Australasian Science Magazine

A remote-operated vehicle explores the deep reef. © Catlin Seaview Survey

By Christopher Doyle

The shallow reefs of the Great Barrier Reef are under stress, with a recent report estimating that half of the coral cover has been lost within the past three decades. However, scientists are now discovering healthy coral communities lying in deeper waters adjacent to these disturbed areas. Could these deep reefs hold the key to the survival of the Great Barrier Reef?

A recent report by the Australian Institute of Marine Science estimated that half of the coral cover on the shallow reefs of the Great Barrier Reef has been lost within the last 27 years. Storm damage and infestations by crown-of -thorns starfish were responsible for the greatest losses, with coral bleaching also contributing to a lesser extent. The report warned that if the current rate of loss continued, the amount of coral cover could be halved again within the next decade.

However, scientists exploring the deep water of the Great Barrier Reef and Coral Sea have recently made an important discovery. They have found that at depths well beyond the normal reach of scuba divers, coral communities are flourishing. The discovery was made as part of the Catlin Seaview Survey, an ambitious project designed to assess the condition of deep coral communities that until now have gone largely unexplored by scientists. The survey is being sponsored by Catlin, a global insurance company.

Professor Ove Hoegh-Guldberg, Chief Scientist for the Catlin Seaview Survey and Director of the University of Queensland’s Global Change Institute, said the findings are shedding new light on the health of the Great Barrier Reef. “Up until now our knowledge was limited to the shallow reefs accessible by scuba diving. In reality that provided us with an incomplete picture. Now, using ROVs (Remote Operated Vehicles), we are able to get below 30 metres and down to 100 metres, revealing a wholly different picture which now includes the deep reef environment.”

Dr Pim Bongaerts, a Postdoctoral Research Fellow at the Global Change Institute and leader of the deep reef survey team, is amazed by the findings so far. “The most striking thing is the abundance of coral on the deep reef. What has blown me away is to see that even 70–80 metres down there are significant coral populations.”

The scientists are using ROVs and innovative dive robots to explore the deep coral reefs, and this has greatly expanded their capacity to study the deep reefs beyond what they could by scuba diving alone.

Bongaerts knows too well the difficulties of diving at depth. When diving to a depth of 40 metres, the deepest he and his team are diving on this expedition, the team have only 8 minutes to do what they need to before ascending to shallower waters. “We have planning sessions of up to an hour in the morning to work out exactly where everybody in the team is going to be at each minute in time to really maximise that 8 minutes,” he says. “It is amazing how much you can get done in that 8 minutes if you plan it correctly. It is a real challenge.”

With the ROVs, however, Bongaerts and his team can survey communities as deep as 100 metres with no restriction on time. The ROVs are fitted with high-definition cameras that relay images to screens on the survey boat above, from where the ROVs are controlled.

Speaking with Hoegh-Guldberg and Bongaerts leaves no doubt that they are excited by what they are seeing in the depths below. “When you look at the shallow reef systems, like off Heron Island for example, you can pretty much guarantee that someone has swum over every piece of coral that you find there,” Hoegh-Guldberg says. “But when you look at the deep reefs, we are looking at corals that no human has ever laid eyes on before.”

So far the team has completed four of its ten planned surveys at areas along the lengths of the Great Barrier Reef and its outlying atolls. While the deep coral communities observed so far appear to be healthy, Bongaerts cautions that this is the first time that the deep reefs have been explored: “This is the first point in time that we are looking at the deep reef communities, so it’s really hard to extrapolate their health without having a comparison”.

However, the coral is much more abundant and diverse than the impacted shallow reefs above them. “In the Coral Sea at Flinders Reef and Holmes Reef, where we did find very impacted communities, we find coral reefs immediately adjacent in deeper water that had no signs of disturbance.”

The deep reefs are out of the impact zone for storms and are relatively buffered from the elevated sea surface temperatures and ultraviolet light stresses that shallow coral communities are exposed to, conditions that contribute to coral bleaching.

However, Bongaerts notes there is still the possibility of the crown-of-thorns starfish impacting on deep reef communities. These unusually large starfish eat coral at an alarming rate, and infestations have decimated large areas of shallow reef. “We know very little about the occurrence of crown-of-thorns on the deep reefs and to what extent those communities may be affected. So far we haven’t really been seeing the crown-of-thorns on the deep reef, but that’s because the areas we’ve been to are also not renowned for having large numbers of crown-of-thorns in the shallows either.”

The survey team has so far observed that while some coral species are only found in deep water, others are found over the whole depth range, albeit with strikingly different appearances. “As you work your way from the shallow reefs all the way down the depth gradient, it is really interesting to see that the morphology of the corals goes from a branching and boulder shape to exclusively plates,” Bongaerts explains. With so little light penetrating to the deep reef, the corals grow in a way that maximises their surface area to the light, and this is presenting a unique challenge to the survey team. “Everything looks different in the deep reef, so it is not unusual for us to collect a coral specimen that looks completely different, that looks like a new species, but it is the same species that you find in the shallows just with a very distinct morphology.”

With many coral species being found in both the shallow and deep water environments, Hoegh-Guldberg and Bongaerts are hopeful that the deep reefs may provide a refuge for shallow water corals that are under stress and may eventually repopulate the damaged reefs that lie above. Research has already begun into this interesting topic. “We’ve looked at a few species within the one family,” Hoegh-Guldberg says, “and what we have found is that when you look at their genetic structure, so to speak, they are actually different populations”.

This result suggests that, for the species studied at least, larvae are not moving from the deep reefs to the shallows, and vice versa. Despite this, Hoegh-Guldberg and Bongaerts are still optimistic that for other species the deep corals do indeed provide recruits to the shallows. “Because you have so many different life histories, so many different coral species and, on top of that, so many different environmental conditions, it is really going to be determined on a case-by-case basis,” Bongaerts explains. “I think there is definitely potential for deep reefs to act as a so-called reproductive source for the shallow reefs, but it will be dependent on how much overlap there is between species in shallow and deep water, and what sort of disturbance you are looking at.”

That is not to say that the deep coral reefs are themselves free from potential disturbances. While the deep reefs surveyed so far seem to be unaffected from the storms, bleaching events and starfish infestations that have impacted the shallow reefs, there is the ever-present threat of rising sea levels and warmer ocean temperatures due to climate change. Rising sea levels may “drown out” certain deep water corals, but Bongaerts notes it is the rate of sea level rise that will determine if the deep coral communities can survive.

“Coral species seem to be adapted to the environmental conditions of particular depth ranges,” he says. “A significant rise in sea level can therefore place individual corals outside the depth range where they can properly function. However, if the increase in sea level is gradual enough, through reproduction, the next generations of these corals could move to more favourable spots on the reef and, as such, counteract the effect of sea level rise.”

As for warmer ocean temperatures, there may also be a shuffling of corals across different depths of the reef, but again this will be dependent on the rate of change, as well as the extent of the temperature rise. “The main issue with a warming ocean is that temperature anomalies become more pronounced as well,” says Bongaerts. “High temperature anomalies during summer months have the potential to lead to large-scale bleaching on coral reefs.”

With more than 90% of the Great Barrier Reef World Heritage Area lying at depths of 30 metres or greater, the Catlin Seaview Survey is providing valuable insights into the state of this immensely important ecosystem. “It is a vast area that harbours all these unique species,” Bongaerts explains. “It is surprising in this day and age that below some of the most well-known reefs which are so popular with divers there is an almost entirely unexplored world and, as a result, an enormous amount of science to be done.”