Coral and oyster reefs are considered to be types of coastal wetlands. As a natural coastal defense they are a buffer for coastlines against waves. Reefs are threatened by rapid environmental change, making it very important to protect and restore reefs.
Coral reefs are shallow-water marine ecosystems characterized by massive calcium carbonate formations secreted by colonies of coral polyps and algae living in their tissues (Sheppard et al. 2005). Reefs build up as each coral species secretes uniquely shaped carbonate skeletons over older skeletal remains. The foundations of older reef structures are riddled with tunnels and channels created by physical and chemical erosion and the effects of reef inhabitants. Coral reefs are home to high fish and invertebrate biodiversity, all uniquely adapted to reef life, yet fundamentally dependent on coral survival. They tolerate little environmental variation and are particularly vulnerable to small changes in water quality, but they can recover once adverse events end, as long as local sources of colonizing organisms and suitable substrates are available (UNEP-WCMC 2006).
In their natural setting, oyster reefs are often found seaward of salt marshes (Scyphers et al. 2011) and are a source of valuable services both to ecosystems and humans. It is estimated that up to 80 per cent of the world’s oyster reefs have been lost, a rate unprecedented for any other marine habitat (TNC 2012). This loss also represents an enormous reduction in the ecosystem services provided by these reefs including food, habitat for bird and marine species and a buffer for coastlines against waves.
The sustainability of both coral and oyster reefs is also threatened by rapid environmental change, which overwhelms reef-species adaptation and resilience following destructive events. The pressures on reefs include human activities (such as sedimentation, water pollution, resources extraction and commercial fishing) (Waddell 2005; Burke et al. 2011), as well as the effects of climate change. Among the latter is the increasing atmospheric concentrations of carbon dioxide and excessive heat, which cause intolerable acidity and water temperatures (Hoegh-Guldberg et al. 2007; De’ath et al. 2009).
Where possible, conservation measures can be put in place and actions taken to deal with the pressures and causes of degradation of coral and oyster reefs. In addition to eliminating or mitigating the source of reef impact, methods of oyster reef restoration and coral transplantation are often used to increase the rate of coral and oyster colonization at damaged sites (Epstein et al. 2001).
For coral reefs, transplantation ideally starts with collecting fragments of living coral rock as soon as possible and storing them in a suitable location until they can be moved to the restoration site (Japp 2000; Epstein et al. 2001). Fragments can then be attached to suitable substrate. In difficult locations, artificial structures can be installed to provide a stable foundation for coral transplants (Japp 2000). Transplantation success depends on the species, transplant shape and type, status of substrate attachment and environmental conditions (Japp 2000). Where that is not possible, coral may be transplanted from nearby reef locations or from coral nurseries prepared in advance for restoration needs.
Similarly, oyster reefs can be constructed artificially to replicate their natural functions. Case studies show that creating large-scale man-made coral reefs is possible, and they are able to replicate many of the functions provided by naturally occurring coral reefs (TNC 2012).
For a long time, grey solutions have been dominant in coping with coastal hazards. Approaches include artificially hardening the shoreline or creating artificial barriers by dumping gabions made of cement and rock into the water (World Risk Report 2012). This is not only damaging to marine ecosystems, but can also shift the impacts of storms to communities down shore, increasing the need for additional defence structures.
There has been growing awareness and evidence of coral and oyster reefs playing a major role in coastal stabilization and coastal defence (World Risk Report 2012). Coral reefs provide natural breakwaters that can mitigate flooding and the erosive effects of storms along low-lying shores (Japp 2000; UNEP-WCMC 2006). They have shown to reduce the wave energy and height that impacts coastlines (Sheppard et al. 2005) attenuating and reducing more than 85 per cent of incoming wave energy (World Risk Report 2012). By forming a natural barrier, the reefs are the first line of coastal defence from the damaging impacts of waves, erosion and flooding.
Like coral reefs, oyster reefs protect from coastal erosion and wave erosion (TNC 2012). Evidence oyster reefs also prevent coastal marsh retreat (Scyphers et al. 2011). Due to their complex structure, these natural barriers reduce water velocities, increase sedimentation rates and provide improved conditions for settlement and retention of propagules, thereby improving the chances of species survival (Scyphers et al. 2011).
Coral reefs and oyster reefs have enormous significance in the lives of millions people. Tropic coastal populations in particular depend heavily on the resources provided by these ecosystems (World Risk Report 2012), where many reef species support fisheries and other livelihood sources (Burke et al. 2011). The reefs also play an important role in sustaining traditional lifestyles and carry cultural significance to local communities. In addition, they are home to rare species with relevance to e.g. production of medicinal products. Coral reefs are also popular tourist attractions (Burke et al. 2011) creating basis for significant income from the tourism sector, such as recreational scuba diving and snorkelling.
