Warming of the oceans and acidification due to increased carbon absorption are destroying the world’s coral reefs and the diverse ecosystem they support. It has been estimated that coral reef habitats have already declined by an estimated 30 – 50 percent since the 1980s, and some coral reef researchers fear that reefs may completely disappear within 20 years if nothing is done to reduce the impacts of climate change.

Coral Reefs Under Threat

In March 2020, Australia’s Great Barrier Reef suffered its most widespread bleaching event to date. Coral bleaching occurs when corals are stressed by changes in their environment, such as temperature, light or nutrient changes. This stress causes the corals to expel the symbiotic algae that live in their tissues, causing them to turn completely white. If corals remain bleached for prolonged periods of time, they eventually die. In March 2020, sixty percent of the reef underwent moderate to severe bleaching, with some corals permanently dying.

There is another big threat to coral reef ecosystems, one that has only recently been studied, with the potential to be more serious than increases in sea water temperatures and increased acidification. According to a study published in Nature Climate Change, corals and other organisms living in the reef could also be suffering from deoxygenation, which could greatly impact the successful recovery of a reef after a bleaching event or when stressed. Loss of oxygen in the ocean is accelerating globally due to climate change and eutrophication. Acute deoxygenation events, and the resultant mass mortality of organisms and formation of dead zones in coastal ecosystems, are increasing in frequency and severity, as a result of human-induced impacts, such as eutrophication, as well as climate change.

Eutrophication begins as an increased load of nutrients in estuaries and coastal waters, resulting in harmful algal blooms, dead zones and fish kills. Eutrophication sets off a chain reaction in the ecosystem. It starts with increased nutrients (predominantly due to increased use of fertilizers on land that end up in the ocean), causing an overabundance of algae and plants to bloom in the sea. The excess algae and plants eventually die and decompose, producing large levels of carbon dioxide. This results in ocean acidification, as it lowers the pH of seawater, which has a further negative impact on the marine ecosystem.

A Lack of Oxygen

All the threats to corals are interlinked. As waters warm, corals and other organisms living in the reef require more oxygen to breathe. Warm water holds less oxygen than cold water, and as corals and other creatures struggle to get the oxygen they require, they consume more oxygen in the process. Therefore, when corals are undergoing stress and bleaching due to warming ocean temperatures, it is quite possible that deoxygenation is also playing a role in the stress response that can be observed in corals.

According to a 2019 report by the International Union for Conservation of Nature (IUCN) titled “Ocean deoxygenation: Everyone’s problem”, oxygen in the world’s oceans has decreased by 2% since the middle of the 20th century. Although agricultural runoff, human waste and pollution play a big role in reducing the oxygen levels in the ocean, the main driver is climate change.

There are many studies that look at the effects of deoxygenation in the open ocean and cold water habitats, however there are still large gaps in knowledge and research when it comes to deoxygenation in coastal coral reefs and tropical environments. In temperature systems, it has been observed that deoxygenation events have caused ecosystem collapse and a loss of ecosystem goods and services. Therefore, studies like the one published in Nature Climate Change and another published in Nature Communications are important to investigate the biological, ecological and geochemical impacts of low  oxygen levels on coral reef decline.

In order to conserve coral reef habitats, it is important to understand “sub-lethal thresholds” – meaning thresholds that when exceeded result in death of the organism. In cold water ecosystems, it is well-known that small changes in oxygen levels can have large impacts on animal behavior, growth rates, reproductive success and susceptibility to disease. It therefore seems fair to suspect similar impacts may be observed in coral reef habitats and tropical environments.

Although not thoroughly studied yet, it remains a real possibility that ocean deoxygenation could hold a larger, yet silent, threat to corals than climate change. When organisms cannot access oxygen, they die very quickly. Whereas corals can still recover after bleaching events due to warming waters, the same corals will not be able to remain alive in low oxygen conditions.

How to Prevent Deoxygenation

One way to curb deoxygenation in the ocean is to better manage our systems on land. Agricultural runoff – including contaminated water and fertilizers – play a large role in eutrophication and resultant deoxygenation, especially in places where coral reefs are in close proximity. Similar to other threats to ocean life, the most effective way to stop ocean deoxygenation would be to lower global carbon emissions and reduce the impacts of climate change. Although this seems impossible to do, coral reef conservationists are hopeful that the ongoing COVID-19 pandemic and lockdowns can provide an important paradigm shift. The pandemic has proven that the world can come together to deal with a global problem. Conservationists are therefore hopeful that the same approach might one day be taken to address climate change and the many environmental issues it is causing. In areas surrounding the Great Barrer Reef, resources are being mobilized to improve agricultural and catchment management in an attempt to ensure the oxygen stocks of coastal reefs.

Deoxygenation has long been acknowledged as a leading threat to coastal marine ecosystems, but has only recently emerged as a critical threat in the tropics. Coral reefs are ecologically and economically valuable ecosystems, and improved knowledge of the threats these systems face will improve coral reef conservation. Although bleaching events have always been blamed on warming waters, recent research indicates that a lack of oxygen may also initiate coral bleaching.