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Deeper zones of global coral reefs are currently safe from warming waters

The Great Barrier Reef, our planet’s precious underwater ecosystem, has faced years of devastation due to rising water temperatures. Sadly, “coral bleaching,” a stark visual sign of reef damage from warming waters, has become far too frequent in many areas and zones of coral reefs around the world.

However, a new study brings a complex mix of hope and warning: deeper parts of the reef seem to be somewhat shielded from the heat, but only up to a certain point.

Distinct coral reef zones

Coral reefs consist of distinct zones. Each zone has conditions tied to light levels, water temperature, and wave energy.

They also differ in aspects of warming and nutrient availability. These varying conditions shape the types of corals and other organisms that can thrive in each zone.

Shallow reefs

The uppermost zone, bathed in sunlight, is where most people think of when they picture a coral reef. This area is crucial for photosynthesis, allowing corals to partner with algae for energy. However, this zone is most vulnerable to extreme heat events driven by climate change.

Mesophotic reefs

Scientists from the University of Exeter and the University of Queensland focused their research on a deeper zone called the “mesophotic coral reef.”

This region exists in a twilight zone roughly 30-50 meters below the surface. Dimmer light penetrates to this depth, but it’s significantly less than the shallow reefs receive.

Ocean water isn’t static – currents, tides, and winds affect how different layers interact. Warmer surface water, driven hotter by climate change, doesn’t always blend thoroughly with the naturally cooler water in deeper regions.

Fortunately, this incomplete mixing can create a temperature buffer, temporarily shielding mesophotic reefs from some of the immediate effects of surface-level warming.

The hope (and the catch)

“Coral reefs are a stark indicator of the widespread damage climate change causes to ecosystems,” says Dr. Jennifer McWhorter of NOAA’s Atlantic Oceanographic & Meteorological Laboratory (AOML), who led the study. “Our study highlights a potential area of resilience within the reefs, but also a clear limit to that resilience if warming continues.”

The news from the study isn’t all positive. There’s a crucial caveat: this natural buffer zone provided by incomplete water mixing won’t last indefinitely. The research team warns that if global warming surpasses 3°C above pre-industrial levels, the protective effect will disappear.

Maintaining coral reef zone temperature differences

As global temperatures rise, the difference in temperature between surface and deep water layers lessens. This weaker thermal gradient reduces the resistance to mixing, allowing warmer surface water to penetrate deeper.

The result? Mesophotic reefs, once shielded, would become exposed to the same harmful heat currently stressing shallow reefs.

The study suggests that if global warming reaches 3°C, temperatures in mesophotic zones of the Great Barrier Reef would surpass 30°C. This is a critical threshold for many coral species.

Extended exposure to such temperatures can trigger widespread coral bleaching and mortality events. These die-offs could significantly disrupt the reef ecosystem, impacting the entire food chain and potentially leading to reef collapse.

Deep coral reef zones and warming

While the existence of mesophotic reefs zones offers a glimmer of hope to warming, it’s important to understand the limitations of this potential refuge. The shallow and deeper zones of a coral reef ecosystem are not interchangeable.

Certain coral species are specifically adapted to the high light levels found in the shallows. These sunlight-dependent corals, often vibrantly colored and brimming with symbiotic algae, wouldn’t be able to survive in the dimmer environment of the mesophotic zone.

Even if the deeper communities persevere through rising temperatures, the loss of these shallow-water species would still inflict a devastating blow on the overall biodiversity and health of the reef ecosystem.

Imagine a lush rainforest losing all its flowering trees – the ecosystem would still exist, but its character and ecological functions would be fundamentally altered. In essence, the potential survival of mesophotic reefs wouldn’t guarantee the complete restoration of the entire coral reef ecosystem.

“Some shallow-water species are not found in deeper areas, so mesophotic reefs can’t fully replace them as shallow reefs are degraded,” notes Dr. McWhorter.

Warming risks on mesophotic reefs

To assess the future risk to mesophotic reefs, scientists employed a multi-faceted approach. They weaved together climate change projections, which predict the overall rise in global temperatures, with specific details about how the ocean layers in this region typically mix.

Factors like currents, tides, and wind patterns complexly influence this water mixing phenomenon. By incorporating these layers and their mixing tendencies, scientists were able to create a more nuanced picture of how future warming might specifically affect the mesophotic zone of the Great Barrier Reef.

Their calculations paint a concerning scenario: even under the most optimistic outlook with reduced greenhouse gas emissions, these deeper zones are predicted to warm by at least 0.5°C by 2050-60.

This warming effect intensifies significantly if emissions remain high, with projections suggesting a temperature increase of 1.2-1.7°C in the mesophotic zone.

Study significance

This research highlights the urgency of fighting climate change and the complex ways it disrupts ocean life. To save coral reef ecosystems, we need a multifaceted strategy:

The study reaffirms the non-negotiable priority of drastically reducing greenhouse gas emissions. Addressing the root cause of rising ocean temperatures is paramount for the survival of coral reefs, both shallow and mesophotic.

The longer we delay action on this front, the more widespread and severe the damage will be. By reducing emissions swiftly, we can buy precious time, allowing more areas of the reef to maintain the natural protection offered by their current temperature differences.

Conservation of reefs from warming

Dr. Paul Halloran of Exeter’s Global Systems Institute (GSI) stresses the need for targeted conservation efforts. This means focusing resources and management strategies on the reefs that exhibit the best chances of survival in a warming world.

Identifying these potentially more resilient areas involves careful study of local conditions that might influence ocean mixing, as well as factors that can contribute to reef health even in the face of some warming.

By adopting this strategic approach, we can increase the likelihood of maintaining some pockets of healthy reef ecosystems despite the broader challenges.

Knowledge is power

Professor Peter Mumby of the University of Queensland emphasizes the vast amount we still have to learn about the complex environments of deeper, tropical coral reefs. Expanding our scientific understanding of these ecosystems is crucial.

Identifying how they differ from shallow reefs is crucial. We need to understand why certain areas are more resilient. Discovering how they might be interconnected will equip us with the knowledge needed.

This knowledge is essential for informed and effective protection efforts. Research and ongoing monitoring will be vital tools in the fight to conserve coral reefs.

The Great Barrier Reef is a wonder of the world, and its plight is a call to action. This study provides a nuanced look at the situation, highlighting both some temporary resilience and the urgency of addressing the root cause.

The paper is published in the journal Proceedings of the National Academy of Sciences.


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