Coral reef degradation increases coastal flooding and erosion risks 

Author: Jill Ashey

Editor: Skylar Collins

Paper: Carlot et al. (2023). Coral reef structural complexity loss exposes coastlines to waves. 

Coral reefs are known throughout the world for the many ecosystem services that they provide, one of them being actors of coastal protection. Reef structures naturally buffer shorelines against waves, storms and floods, mitigating property damage and erosion. Due to their proximity to the coast, coral reefs act as natural barriers, dissipating wave energy before it reaches the shoreline. 

More than 500 million people are protected by coral reefs that act as natural breakwaters. However, coral reefs are being increasingly affected by anthropogenic stressors such as higher ocean temperatures, ocean acidification, pollution, rising seas, and sedimentation. These stressors can degrade coral reefs, making them less effective as coastal protectors and leaving the coast vulnerable to large storms and floods. Given the importance of coral reefs in facilitating coastal protection, Carlot et al. (2023) investigated the natural protection of coral reefs in Mo’orea, French Polynesia. Specifically, they examined how the structural complexity of coral reefs dissipates incoming wave energy and assessed the protective capacity of reefs under real world and future conditions.

How did they do it? 

In order to quantify how reef structural complexity affects wave energy, researchers combined coral disturbance-recovery observations with hydrodynamic models. They focused primarily on reefs in Ha’apiti, a site with southwest orientation with high wave energy. Ecological sampling and structural complexity profiles were utilized in this study, with surveys taking place in 2005 and from 2008 to 2016. Each survey documented changes in coral colony abundance and size distributions for the three most prevalent coral taxa (Acropora, Pocillopora, Porites) along a 10 m2 transect. Using the survey data, a 3D model of coral assemblages was constructed and structural complexity of the 3D model was estimated. 

In-situ hydrodynamic and topographic (beach profile, reef morphology) measurements were also taken at the site to link the levels of structural complexities with wave energy dissipation capacity of reefs. These measurements were used to build and calibrate a Boussinesq Wave Propagation Model, a mathematical equation used to describe behavior of waves in shallow water. This model was utilized to assess a coral reef’s ability to attenuate incoming wave energy under all possible conditions for Ha’apiti. 

What did they find? 

The authors found that reefs in Ha’apiti can absorb 77-91% of incoming wave energy, affording the island of Mo’orea, French Polynesia tremendous protection. A strong correlation between reef structural complexity and wave dissipation was also identified, as has been demonstrated in previous work. Between 2006 to 2010, Mo’orea experienced a predatory sea star outbreak and an extremely destructive cyclone, which reduced living coral cover from 50% to 3% (Figure 1). Not only was coral cover reduced, but reef structural complexity was halved, leading to decreased coastal protection. 

Figure 1: Timeline of the structural complexity from 2005 to 2016 on the west side of the island of Mo’orea, French Polynesia. Ecological disruptions included a predatory sea star outbreak from 2006 to 2009 and a cyclone in 2010. Photographs illustrate the reefs in (a) 2005, (b) 2011 (classified as an unhealthy reef), and (c) 2016 (classified as a healthy reef). 

The Boussinesq Wave Propagation Model examined how wave run-up height, or the maximum onshore elevation reached by waves relative to the shoreline position in the absence of waves, was affected by wave height and reef structural complexity. The model found that wave run-up height increases with offshore wave height, and reduced reef structural complexity further escalates this pattern (Figure 2). Therefore, deterioration of coral cover will lead to higher frequency of extreme flooding. 

Figure 2: Residual run-up wave height according to reef structural complexity loss and the magnitude of the event. 

Why is this research important? 

This research provides important insights into the natural protection offered by coral reefs and how their structural complexity dissipates incoming wave energy. The findings of this research have practical implications for policy makers, as they can be used to make informed decisions about effective coastal protection schemes which preserve or strengthen existing ecosystem services provided by coral reefs. Additionally, these results highlight the need for urgent action to address the anthropogenic stressors that are rapidly degrading coral reef ecosystems that provide coastal protection for millions of people. 

Further reading

  • Beck, M. W.  et al. The global flood protection savings provided by coral reefs. Nat. Commun. 9, 2186 (2018).
  • Harris, D. L.  et al. Coral reef structural complexity provides important coastal protection from waves under rising sea levels. Sci.  Adv. 4 (2018).
  • Woodhead, A. J., Hicks, C. C., Norström, A. V., Williams, G. J. & Graham, N. A. J. Coral reef ecosystem services in the Anthropocene. Funct. Ecol. 33(1023), 1034 (2019).

Featured image: photo by Paul Selvaggion. 

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