Impacts of Ocean Acidification on Coral Reef Resilience

Writer: Bejan Yilmaz

Ocean acidification is one of the most significant environmental challenges facing coral reefs today, despite them being one of the planet's most diverse and vital ecosystems. With increased levels of carbon dioxide, the chemistry of saltwater changes, making corals weaker and less able to grow, heal, and resist disturbances. This article has shown that ocean acidification decreases coral reef resilience by impairing calcification, disrupting reproduction, increasing the vulnerability of these organisms to bleaching, and causing damage to species that rely upon reefs, with severe social and economic impacts. Understanding this effect is crucial for protecting the reefs and the communities that depend on them.

What is ocean acidification?

Ocean acidification begins with atmospheric carbon dioxide dissolving into seawater. It forms carbonic acid through the reaction of carbon dioxide with water, which then breaks down into hydrogen ions (H⁺). With more hydrogen ions in the water, the pH level decreases, resulting in more acidic water. These hydrogen ions also bind to carbonate ions, producing bicarbonate. Because corals require carbonate ions to form their skeletons of calcium carbonate, this reaction is detrimental to corals. When there is a lack of carbonate ions, corals cannot calcify well. This weakens the structural foundation of reefs. (Reef Resilience Network, n.d.).

How Acidification Weakens Coral Calcification

One of the most direct and harmful impacts of ocean acidification is reduced calcification. The corals form their skeletons through a process that needs high levels of carbonate ions. When these ion concentrations decrease, the rate of skeleton formation declines. Studies have documented that coral calcification can decline sharply under predicted future acidification conditions.

Large-scale ecosystem modeling further shows that the addition of acidification to ocean warming synergizes the problem. Together, these two stressors synergistically increase coral mortality, decrease rates of regrowth, and overall reduce the capacity of a reef to recover following stress events such as bleaching or storms. According to Anthony et al. (2011, PMC), this weakening of the physical reef structure makes the ecosystem more fragile and less capable of supporting marine life.

Effects on Coral Reproduction and Recruitment

Moreover, ocean acidification impairs coral reproduction. The early stages of coral life tend to be sensitive to changes in pH. Many larvae of coral fail to settle on substrates in the more acidified water, hence reducing the juveniles that can grow and join the reef. When fewer juveniles survive, the recovery of reefs also becomes significantly slower.

This reduced recruitment then becomes a serious problem after disturbances: for example, after a bleaching event or a storm, reefs depend on new coral settlers to rebuild the damaged areas. If acidification prevents successful settlement, recovery may take decades or may not occur at all.

Symbiosis Breakdown and Increased Bleaching

Corals survive through a fragile symbiotic relationship with tiny algae, known as zooxanthellae. Living inside the tissues of coral, the algae are essential and make energy with the use of sunlight, thus helping the growth of corals. Acidification disrupts this. Stress induced by changes in seawater chemistry makes the corals more vulnerable to the bleaching process, wherein corals excrete their algae (Hughes et al., 2017).

Bleaching severely depletes the energy available to corals. Additionally, acidification slows their ability to rebuild their skeletons following bleaching. For that reason, the chance of survival of the corals exposed to both high temperature and more acidic water is much lower. Repeated bleaching events under acidified conditions can even lead to widespread decline of the reef.

Impacts on Reef-Dependent Species and Ecosystem Stability

Coral reefs support thousands of marine species, many of which also depend on carbonate ions to build shells or skeletons. Mollusks, urchins, and some plankton species are especially sensitive. These populations decline as a result of acidification spreading throughout the entire food chain.

Those fish that use the coral reefs for protection and/or as a source of breeding and feeding grounds would also be in jeopardy. Research has shown that high levels of CO₂ impair the ability of fish's senses to operate, shifting their ability to detect predators and orient appropriately. (Munday et al., 2009) Widespread bleaching due to rising temperatures usually occurs along with increasing levels of atmospheric CO₂. Weakening corals and reduced biodiversity make the whole reef ecosystem even less resilient and stable.

Social and Economic Consequences

Besides underpinning fisheries and protecting coastlines from storms, healthy coral reefs also attract millions of tourists each year. Acidification compromises all of these services. According to the Centre Scientifique de Monaco, mass loss of coral reefs could result in devastating economic costs and significantly affect communities dependent on fisheries and tourism. Reefs also function as coastal breakwaters, reducing erosion and storm damage along coastlines. With the degradation of reef frameworks due to acidification, for many countries, the cost of coastal protection could increase enormously.

Real-World Evidence: The Case of Comoros

In some places, severe effects of acidification can already be observed. A drop in pH level is causing a decline in coral growth in Comoros, a small island nation in the Indian Ocean, which threatens the stability of local reefs. Such changes are reportedly affecting fisheries and tourism, two key sources of income for the country. (IOSD, 2025) The Comoros case shows very clearly that acidification is not only a global environmental problem but also one of the current major social and economic issues for the most vulnerable nations.

Scientific and Policy Responses

The acidification of coral reefs is being explored globally by scientists. Models have allowed predictions concerning the degradation of reef health that may be expected from warming and acidification. International institutions, such as the Centre Scientifique de Monaco, continue to organize workshops to connect researchers and policymakers and to propose solutions.

Local management is also important. Pollution reduction, overfishing control, protection of herbivores, and expansion of marine protected areas allow the reefs to resist stress (Reef Resilience Network, n.d.). Yet, all agree that the long-run major solution involves reducing global CO₂ emissions.

Among the many threats to coral reef resilience, ocean acidification is arguably one of the most serious. It impairs calcification, interferes with reproduction, exacerbates bleaching, and degrades the ensembles of species dependent on reefs. These ecological impacts become human problems when fisheries collapse, tourism declines, and coastal areas lose their natural protection. The safeguarding of coral reefs requires science-based policies, strong local management, and actions from the global community to reduce carbon emissions. Without such efforts, many coral reefs may degrade in strength and diversity this century. The persistence of coral ecosystems and the livelihoods of humans that depend upon them requires urgent and concerted action.

References

• Allemand, D., & Osborn, D. (2019). Ocean acidification impacts on coral reefs: From science to solutions. Regional Studies in Marine Science, 28, 100558.

• Anthony, K. R. N., Maynard, J. A., Diaz-Pulido, G., Mumby, P. J., Marshall, P. A., Cao, L., & Hoegh-Guldberg, O. (2011). Ocean acidification and warming will lower coral reef resilience. Global Change Biology, 17(5), 1798–1808.

• Anthony, K. R. N., et al. (2011). Modeling coral reef futures under ocean acidification and warming. Philosophical Transactions of the Royal Society B. (PMC study)

• Centre Scientifique de Monaco. (2017). Ocean Acidification Impacts on Coral Reefs: Workshop Report.

• Doropoulos, C., Ward, S., Marshell, A., Diaz-Pulido, G., & Mumby, P. (2012). Ocean acidification reduces coral recruitment. Proceedings of the Royal Society B, 279(1739), 1299–1308.

• Hughes, T., et al. (2017). Global warming and recurrent mass bleaching of corals. Nature, 543, 373–377.

• IOSD. (2025). Ocean Acidification Threatens Coral Reefs: The Growing Crisis in Comoros Waters. International Organization for Sustainable Development.

• Knowlton, N., & Jackson, J. (2008). Shifting baselines and the decline of coral reefs. Science, 321(5880), 560–563.

• Munday, P., et al. (2009). Ocean acidification impairs olfactory discrimination in fish. Proceedings of the National Academy of Sciences, 106(6), 1848–1852.