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Ocean deoxygenation could be silently killing coral reefs, scientists say

  • A new perspective paper argues that ocean deoxygenation is the biggest threat to coral reef survival, perhaps even more so than warming sea temperatures and acidification.
  • Oxygen in the world’s oceans has decreased by 2% since the middle of the last century, due largely to climate change, agricultural runoff and human waste.
  • A growing body of work examines deoxygenation in the open ocean, but little research has been done on the effects of decreased oxygen on coastal coral reefs systems in tropical environments, and this paper begins filling that gap.
  • The lead author and his colleagues are currently collecting data off the coast of Heron Island on the Great Barrier Reef to understand the effects of deoxygenation on the surrounding reefs.

In March, Australia’s Great Barrier Reef suffered its most widespread bleaching event to date. Sixty percent of the reef underwent moderate to severe bleaching, and some corals may never recover.

The cause of this bleaching event was climate change, which brought unusually warm waters to the Great Barrier Reef in February, and disrupted the delicate, symbiotic relationship between the corals and their life-sustaining algae. In general, when sea temperatures rise, corals become stressed and expel algae from their tissues. Without this algae, the corals turn ghostly white and slowly starve.

An Acropora coral reef in Chuuk, Micronesia. Image by David Burdick, NOAA.

But there’s another big threat to coral reef systems, one that could be more serious than sea temperature rise and acidification. According to David Hughes, lead author of a new study published in Nature Climate Change, corals and other organisms living on the Great Barrier Reef could also be suffering from deoxygenation, and this could greatly impede the reef’s recovery.

“We know that the stresses of warming and the deoxygenation are likely to interact with each other,” Hughes, a research associate at the University of Technology Sydney’s Climate Change Cluster, told Mongabay. He explained that as water warms, corals and other organisms need more oxygen to breathe. But warm water holds less oxygen than cold water. As corals and other organisms struggle to get the oxygen they need, they consume more oxygen in the process.

Scientists assess coral mortality on Zenith Reef following the bleaching event, Northern Great Barrier Reef, November 2016. Photo credit: Andreas Dietzel, ARC Centre of Excellence for Coral Reef Studies.

“For sure, whenever there are these bleaching events due to warm waters, it’s quite possible that the deoxygenation is also playing a role in the stress response that you see in the coral community,” Hughes said. “The other thing is that when corals die due to bleaching — or whenever anything dies within the water — that also consumes oxygen because of the bacterial breakdown of decaying organic matter, so it can create a cascading effect.”

Oxygen in the world’s oceans has decreased by 2% since the middle of the 20th century, according to a report by the IUCN. Agricultural runoff and human waste play a major role in lowering oxygen levels in the ocean, but the main source is climate change.

A diver swims over bleached coral at Heron Island, Australia, in 2016. When ocean temperatures rise, corals under stress expel algae, causing them to turn bone white and suffer increased vulnerability to collapse if the stress is prolonged. Image courtesy of The Ocean Agency / XL Catlin Seaview Survey / Richard Vevers via Flickr (CC BY 2.0).

A growing body of research looks at the effects of deoxygenation in the open ocean and cold water environments, but little is known about deoxygenation in coastal coral reefs systems in tropical environments. This new paper investigates existing biological, ecological and geochemical studies to theorize that low oxygen levels could be contributing to coral reef decline, but notes that a lot more research is needed.

“One of the key messages of the paper is that we really need to understand what we call ‘sub-lethal thresholds’ that impact organisms before they die,” Hughes said. “In cold water ecosystems, we know that animal behavior, growth rates, reproductive success, and even susceptibility to disease is all affected by very small changes in oxygen, so we have good reasons to suspect that the same would apply to coral reefs, but we just don’t have the data on that yet.”

A Hawaiian coral which sustained bleaching in 2015. Image by Greg Asner.

Hughes said he believes that ocean deoxygenation could be the most immediate threat to coral reef survival.

“When you have these very acute deoxygenation events …. corals can’t breathe, and when things can’t breathe, they die very quickly,” Hughes said. “So in that sense, it can be more of an immediate threat to coral reefs than warming.”

“I suppose it’s the same with human beings,” he added. “You have the rule of three — you can survive three weeks without food, three days without water, but only three minutes without oxygen. The same kind of rule applies to coral reefs. When you have a complete lack of oxygen, then that is the critical factor on the reef at that particular time, more so than temperature and acidification.”

Marine biologist David Hughes set up an oxygen sensor on the reef system off the coast of Heron Island. Image by Trent Haydon.

Hughes and colleagues are currently collecting data off the coast of Heron Island on the Great Barrier Reef to further understand the effects of deoxygenation on the island’s coral reef system. The oxygen sensors were in place when the mass bleaching event happened, so Hughes said he anticipates receiving some interesting data. But neither Hughes nor his colleagues are able to collect the sensors during the COVID-19 crisis.

“How soon we can get that data is a bit of a guess at the moment,” Hughes said.

One way to curb deoxygenation is to better manage land and agricultural runoff, especially in places where coral reefs are in close proximity, Hughes said. But the most effective way to stop ocean deoxygenation is to lower global carbon emissions. This task might feel insurmountable, but Hughes said he thinks the COVID-19 crisis provides an important paradigm shift.

Fish and corals in Bunaken, North Sulawesi. Image by Matt Kiefer/Flickr.

“It clearly shows that we can all pull together when there’s a global problem,” Hughes said. “If we can apply the same approach [to dealing with COVID-19] to addressing climate change, then we have a good chance of actually tackling the issue.”

Citation:

Hughes, D. J., Alderdice, R., Cooney, C., Kühl, M., Pernice, M., Voolstra, C. R., & Suggett, D. J. (2020). Coral reef survival under accelerating ocean deoxygenation. Nature Climate Change, 10, 296-307. doi:10.1038/s41558-020-0737-9

Banner image caption: A coral reef system along the coast of northern Mozambique, one of nearly 300 coral reefs in the Western Indian Ocean examined by researchers from the Wildlife Conservation Society and other organizations in a 10-year study that focused on coral diversity and rates from bleaching events. Photo credit: Emily Darling/WCS.