Deep Water No Haven for Barrier Reef
Coral Bleaching: An Event Gone Far Deeper Than Thought

The recent event of mass coral bleaching has gone far deeper than thought. These deep-water reefs were previously considered as a safe haven from the killer events.

Dr. Pim Bongaerts, curator of invertebrate zoology and Hope for Reefs co-leader at the California Academy of Sciences and co-author of the study stated, “During the bleaching event, cold-water upwelling initially provided cooler conditions on the deep reef. However, when this upwelling stopped towards the end of summer, temperatures rose to record-high levels even at depth.”

The research team was surprised to discover the bleached coral colonies 131 feet beneath the ocean’s surface said the lead author Dr. Pedro Frade from the Center of Marine Sciences (CCMAR)

“It was a shock to see that the impacts extended to these dimly lit reefs, as we were hoping their depth may have provided protection from this devastating event.”

To characterize how temperature conditions at depth differ from those in shallow habitats, the team positioned sensors 328 feet beneath the surface with the help of remotely operated vehicles (ROVs).

The survey was conducted by a team of divers during the height of bleaching across several sites on the northern Great Barrier Reef.

They noticed that while confirming previous reports of impacts on close to half the shallower corals, the overall mortality and bleaching affected more than half of corals at the deep sampling points.

Dr. Ove Hoegh-Guldberg from The University of Queensland, where the study was conducted said, “Unfortunately, this research further stresses the vulnerability of the Great Barrier Reef. We already established that the refuge role of deep reefs is generally restricted by the limited overlap in species with the shallow reef. However, this adds an extra limitation by demonstrating that the deep reefs themselves are also impacted by higher water temperatures.”

However, the researchers will carry on the research over the variation between the recovery process between deep and shallow reefs.

The study that is published in Nature Geoscience is the first of its kind to renovate the evolution of the reef over the past 30 millennia in response to abrupt and major environmental change.

The 10-year transnational endeavor has proved that the reef is more resilient to major environmental vicissitudes like sea-temperature change and sea-level rise than expected, however, it also showed a high sensitivity to poor water quality and sediment input.

Jody Webster, Associate Professor at the University of Sydney said, “Our study shows the reef has been able to bounce back from past death events during the last glaciation and deglaciation. However, we found it is also highly sensitive to increased sediment input, which is of concern given current land-use practices.”

Professor Webster found this rate to be unlikely to survive the current rates of sharp declines in coral coverages, increased sediment flux, decreasing water quality, year-on-year coral bleaching, and rise in sea surface temperature since European settlement.

He said, “I have grave concerns about the ability of the reef in its current form to survive the pace of change caused by the many current stresses and those projected into the near future.”

Associate Professor Webster stated that the previous studies have verified the fact of rising sea temperature by a couple of degrees over a period of 10,000 years. Nonetheless, the current predictions of sea surface temperature show the temperature change around 0.7 degrees in a century.

He said, “Our study shows that as well as responding to sea-level changes, the reef has been particularly sensitive to sediment fluxes in the past and that means, in the current period, we need to understand how practices from primary industry are affecting sediment input and water quality on the reef.”