Suffocating seas: low oxygen levels emerging as third major threat to tropical coral reefs

Corals in low-oxygen seawater may not show visible signs of stress. Mike Workman/Shutterstock

Coral reef research has focused on the twin evils birthed by record-high greenhouse gas emissions: warming oceans and increasingly acidic seawater. These global threats are caused by seawater absorbing the excess heat and carbon dioxide that fossil fuel burning has added to the atmosphere. But there is another consequence that is seldom discussed.

Globally, oceanic oxygen is being depleted because seawater holds less oxygen as it heats up. In the warm coastal waters where tropical coral reefs grow, the immediate effects of low oxygen concentrations can be catastrophic. Short-term hypoxia events are increasingly reported in which dissolved oxygen levels suddenly plummet – often triggered or exacerbated by chemical pollution running off the land, like nutrient-rich fertilisers – which can kill entire coral communities and decimate reefs within days.

Corals are animals, and like other aquatic animals, they breathe in oxygen from the water to fuel their metabolism. Thanks to a symbiotic relationship with microscopic algae, corals also turn the Sun’s energy into food – oxygen is the byproduct.

Oxygen levels on coral reefs naturally fluctuate in a daily cycle, with dissolved oxygen peaking around noon and gradually falling as the light fades. At night when photosynthesis stops, corals continue to respire (consume oxygen), and seawater oxygen is depleted.

This cyclic rise and fall in oxygen means that some corals have already evolved strategies to withstand changes in dissolved oxygen. When the amount of oxygen available to corals falls below this natural range, corals can get stressed and their normal biological processes are disrupted, in many cases leading to death.

Just like us, corals need oxygen to survive. But I (Jennifer Mallon) discovered that the effects of low oxygen on corals are not always obvious to the naked eye, and that juvenile corals may be especially vulnerable.

Hard to spot signs

To understand the effects of low oxygen levels on corals I travelled to the Smithsonian Marine Station in Florida, as part of a research project led by the University of Florida’s Andrew Altieri and the Smithsonian’s Maggie Johnson and Valerie Paul.

At the Smithsonian, 24 climate-controlled seawater tanks simulate varying levels of deoxygenation already present on coral reefs around the world, ranging from severe deoxygenation, which our research observed on the Caribbean coast of Panama, to normal conditions, such as those replicated in aquariums around the world.

Researchers recreated environmental conditions for corals in the lab. Jennifer Mallon

While some corals, like the Caribbean staghorn coral (Acropora cervicornis), died within a few days of severe deoxygenation, other important reef-building species such as the mountainous star coral (Orbicella faveolata) survived, demonstrating that tolerance of low oxygen was different between species.

When we studied the corals that survived deoxygenation, we discovered that hypoxic stress may not always be visible. Even when exposed to deoxygenation for two weeks, some corals showed no signs of bleaching, which is when the colourful algae depart and corals turn a ghostly white. More detailed measurements revealed something worrying: despite outward appearances, low oxygen exposure had impaired coral metabolism, potentially stunting their growth and reef-building abilities.

Existing methods for measuring coral health in the field are mainly visual, and include assessments by trained divers who search for signs of paling or bleaching corals. The hypoxic stress responses we saw in our experiment could be going under the radar.

Baby corals at risk

We also wanted to know how deoxygenation affects a coral’s ability to breed.

Coral sexual reproduction is already a tricky business. Spawning events, when corals release egg bundles into the water, occur just a few nights a year, and the resulting larvae are highly vulnerable. Few survive the multi-day swim to the reef where they settle and metamorphose into juvenile corals.

On modern Caribbean reefs, wild juvenile corals are rare. People involved in restoring reefs help corals to sexually reproduce in the lab and rear the juveniles in order to later transplant them onto the reef.

Juvenile corals often settle in reef crevices where they are exposed to lower oxygen levels for longer than in open water, because less water flows over them. When we incubated coral larvae in deoxygenated water throughout the settlement process, we found that initial rates of larval survival and settlement were not significantly affected.

Things changed once the larvae had settled and begun to form juvenile corals. Early-stage juvenile corals, known as primary polyps, lack symbiotic algae to help them meet their nutritional needs via photosynthesis and so rely on respiration for energy. Without enough oxygen, they cannot respire properly and begin to die off.

A coral spawning event off the coast of Queensland, Australia. Coral Brunner/Shutterstock

Coral conservation in breathless waters

Our research can help those involved in restoring reefs understand the oxygen needs of corals, as well as highlight a previously overlooked threat.

Even corals that survive deoxygenation show signs of a weaker metabolism that will make it harder to conserve healthy reefs, as restoration relies on healthy coral growth to regenerate what is damaged.

As a next step, field measurements of coral metabolism will be carried out on Florida’s barrier reef tract when oxygen levels are predicted to drop during the warm summer months, to capture the real impact of deoxygenation on coral health.

Dissolved oxygen data has not always been collected as part of reef monitoring, even during warm water bleaching events when oxygen is low. As the climate crisis worsens, it will be imperative to do more of this monitoring in tropical coastal waters. Further research into how distinct coral species respond to hypoxia is also essential for targeted conservation strategies.

By confronting the silent threat of deoxygenation head on, we can safeguard the future of coral reefs and the countless marine species that depend on them.