in tropical coastal systems can make a first-order assessment. To fully assess the suitability for corals reefs one should have additional ecological expertise on corals.
The added value of this tool within Building with Nature-type (BwN) projects is that it enables the creation of natural coastal barriers with reef-building ecosystem engineers. Ecosystem engineers are certain key species that form structurally complex habitats. They aim to combine the intended function, for example, coastal protection and/or associated economic and social returns with habitat restoration/conservation by for example enhancing biodiversity.
This Building Block is mainly applicable in the planning and design phase and in the operation and maintenance phase. If the execution of the works requires relocation of corals, information in this Building Block is also relevant in the construction phase. An example where coral relocation was successfully applied is the Cruise Ship Terminal in Jamaica.
Tip: In the figure below the light blue boxes show the advantages of implementing corals. In the black boxes some of the main necessities for creating a coral community are depicted. Clicking on the image will give you more in depth information about corals.
Habitat requirements corals
This Building Block describes the habitat requirements for corals 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 constitutes all living aspects of the coral environment, and includes intrinsic (e.g. species tolerances) as well as extrinsic (e.g. competition with algae) factors. The hydrosphere relates to the liquid environment, and includes hydrodynamics and pollutants. The lithosphere represents the solid environment (e.g. substratum), while the atmosphere relates to climatic and meteorological conditions.
For each sphere the key parameters are described, and minimum and maximum values for coral persistence are provided where possible. These values are based on literature, but should not be prioritized over available local information. The values provide an overall guideline for the potential establishment of coral communities.
The diagram gives an overview of the environmental spheres, with the critical parameters highlighted in red. It should be noted that the thresholds for each parameter vary with species, life stage and exposure history and may be further modified by interactions with other stressors.
Tip:select a 'sphere' in below diagram 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 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 system?
- 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 coral reef are needed to reduce erosion or stabilize sediment? And what dimensions to 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 when including those ecosystem engineers in the design?
In this Building Block, the focus lies on the habitat requirements for corals. The determination flowchart, see Figure 6, gives a first answer to the suitability of the project area as a habitat for corals. Other questions can be elaborated in subsequent or parallel steps.
The determination flowchart is based on the habitat requirements described in the general introduction and helps the user to define the possibilities for coral establishment and/or restoration. It also provides an indication whether the area could be considered suitable for corals to provide an ecosystem service for coastal protection. The goal of the determination flowchart is to give a low-threshold and low-cost first indication, usually in an early stage of a project, whether corals are a viable option for coastal protection. It can also be employed to determine if restoring or improving existing coral reefs is an option, as it can suggest due to which problem(s) coral development stagnates.
The potential regulatory ecosystem services of corals are:
• The coastal protection value, e.g. erosion control, storm and flood protection by dissipating wave energy.
• Providing a sustainable barrier growing with (relative) sea-level rise.
• Enhancing other ecosystems by interactions of physical (hydrodynamic energy), chemical (sediment and nutrient) and biological (organisms) factors.
Other ecosystem services, such as the provisional (e.g. food), cultural and supporting services are not evaluated here. More information on these services can be found on the wiki Environment Page 'Tropical Shelf Seas and Shores'.
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. 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 proper 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 a present (eco)system can provide other functions, such as cultural or supporting services, that may be lost by constructing something new. For example: creating a coral reef might be unsuitable in an anchorage area. If losing the current state is considered acceptable and if it is replaced by a (new) coral reef that provides the desired ecosystem service, the determination flowchart shown in Figure 6 can be utilized.
The goal of the flowchart is to address the following question:
‘Does the intended project area have potential to (re-)establish a sustainable coral reef which provides the desired ecosystem services?’
The flowchart aims to provide an initial indication, as the dynamics are too complex to comprise in a basic and generic tool like this. If the flowchart indicates potential, an expert should be consulted to allow for more detailed knowledge to incorporate corals in the engineering stage of a project. Information on the creation on artificial reefs can be found on the page Artificial Reefs.
Before employing the flowchart, several items have to be considered. First of all, corals mainly occur in tropical environments. Secondly, contamination will hamper coral establishment and growth. The possibility to remediate the soil or clean the water should be considered first, before going through the flowchart. Finally, fishing can be a threat to coral reefs, especially cyanide fishing, dynamite fishing and trawling. This means that in some cases regulations should be made or (re-)enforced to prevent these ecosystems being damaged.
When going through the flowchart, note that comments are available to interpret the values provided in the flowchart. Keep in mind that every location is unique and has characteristic dynamics. In addition, coral reef ecosystems are very complicated. This makes it challenging to give strict thresholds, which therefore often are not specified.
Another aspect to consider before using the flowchart is the historic ecosystem presence in the area. If coral reefs were present in the past but absent or bleached in the present state, the history of the area is important to determine the cause and the chances of success in the future. Possible causes can be large scale climatological infrequent events or historic or current forms of (anthropogenic) pollution or unsustainable fishing methods (trawling, cyanide fishing, blasting).
