Coral Bleaching Awareness Month – Understanding the Crisis Beneath the Waves

Coral reefs are crucial, vibrant ecosystems often described as the rainforests of the ocean. Despite covering less than 1% of the ocean floor, they provide immense ecological and economic value: offering shelter and food for thousands of species, including a quarter of all known marine fish species (Spalding et al., 2001). These complex habitats generate an estimated $9.9 trillion annually through tourism, fisheries, and coastal protection (Spalding et al., 2001; Burdett et al., 2024).

However, coral bleaching is accelerating reef decline. Half of the world’s coral reefs have already been lost (Wilkinson, 2008), threatening biodiversity, ecosystem stability, and the livelihoods and safety of coastal communities.

This week, as part of Coral Bleaching Awareness Month, we’re bringing attention to the science behind coral bleaching and the impact it is having on our planet.

What is Coral Bleaching?

Figure 1: Partially bleached coral colony showing labelled coral polyps and associated zooxanthellae.

Coral bleaching is the process by which corals expel the microscopic algae, known as zooxanthellae, from their tissues. These algae provide the corals with 90-95% of their energy (Anthony et al., 2009) and give them their vibrant colour. Without them, corals enter a starved state and appear white as their calcium carbonate skeletons become visible through their transparent tissue. In this weakened state, they become more vulnerable to additional stressors such as disease, predation, and pollution and are at higher risk of mortality.

It's important to note that bleached corals are not dead. If conditions improve quickly; they can regain their algae and recover. However, if not, they will die, which can trigger a ‘phase-shift’ where the composition of the ecosystem can be irreversibly altered. The intricate architecture of the reef may be completely lost as the dead coral skeletons break down and collapse into rubble, or the reef is overgrown by algae.

What Causes Coral Bleaching?

The primary trigger of coral bleaching is elevated seawater temperatures; however, it is not the only cause. Other stressors include:

• High levels of sunlight

• Cold seawater temperatures

• Sudden changes in salinity or pH

• Poor water quality, sedimentation, and land-based runoff

These stressors often act together, amplifying coral stress and reducing their recovery window - resulting in an increased risk of mortality and ecosystem collapse.

Although more and more is being understood about the causes of coral bleaching, researchers are continuing to investigate the physiological mechanisms behind it—including emerging theories involving oxidative stress and cellular breakdown. These insights are vital to shaping how we respond to future threats facing coral reef ecosystems.

Measuring Heat Stress: Degree Heating Weeks

Scientific literature often refers to Degree Heating Weeks (DHW) when discussing coral bleaching as it is a common method for estimating how much thermal stress corals experience over time.

DHW measure how much, and for how long, seawater temperatures exceed the average warmest monthly temperature. The higher the DHW, the greater the stress, and the higher the risk of coral bleaching and mortality.

Figure 2: Key DHW thresholds and associated coral bleaching response (Heron et al., 2016; Hughes et al., 2018).

During the current global coral bleaching event, some regions have recorded DHW values exceeding 12°C-weeks, indicating extreme and prolonged stress.

Can Coral Reefs Recover?

Yes – if stressors are reduced and corals are given enough time to recover. But time is running out, and some species may already be beyond recovery.

However, scientists and practitioners around the world are designing and implementing cutting-edge techniques to preserve and restore coral reef ecosystems. From active restoration to assisted evolution, these efforts aim to accelerate recovery, enhance resilience, and prevent irreversible ecological loss.

At Ocean Revive, we’re contributing to this global effort by restoring coral resilience through science, innovation, and collaboration.

Learn More About Coral Bleaching

Follow us throughout Coral Bleaching Awareness Month for:

• A summary of the latest global coral bleaching event to provide context for our current situation

• An overview of current methods being used by scientists to combat its effects on reef ecosystems

• A practical guide for practitioners, policymakers and members of the public to take action

Together, we can bring lasting colour back to coral reefs.

References

• Anthony, K.R., Hoogenboom, M.O., Maynard, J.A., Grottoli, A.G. and Middlebrook, R., 2009. Energetics approach to predicting mortality risk from environmental stress: a case study of coral bleaching. Functional ecology, 23(3), pp.539-550.

• Burdett, H.L., Albright, R., Foster, G.L., Mass, T., Page, T.M., Rinkevich, B., Schoepf, V., Silverman, J. and Kamenos, N.A., 2024. Including environmental and climatic considerations for sustainable coral reef restoration. PLoS biology, 22(3), p.e3002542.

• Heron, S.F., Maynard, J.A., Van Hooidonk, R. and Eakin, C.M., 2016. Warming trends and bleaching stress of the world’s coral reefs 1985–2012. Scientific reports, 6(1), p.38402.

• Hughes, T.P., Anderson, K.D., Connolly, S.R., Heron, S.F., Kerry, J.T., Lough, J.M., Baird, A.H., Baum, J.K., Berumen, M.L., Bridge, T.C. and Claar, D.C., 2018. Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science, 359(6371), pp.80-83.

• Spalding, M.D. et al. (2001). World Atlas of Coral Reefs. UNEP-WCMC.

• Wilkinson, C.C., 2004. Status of coral reefs of the world: 2004. Australian Institute of Marine Science (AIMS).