Extreme Corals: Resilience in the world of intertidal reef flats

Written by Louise Anderson

Coral reef ecologists are generally envisaged working on the classically beautiful, tropical reefs of the kind portrayed in Blue Planet II. Less well-known is that there is also ample opportunity to paddle around in the strange, diverse world of intertidal coral reef flats. The Thai island of Phuket is home to several excellent examples of this kind of habitat. These sites completely changed my own perception of coral reefs, and the remarkably hardy corals that live in this environment are particularly exciting to reef scientists because of their ability to thrive under such challenging conditions.

Figure 1. Exposed corals at low tide. Notice that some of them have bleached pale or white

Readers who were lucky enough to get to the European Coral Reef Symposium in December last year will remember Professor Barbara Brown’s plenary speech about the importance and challenges of long-term monitoring of coral reefs, namely on the shallow slope and reef flat habitats of the west coast of Thailand where she has spent much of her career. These habitats are so interesting because they represent some of the extreme conditions that corals can tolerate. Studying intertidal reef flats can provide useful insight into how reef ecosystems may respond to a changing climate (Camp et al. 2018). One of these sites, called Ao Tang Khen, has yielded all sorts of exciting information about the physiological tolerances of corals, as a consequence of the Phuket Marine Biological Centre’s (PMBC) long-term monitoring efforts.

Ao Tang Khen experiences an exceptional range of extreme conditions, largely due to the low spring tides that expose the reef flat to the heat and sun for hours at a time. In addition to these low tides, the water itself is warm and turbid, and local impacts of human activity such as dredging and construction work exacerbate this stressful environment. Despite unfavorable conditions, coral cover is high (around 60-70%), and a surprisingly diverse coral community (up to 60 species) manages to thrive on this reef flat that extends 200m offshore.

One of the great joys of this particular site is how accessible it is. Walking out across the flat at low tide to lay transects in the sunshine is a real treat, with the occasional sea snake adding to the excitement. Other creatures to keep an eye out for include mantis shrimp, the odd moray eel and a dizzying number of crabs.

Figure 2. Ao Tang Khen at low tide during the 2016 coral bleaching.

Environment by numbers

  • There is a 3-4m tidal range
  • Corals are aerially exposed for up to 4h on a spring tide
  • Temperatures can fluctuate by 15°C on these tides
  • Sediment loads are 20-40mg/l
  • Seawater temperatures are generally around 29-31°C

Ao Tang Khen and Climate Change

Ao Tang Khen has weathered the sedimentation produced by nearby dredging, the operation of a tin smelting facility, changes in sea level and tsunamis. This is a testament to its resilience, but Ao Tang Khen and other reefs like it have not been immune to the impacts of global climate change. The site has been affected by a number of coral bleaching events, including the most recent 2014-16 global bleaching. Coral bleaching is a stress response, usually as a result of high temperatures, where the coral animal expels symbiotic algae from its tissue and turn white in the process. Corals that are bleached will generally die unless the stressor is quickly removed. Corals at Ao Tang Khen have experienced bleaching several times since monitoring began in 1979. Researchers suspect that there is a level of resilience at the reef flat not shared by nearby subtidal coral communities, due to rapid recovery trajectories recorded between bleaching events. The exact nature of this apparent resilience, from long-term community changes to the resistance of key coral species on the reef flat, is an active area of research. Continuing this monitoring is therefore an important part of understanding how corals can survive in changing conditions.

Why do intertidal sites matter?

It is important to highlight the invaluable work of the PMBC and others in ensuring that this area (and others like it) continues to be monitored. High-quality, long-term monitoring for coral reefs can be difficult to accomplish, and the importance of this resource will continue to be felt as we increasingly use our understanding of how corals live in the hot, turbid and intertidal habitats at the margins of their tolerances, to inform how climate change is likely to affect reefs in to the future.

Find out more:

  • PMBC Publications: http://dmcrth.dmcr.go.th/pmbc/download/
  • See Camp et al. (2018)’s recent review of what we can learn about corals in extreme environments, and Roche et al. (2018) to find out more about what ecological resilience is and why it’s important for coral reefs.


Brown, B. and N. Phongsuwan (2012). ‘Delayed mortality in bleached massive corals on intertidal reef flats around Phuket, Andaman Sea, Thailand’, Phuket mar. biol. Cent. Res. Bull. 71: 43-18 [online]. Available at: https://www.researchgate.net/publication/268422928_Delayed_mortality_in_bleached_massive_corals_on_intertidal_reef_flats_around_Phuket_Andaman_Sea_Thailand (Accessed: 4th April 2018)

Brown, B. E., R. P. Dunne, A. J. Edwards, M. J. Sweet and N. Phongsuwan (2015). ‘Decadal environmental ‘memory’ in a reef coral?’, Marine Biology, 162(2): 479-483 [online]. Available at: https://doi.org/10.1007/s00227-014-2596-2 (Accessed: 4th April 2018)

Camp, E. F., V. Schoepf, P. J. Mumby, L. A. Hardtke, R. Rodolfo-Metalpa, D. J. Smith and D. J. Suggett (2018). ‘The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments’, Frontiers in Marine Science, 5(4) [online]. Available at: https://doi.org/10.3389/fmars.2018.00004 (Accessed: 2nd April 2018)

Roche, R. C., G. J. Williams and J. R. Turner (2018). ‘Towards Developing a Mechanistic Understanding of Coral Reef Resilience to Thermal Stress Across Multiple Scales’, Current Climate Change Reports, [online]. Available at: https://doi.org/10.1007/s40641-018-0087-0 (Accessed: 2nd April 2018)



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