In this section, a practical application is shown in which the above theory is applied to a real-life situation. The study area for this application is East Coast Park (ECP), a recreational coastal park in Singapore, see Figure 8. The whole park is located on reclaimed land which is subject to coastal retreat since reclamation in the 1970s. Numerous hard structures, mainly headland breakwaters, were constructed at ECP to combat these erosion problems. More information on East Coast Park can be found on the page Enhance the coastal values of coastal protection measures (Singapore).
There is an opportunity for a Building with Nature (BwN) approach to mitigate erosion at ECP in combination with the opportunity to enhance biodiversity in the area. Currently, ecosystems like coral reefs, seagrass meadows, oyster reefs or mangrove forests are rare or completely absent at ECP, although they may have naturally occured there. With a BwN design a suitable habitat for these ecosystems can be created or restored. Another advantage of a BwN design over traditional hard structures is that natural systems can grow with relative sea level rise. A BwN design may present challenges, for instance, creating habitats for undesired species. An example is the concern of the creation of still water pools which may attract mosquitoes. With a balanced and well thought-out design, these issues can be mitigated or prevented.
Corals are found to a limited extent on the ECP shoreline and further east, at locations where a hard substratum is present. Near the Tanah Merah ferry terminal, for instance, coral occurs at the seaward side of the shore-parallel breakwater. Also near headland breakwaters some corals are found. This blogspot shows pictures of corals at ECP, young colonies with soft and hard corals. The hard corals found at ECP are probably better adapted to low salinities and low light levels. Corals require hard substratum for attachment. At ECP, most hard substratum is formed by breakwaters, since the reclaimed coastline consists of sand and the original deposits are clayey. The breakwaters, however, have their toe at the low water line, which limits the space for coral growth, since corals need to be submerged most of the time. In the past, prior to the reclamation works, intertidal corals were found at the rocky cliffs near the ECP coastline, see Figure 9. (Hilton & Manning, 1995) This is a first indication that coral establishment at ECP may be possible, but that the habitat has become less favourable over time. The possibilities and limitations for coral growth at ECP are explored using the determination flowchart.
Following the determination flowchart, see Figure 10, the aim is to determine the reason for poor presence of corals at ECP and how this situation can be improved. The goal is to give an indication if corals can be considered as ecosystem engineers in a BwN design for ECP. So far no detailed design for ECP has been decided on, which provides the opportunity to adapt the design in such a way that an optimal habitat for corals is created. Later on in the design process, the possibilities for adaptation of the design will decrease as more aspects of the design have become final and cannot be changed anymore. Still, the possibility to incorporate coral in the final design can be considered (using the flowchart).
For each parameter in the determination flowchart, site-specific information is needed. Sources of information that can be considered to obtain site-specific values for these parameters are, for instance:
• Current state of the ecosystem at the site (occurrence of species / health of species)
• Available literature
• Computational modelling
• Data collection (measurements/field work)
The sequence in this list represents a hierarchy, in that the amount of effort needed to obtain information from the specific source usually increases from top to bottom. Yet, the effort can vary significantly due to knowledge on the site, circumstances on site, availability of measurement instruments, etc.
In the following section, the determination flowchart is discussed from top to bottom to give the rationale behind each habitat requirement.
Bare hard substratum - Coral needs hard substratum for attachment. The substratum must be free from sediment and other loose material, and also from precolonising organisms such as macroalgae and sponges, which can exclude coral settlement. At ECP, the presence of hard substratum is poor. The available hard substratum is mainly intertidal, which is not in line with the preferred inundation levels. Much of the hard substratum in the lower intertidal is colonised by several types of macroalgae and filamentous algae (e.g. Enteromorpha) and encrusting sessile animals such as barnacles (Lee et al. 2009). Sponges were absent on the breakwaters, probably due to unsuitable inundation regimes (Lee et al. 2009). At some locations corals do settle at ECP, which indicates that precolonising is probably not a major problem. In sum, hard substratum is not abundantly present at ECP, which limits coral settlement.
Sedimentation - Corals can withstand occasional sedimentation as result of an event (e.g. a storm) of 250 mg/cm2/day. Some coral species have special techniques to free themselves from sedimentation. If the sediment cover is too thick or remains on the corals permanently, no photosynthesis can take place and the corals start to bleach, see Figure 11. In an environment with waves, resuspension and subsequent settlement of sediment on corals may occur at shorter time intervals. More information is needed to check whether this would prevent coral establishment at ECP. Sedimentation of eroded beach sediment and sedimentation of fines from the drains at ECP might cause a problem. Concluding, if corals are placed at a location where wave-induced sediment transport takes place, the stress they experience from settlement and resuspension of sediment has to be investigated. During the design phase, the (changes in) sedimentation and erosion patterns caused by the new coastal protection scheme should be elaborated.