Oyster reefs are shown to provide food and shelter for crabs and fish species, which in turn increases the catch for fisheries. They also have shown to remove nitrogen from coastal waters, preventing algal blooms and dead zones (TNC 2012).
Restoring coral reefs is usually a very expensive and technologically complex exercise. The critical features making coral reefs such effective protection barriers, are the size, height, hardness and structural complexity of the reefs (i.e. friction) (World Risk Report 2012). Once lost, such features are very difficult and expensive to replicate. The best approach, therefore, is to protect reefs from external stressors before they are degraded, focusing on the sources of human impact. The creation of no-fishing zones at reefs, for example, appears to restore reef resilience and may make them somewhat less susceptible to increases in global temperature and carbon dioxide (Mumby and Harborne 2010; Selig and Bruno 2010).
A study on oyster reef restoration projects in the Gulf of Mexico, for instance, has shown that investments in restoration activities could yield a several-fold return on investment through gains in fisheries and avoided damage for properties and public infrastructure. The case study assessed a USD 150 million investment over ten years in restoring 160 km of oyster reefs in the Northern Gulf of Mexico. The assessment showed that the initial investment would be returned twofold in the period via new jobs and goods and services delivered to the local communities (TNC 2012).
The accelerated rate of global climate change requires particular consideration in relation to the long- term fate of restored reefs. The risks of eventual coral reef loss at the warmest edges of coral reef ranges, for example, can occur regardless of the success of the restoration efforts and need to be considered in connection with investment decisions. A troublesome concern is also elevated ocean acidity from increasing carbon dioxide (Hoegh-Guldberg et al. 2007). However, many studies have shown that in most cases investments in coral and oyster reef protection yield manifold benefits, once the socio- economic co-benefits are considered.
Burke, L., Raytar, K.,Spalding, M. and Perry, A. (2011). Reefs at risk revisited. Washington, D.C.: World Resources Institute.
De’ath, G., Lough, J.M and Fabricius, K.E. (2009). Declining coral calcification on the Great Barrier Reef, Science, vol. 323, pp. 116-119.
Epstein, N., Bak, R.P.M. and Rinkevich, B. (2001). Strategies for gardening denuded coral reef areas: The application of different types of coral material for reef restoration. Restoration Ecology, vol. 9, pp. 432-442.
Hoegh-Guldberg, O., Mumby, P. J., Hooten, A. J. et al. (2007). Coral reefs under rapid climate change and ocean acidification. Science, vol. 318, pp. 1737-1742.
Japp, W.C. (2000). Coral reef restoration, Ecological Engineering, vol. 15, pp. 345-364.
Mumby, P. J. and Harborne, A.R. (2010). Marine reserves enhance the recovery of corals on Caribbean reefs. PLoSONE, vol. 5, pp. e8657.
Scyphers, S.B., Powers, S.P., Heck Jr., K.L. and Byron, D. (2011). Oyster Reefs as Natural Breakwaters Mitigate Shoreline Loss and Facilitate Fisheries. PLoS ONE, vol. 6, p. 8.
Selig, E.R, and Bruno, J.F. (2010). A global analysis of the effectiveness of marine protected areas in preventing coral loss. PLoS ONE, vol. 5, p. e9278
Sheppard, C., Dixon, D.J., Gourlay, M., Sheppard, A. and Payet, A. (2005). Coral mortality increases wave energy reaching shores protected by reef flats: Examples from the Seychelles. Estuarine, Coastal and Shelf Science, vol. 64, p. 223–234.
TNC (2012). The Nature Conservancy, The Economics of Oyster Reef Restoration in the Gulf of Mexico: A Case Study in Mobile Bay, Alabama.
UNEP-WCMC (United Nations Environmental Programme/World Conservation Monitoring Center. (2006). In the front line: shoreline protection and other ecosystem services from mangroves and coral reefs. Cambridge UK: WCMC/UNEP
Waddell, J.E. (2005). The State of the Coral Reef Ecosystems of the United States and Pacific Freely Associated States: 2005. NOAA Technical Memorandum NOS NCCOS 11. Silver Springs, Maryland: NOAA/NCCOS Center for Coastal Monitoring and Assessment’s Biogeography Team.
World Risk Report (2012). Available from http://www.worldriskreport.com/.