Could coral reefs survive in the depths?

I bet you know that there are incredible coral reefs in the tropics that you can see while snorkeling. Did you know that corals go further than 150 m?

Mesophotic reefs (medium light) are found essentially in the same places as shallow reefs, but deeper than 30 to 40 m. Surprisingly, on these reefs, corals can be even more abundant than in shallow waters and still use the sun for energy. Surviving in the depths could provide a lifeline to coral reefs around the world, but why do some scientists think so?

Coral reefs are threatened. They face physical injuries caused by storms, poisoned by excess nitrogen and starved by bleaching. Bleaching occurs when corals eject their symbiotic algae in response to cooking by warming oceans and "sunburn" by bright light. Since mesophotic reefs are found in deeper waters, they are darker and normally cooler than shallow reefs. They are also often further away from the shore, which means that these threats may be weaker.

Despite this, some scientists are skeptical. We know that corals get used to normal conditions and are damaged by extremes. If a coral is used to being colder and darker, perhaps they are more sensitive to any changes? We also found that mesophotic reefs may not prevent total warming of the oceans. When the days get warmer, the shallow water warms up first because it is closer to the sun. The deeper water can only find its delayed heating because it takes time to spread in the water column. Strong storms have also broken the mesophotic reefs in the past.

In the summer of 2015, scientists from the Thinking Deep team at Oxford University conducted an experiment on the reefs of Honduras (Laverick and Rogers, 2018). Small cuttings were taken from deep coral colonies, called Agaricia lamarcki. These samples came from two different locations and depths ranging from shallow reefs to mesophotic reefs. These samples were then mixed and returned to a third reef at shallow or mesophotic depths. This set-up allowed researchers to unravel the effects of what the coral was experiencing and what it was used to in the past.

After a few weeks, sea surface temperatures began to rise and whitening corals were observed on the reef. The researchers observed their experimental corals until the end of their season in the field, taking pictures as they checked. When the data were analyzed, it was clear that the corals placed on the mesophotic reefs were less laundered and that they were almost 20% more likely to survive. Interestingly, there is no indication that exposure at different depths or locations has made the difference. In this case, it seemed that the corals of the mesophotic reefs were in a less stressful environment.

We can therefore expect that the mesophotic reefs will suffer less when the shallow reefs are threatened by multiple threats. Massive coral bleaching has become a global problem. Famous, in 2016, 93% of the Great Barrier Reef was affected by bleaching. This caused the passage of an obituary article on the reef, despite the fact that scientists thought that the reef could still survive. This investigative work was conducted by scientists from the ARC Center of Excellence for Coral Reef Studies (Hughes et al., 2017), leaving open the question of how the bleaching event hit the mesophotic reefs.

As suggested by the Honduran experience, follow-up research by Frade et al. (2018) on the heavily impacted northern part of the Great Barrier Reef found less bleaching on mesophotics than on shallow reefs. Mesophotic coral corals were 25% less likely to bleach at depths similar to those experienced in Honduras. At 5m, 11.8% of the corals died and only 5.7% died at 40 m. Although the deeper reefs seem to be protected, it is important to keep in mind that there is still significant damage. Yes, they were less likely to bleach, but at 40 m, 40% of the coral was still whitening.

In the future, we will know more about the mesophotic reefs. The Deep Reef Shelter Hypothesis is a key concept that attracts a lot of attention from researchers. Based on the idea that deep reefs are less affected, could they help shallow reefs to recover? To find out, new studies are questioning whether deeper corals are able to reproduce enough to support shallow reefs and whether mesophotic reefs contain their own unique set of species.

Regardless of what this research reveals, it is important that we do everything we can to protect the coral reefs now. 2018 is the third international year of the reef. Events are taking place all over the world to tell the public what reefs do for us as a species and how diverse and beautiful these ecosystems are. Coral reefs may survive longer in depth than on the surface. But the reefs around the world will need our help if they want to keep all their splendor for generations to come.

These findings are described in the paper titled Experimental Data on Agaricia Lamarcki's Mortality Reduction on a Mesophotic Reef recently published in the review. Marine environment research. This work was led by Jack H. Laverick and Alex D. Rogers from the University of Oxford.