Before climate change caused more frequent and deadly bleaching events, cyclones were one of the greatest threats to the health of the Great Barrier Reef.
Scientists estimate that, before 2016, the destruction of tropical hurricanes was responsible for one-third to nearly half of the loss of coral cover on the world's largest reef system over the past 30 years, according to a new study published in the journal Scientific reports. The fading of corals, on the other hand, only contributed 5 to 10 percent of the coral loss.
That all changed two years ago when the first of two unprecedented driving events struck the 1,400 mile long Great Barrier Reef and eventually killed half of the corals (and other tropical reefs devastated worldwide). The new study analyzes how corals have been restored from a combination of cyclones and bleach events of the past 19 years in the Palm Islands Group, a collection of reef rims in the central part of the Great Barrier Reef.
The conclusion: 47.8% of the hard coral covering of the research locations has been lost since 1998 and the composition of the reef ecosystem has changed drastically. This was mainly due to a bleaching event in 1998 and Cyclone Yasi, a Category 5 storm that hit a direct hit on the reefs in 2011. (The bleaching event in 2016 hit the northern part of the Great Barrier Reef the hardest, leaving the Palm Islands Group in the central part relatively unharmed. The impact of the bleaching event 2017 will be published in a soon to be published paper.)
"With predicted outflows between bleaching and cyclones, the probability of another serious disruption event before coral covering and composition assembly approaches historical levels suggests that reefs will continue to erode," scientists with the Australian Research Council Center of Excellence for Coral Reef Studies James Cook University in Townsville, Queensland.
The findings underline the importance of such long-term "longitudinal" studies, given "shifting baselines" about what a healthy coral reef is and the subsequent risk of baseline bias among scientists and the public. For example, in the Palm Islands Group study, 1998 was used as the basis for calculating the decrease in coral coverage in the subsequent 19 years. But, as the authors pointed out, the reefs probably had already acquired an unknown amount of degradation before 1998.
"Shifting baselines are a serious problem," Gergely Torda, one of the lead authors of the study and a senior research fellow from the ARC Center of Excellence for Coral Reef Studies, said in an e-mail from the field in French Polynesia. "From a scientific point of view, the problem is that we have probably missed the opportunity to understand how pristine coral reefs function. We know that coral reefs continued to thrive for tens of millions of years, but whether modern reefs-that have been degraded to varying degrees and probably have a lot of their biological and functional diversity and redundancy lost – being able to survive in the coming decades of dramatic climate change remains elusive. "
This poses major challenges for policy makers and natural resource managers, Torda noted, because conservation goals can also lead to prejudice.
"Tourists come to the degraded parts of the [Great Barrier Reef] still say it is beautiful, and they do not understand what researchers are talking about, "he added. It is because they did not see it when it was still alive and functional. Changing baselines can enlighten us in a comfortable business as usual is good enough -mindset, without noticing how we are moving towards an ecological collapse on a global scale. "
Coral reefs have endured ecological upheaval for thousands of years, have suffered widespread devastation and have eventually rebounded. But the new study describes how the one-two-blow of bleaching and cyclones can remove the resilience of coral and can result in the restructuring of reef ecosystems.
Scientists conducted the coral surveys in 1998, 1999, 2001, 2011, 2014, 2014 and 2014 using identical methods at two edge erosion locations at Orpheus and Pelorus in the Palm Islands Group. The coral coverage decreased from 51.7 percent to 15.4 percent after the Bleaching event in 1998. (Peaks in ocean temperatures caused by the burning of fossil fuels and natural weather phenomena cause bleaching, with the symbiotic zooxanthellae algae in exchange for shelter, feed corals and give them color, become poisonous and be expelled. Corals have turned white and can die unless the water temperature cools down and the zooxanthellae algae return.)
The coral coverage at the research sites recovered to 27.8 percent in the decade after the bleaching event in 1998, but was destroyed by Cyclone Yasi in 2011, when it dropped to 4.1 percent, according to the study. The coral cover continued to decline over the next three years and reached 1.6 percent in 2014. But in a sign of the resilience of a reef under the right conditions, the coral coverage was increased to 27 percent by 2017.
The result of the bleaching event 2017 is the subject of a paper that is currently being assessed. Torda could not discuss the details of the findings, but said: "Anecdotally, I can tell you that on our study sites for the recent article, the bleaching of 2017 was not as bad as in other parts of the power station. [Great Barrier Reef] and did not result in massive mortality (multispecies). "
"In general, the reefs of the Palm Islands were removed with minor damage," he added.
Still, the reef ecosystem that was present in 1998 may have disappeared forever, because some species of coral have deteriorated dramatically and have been displaced by others. For example, complex branched corals that provide habitat for a variety of marine life were virtually wiped out and slowly restored, while massive corals proved to be more resilient and became a dominant species at the research sites.
Torda said that any long-term changes in the reef fish communities that depend on those habitats remain unknown, because the research focused on coral composition. However, research by Torda's colleagues on another part of the Great Barrier Reef did find a loss of fish diversity in the wake of the 2016 bleaching activity.
The researchers found that coral species generally deteriorated for years during the recovery period. "The disruption of the complex ecological feedback loops that exist between corals, their competing algae, their predators and countless species of reef fish can cause further mortality in the aftermath of the disruption and slow down the recovery," wrote the scientists. "It is important that the loss of large colonies of slow-growing species can leave a mark on the assembly composition for decades, if not for centuries."
"The increase in frequency and severity of acute disruptions due to extreme weather conditions associated with global climate change suggests that the exposed reefs of the Orpheus and Pelorus Islands, along with many other coral reefs in the world, may never recover to the state of pre-disturbance, and will continue to erode, "they concluded.
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