Light - At least 6-8% of the surface irradiance is needed for the zooxanthellae to photosynthesize and provide the corals with sufficient food and energy. When light penetration is not sufficient, the cause for light attenuation in the water column can be investigated. This can be due to a high suspended solids concentration or a phytoplankton bloom. If this light attenuation cannot be mitigated, offering hard substratum at shallower water depth might be a solution, although this will require (re-)analyses of temperature and inundation regimes. At ECP, turbidity might be a problem. Coastal erosion brings sediment into suspension and turbidity levels can be high. In addition, several drains discharge at the shoreline, which might increase nutrient availability. The latter may promote phytoplankton growth, increasing not only turbidity, but also competition for light. More information and research is needed to judge what light level is available at ECP at different water depths. In sum, light might be a limiting factor for coral growth at ECP, but more research is needed for firm conclusions.
Water temperature - Optimal water temperature for coral growth is between 28 and 34 degrees Celcius. In the tropics, too high temperatures may be a problem in semi-enclosed basins with long retention times. In the coastal waters of Singapore corals occur and ECP is a coastal area with short retention times. Hence water temperatures at ECP are assumed to be suitable, although the actual temperature regimes arising from the design ought to be considered.
Submergence/Inundation - Intertidal corals do exist, see Figure 9 and 12, but submergence times must be relatively long. This means that corals need to be submerged at least 20 hours/day. In the current situation at ECP, hard substratum is offered with too infrequent and too short durations of submergence. This is a bottleneck for coral establishment and expansion. For an BwN design including corals as ecosystem engineers, hard substratum should be placed at greater water depths with more regular and extended periods of inundation, provided that light levels are high enough. At this moment the major part of the hard substratum at ECP is located in the intertidal zone, with too infrequent and too short durations of inundation. This hampers coral establishment, growth and persistence.
Salinity - Corals need a salinity higher than 15 ppt. For Singapore an upper limit of 36 ppt is advised, as higher salinity levels affect physiology and reproduction. The salinity levels at ECP are not known in detail. Fresh water from the sewage system is filtered and released into the sea at Changi (5 km from the water line at the bed). Also the drains, discharging at the ECP coastline can (periodically) decrease the salinity. It is assumed that the freshwater discharges are mixed and spread in such a way that salinity levels at ECP do not drop below 15 ppt. Hence salinity at ECP is probably not outside the required range.
Pollution - There are no indications that the waters at ECP are polluted. Yet, the proximity of major commercial ship anchorage areas raises the risk of pollution from ship-based sources. The most likely pollutants are oil and toxins from antifouling paints. Additional potential sources include nutrients and sediments from drain discharges and human activity from recreational users of the park. More information is needed to eliminate pollution as a threat for coral reefs.
Availability of larvae - Availability of larvae will enable natural recruitment of corals. Absence of larvae at the site will require introduction of larvae. Larvae availability is dependent on the distance to other corals and on currents that can deliver larvae at the project area. Since corals occur at ECP, larvae availability is not a problem.
The reasons for limited coral presence at ECP is probably a combination of a lack of hard substratum with suitable submergence conditions. The hard substratum present is located high in the intertidal zone, with relatively long emergence periods. Another issue may be the light availability. This may become increasingly important if a BwN design is pursued which aims at coral growth at deeper water. Water temperature, salinity and availability of larvae seem suitable. Further steps towards the BwN design should clarify whether sedimentation is an issue. This initial assessment indicates that coral growth at ECP is possible, if the presently unfulfilled habitat requirements are engineered.
Next steps in developing a BwN for ECP including corals would be:
- Determine the water depth range which meets the habitat requirement for submergence times and light availability for corals at ECP. Offering hard substratum at a greater water depth will meet the requirement for submergence, but may come with too little light availability.
- Investigate the stress caused by sedimentation and resuspension of sediments at ECP.
- Investigate and quantify potential pollutant sources.
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- Fig. 1. Factors and processes that influence the distribution of corals and coral reefs Harricott and Banks, 2002.
- Fig. 2. Coral succession
- Fig 3-5. Habitat requirements tree corals, four spheres.
- Fig. 6. Determination flowchart
- Fig. 7. Coral Reef. Photo: Florida Keys National Marine Sanctuary
- Fig. 8. Location of East Coast Park in Singapore. Adapted Google image.
- Fig 9. Occurence of intertidal coral near ECP coastline in 1953.
Hilton, M.J. & Manning, S.S. (1995). Conversion of coastal habitats in Singapore: indications of unsustainable development. Environmental Conservation 22, no. 4 (winter 1995), pp. 307-322.
- Fig. 10. Walkthrough determination flowchart for case East Coast Park.
- Fig. 11. Example of bleached with healthy corals in the Mariana Island (Guam, Oceania). Photo David Burdick NOAA.
- Fig. 12. Porites lutea. Great Barrier Reef, Australia. An intertidal 'micro-atoll'. Photograph: Charlie Veron.