I bet you know there are amazing coral reefs in the tropics you can see while snorkeling. Did you know corals keep going deeper than 150m?
Mesophotic (middle-light) reefs are found in largely the same places as shallow reefs, but deeper than 30-40m. Amazingly, on these reefs, corals may be even more abundant than in the shallows and still use the sun for energy. Surviving in the deep could provide a lifeline for coral reefs around the world, but why do some scientists think so?
Coral reefs are under threat. They face being ripped apart physically by storms, poisoned by excessive nitrogen, and starved by bleaching. Bleaching happens when corals eject their symbiotic algae in response to being cooked by warming oceans and “sunburnt” by bright light. As mesophotic reefs are found in deeper water, they are darker and normally colder than shallow reefs. They are also often further from the shore, meaning these threats may be weaker.
Despite this, some scientists are skeptical. We know corals get used to normal conditions and are damaged when shocked by extremes. If a coral is used to it being colder and darker, maybe they are more sensitive to any changes? We have also seen that mesophotic reefs might not avoid warming oceans entirely. When days get hotter, the shallow water heats up first as it’s closer to the sun. The deeper water might only find its heating delayed, as it takes time to spread through the water column. Strong storms have also smashed mesophotic reefs in the past.
In the summer of 2015, scientists from the University of Oxford’s Thinking Deep team conducted an experiment on the reefs of Honduras (Laverick & Rogers, 2018). Small cuttings were taken from colonies of a coral with a large depth range, called Agaricia lamarcki. These samples came from two different locations and from depths crossing from shallow into mesophotic reefs. These samples were then mixed and placed back onto a third reef, either at shallow or mesophotic depths. This set up allowed researchers to tease apart the effects of what the coral was currently experiencing, and what it may have been used to in the past.
After a few weeks, sea surface temperatures started to rise and bleaching corals were spotted on the reef. The researchers watched their experimental corals until the end of their field season, taking photos as they checked. When the data was analyzed, it was clear that corals placed on mesophotic reefs were less bleached and were almost 20% more likely to survive. Interestingly, there was no sign that any past exposure to different depths or locations made a difference. It seemed, in this case, that the corals on mesophotic reefs were in a less stressful environment.
We might, therefore, expect mesophotic reefs to suffer less when the shallow reefs are being hammered by multiple threats. Mass coral bleaching has become a global problem. Famously, in 2016, 93% of the Great Barrier Reef was hit by bleaching. This caused an obituary article for the reef to go viral, despite scientists believing the reef could still survive. This survey work was conducted from the air by scientist’s from ARC Centre of Excellence for Coral Reef Studies (Hughes et al., 2017), leaving open the question of how the bleaching event hit mesophotic reefs.
As suggested by the Honduras experiment, follow up research by Frade et al. (2018) on the highly impacted northern section of the Great Barrier Reef found less bleaching on mesophotic than shallow reefs. Corals on mesophotic reefs were 25% less likely to bleach at similar depths to the Honduras experiment. At 5m 11.8% of corals died, with only 5.7% dying at 40m. Though deeper reefs appear to be protected, it is important to remember that there was still significant damage. Yes, they were less likely to bleach, but at 40m, 40% of the coral was still bleaching.
Into the future, we will be finding out more and more about mesophotic reefs. A key concept attracting lots of research attention is the deep reef refugia hypothesis. Building on the idea that deep reefs are less impacted, could they help shallow reefs recover? To find this out, new studies are considering 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 finds, it is important that we do what we can to protect coral reefs now. 2018 is the third International Year of the Reef. Events are happening all over the world to tell the public what reefs do for us as a species, and just how spectacularly diverse and beautiful these ecosystems are. Coral reefs could survive longer in the deep than at the surface. But reefs worldwide will need our help if they are to stand any chance of retaining their full splendor for generations to come.
These findings are described in the article entitled Experimental evidence for reduced mortality of Agaricia lamarcki on a mesophotic reef, recently published in the journal Marine Environmental Research. This work was conducted by Jack H. Laverick and Alex D. Rogers from the University of Oxford.