Coral reefs provide many essential ecosystem services, including a vital defence along coastlines: protection from wave erosion. Loss of diversity in coral reefs causes a reduction in the structural complexity of the habitat, which enables
higher wave erosion of tropical coastlines, threatening communities and terrestrial habitats.
Shallow water coral reefs are natural breakwaters or barriers, capable of attenuating up to 98% of incoming wave energy. Therefore, coral reefs are vital in protecting habitats and breeding grounds for thousands of species, used as
nurseries by juvenile organisms, and habitats that provide economic income and a food supply to millions of people.
Recent research carried out by a team led by Jérémy Carlot and Michalis Vousdoukas from the PSL Université Paris, France, has investigated the effect of coral reef structural complexity loss on coastal erosion rates on the island of
Mo’orea, French Polynesia.
Hydrodynamic modelling was combined with disturbance-recovery observations to assess the effects expected with current coral loss trends. They found that if structural complexity is reduced by 50%, waves of extreme heights reaching shore
will be an estimated 50 times more frequent. Structural complexity helps dissipation of wave energy, preventing damaging surges which abrade costalines at a rapid rate.
Low structural complexity in reefs is associated with poor health and can experience higher wave run-up heights compared to healthy reefsㅡreefs absorb 77% to 91% of the incoming wave energy. As more reefs lose their complexity, more
coastlines may become susceptible to frequent flooding and erosion.
As the model was tested under various conditions it found that, if further reef degradation continues at its current rate, severe weather events which occur once every 10 years, are estimated to increase in frequency to once every yearㅡthis
could cause substantial problems for coastal communities
The recent ocean warming events caused by climate change, combined with ocean acidification, pollution, and degradation by anthropogenic activities, have caused a significant loss of coral reefs globally in the past 100 years. This is of
high concern due to the many livelihoods, species, and ecosystem processes which rely on healthy reefs to function and thrive; hence reef research and conservation are of urgent importance.
The study investigated factors that may have led to unhealthy coral reefs and loss of coral complexity. It found that in one year (2021) unhealthy conditions were created after a predatory sea star outbreak and a cyclone the previous year.
These conditions reduced coral cover from 50% to 3% and halved the overall structural complexity of the reef.
The issues of coral complexity and reduction of wave heights are interconnected. Unhealthy coral reef conditions result in wave run-up heights 9.6% higher compared to healthy reefs.
On the other hand, the presence of a higher complexity profile was associated with healthier reefs. Coral reefs consist of multiple different coral types, which fulfil different roles within the reef system. Bouldering, or massive, corals
are slow-growing, compound and hardy corals, which provide an initial wave-breaking defence at the front of a reef, reducing the wave power reaching the rest of the corals.
‘Coral reefs consist of multiple different coral types, which fulfil different roles within the reef system.’
Foliose corals have an undulating, folded formation of many thin sheets coiled or layered, which resembles a lettuce growth form. These provide nooks and crannies in which many organisms hide, as well as further breaking up the reef and
slowing down water movement, preventing breakage of more delicate corals.
Branching corals, such as Acropora species, grow faster than other coral forms, adding height and shielding to the reef structure, which can help break up currents and create safe ‘still’ zones for juvenile organisms which may
be too weak to swim against strong currents. Fan corals, a form of branching coral, grow at the front of the reef, perpendicular to currents to maximise food capture; this also acts as a wave breaker, helping shelter the reef behind.
Digitate corals are shorter and sturdier than branching, though are often slower growing. These species have a high surface area which creates resistance against water movement, hence providing shelter and safety, or resting spots, for many
species and removing more energy from incoming waves which then are less powerful when they reach the shore.
Plating corals create vast flat plates, which may be textured or smooth, forming a layered canopy which is essential in sheltering organisms from predators, as well as providing additional defence against wave action and currents.
Encrusting corals form thin, flush layers across rocks or the skeletons of other corals, adding further diversity. This variation in types, as well as species, creates not only huge biological diversity but also structural complexity.
Over 500 million people worldwide are protected by reefs, as well as relying on them for fisheries and tourism-based livelihoods. This study was based on reefs at a high-energy wave site on the Ha’apiti shores of French Polynesia, however,
the findings apply on a general scale to many reefs across the globe which share a similar structural complexity.
With the intensity and frequency of tropical storms increasing rapidly as the climate warms, natural coastal protection must be maintained to protect coastlines from damage, flooding, and long-term erosion. Weather events are predicted to
get progressively more extreme in the coming decades, and artificial coastal defences are expensive and can be environmentally damaging, as well as unaffordable for many communities.
Coral reefs are being degraded across the tropics due to global warming, ocean acidification and anthropogenic pollution and disturbance, as well as destructive fishing practices. Reef ecosystems exist in a delicate balance of corals,
sponges and molluscs which provide the structural substrate in which herbivorous and carnivorous species live.
Herbivores graze the algae and prevent overgrowth, whilst carnivorous species control the herbivore species to maintain balance and ensure algae is not overgrazed. Apex predators such as sharks and large fish remove weak, injured, or
diseased organisms from the reef, control carnivore populations, and enhance nutrient cycling by transporting and depositing nutrients across reefs.
‘Natural coastal protection, such as reefs, must be maintained to protect coastlines from damage.’
Reefs which contain a high diversity of coral forms and species have the highest functionality as coastal defence systems, as well as biological habitats, carbon sinks, and nutrient powerhouses. It is of urgent importance that the
structural complexity of reefs across the Earth’s coastlines is conserved to ensure the least possible damage to coastal systems and communities in future weather events.
Research such as this is needed to allow an understanding of the potential consequences of reef loss to be predicted, as well as to gain traction and funding for active conservation, protection, and increase public awareness. Increasing
global reef coverage and health will not only aid in protecting coastlines, but will enhance marine ecosystems through increased biodiversity, which in turn increases resilience against climatic change, supports fisheries, and provides a
more stable future for the earth’s inhabitants.
Carlot, J., Vousdoukas, M., Rovere, A., Karambas, T., Lenihan, H.S., Kayal, M., Adjeroud, M., Pérez-Rosales, G., Hedouin, L. and Parravicini, V. (2023) Coral reef structural complexity loss exposes coastlines to waves. Scientific
Reports. Volume 13, Issue 1, number 1683.