Diving on the remote coral reefs in the north of Western Australia during the world’s worst bleaching event in 2016, the first thing I noticed was the heat. It was like diving into a warm bath, with surface temperatures of 34⁰C.
Then I noticed the expanse of bleached colonies. Their bright white skeletons were visible through the translucent tissue following the loss of the algae with which they share a biological relationship. The coral skeletons had not yet eroded and collapsed, a grim reminder of what it looked like just a few months before.
I spent the past 15 years documenting the recovery of these reefs following the first global coral bleaching event in 1998, only to see them devastated again in the third global bleaching event in 2016.
The WA coral reefs may not be as well known as the Great Barrier Reef, but they’re just as large and diverse. And they too have been affected by cyclones and coral bleaching. Our recent study found many WA reefs now have the lowest coral cover on record.
When my colleague, Rebecca Green, witnessed that mass bleaching for the first time, she asked me how long it would take the reefs to recover.
The worst mass bleaching on record
A similar scene is playing out around the world as researchers document the decline of ecosystems they have spent a lifetime studying.
Our study, published in the journal Coral Reefs, is the first to establish a long-term history of changes in coral cover across eight reef systems, and to document the effects of the 2016 mass bleaching event at 401 sites across WA.
Given the vast expanse of WA coral reefs, our assessment included data from several monitoring programs and researchers from 19 institutions.
These reefs exist in some of the most remote and inaccessible parts of the
world, so our study also relied on important observations of coral bleaching from regional managers, tourist operators and Bardi Jawi Indigenous Rangers in the Kimberley.
Our aim was to establish the effects of climate change on coral reefs along Western Australia’s vast coastline and their current condition.
The heat stress in 2016 was the worst on record, causing mass bleaching and large reductions in coral cover at Christmas Island, Ashmore Reef and Scott Reef. This was also the first time mass bleaching was recorded in the southern parts of the inshore Kimberley region, including in the long oral history of Indigenous Australians who have managed this sea-country for thousands of years.
The mass bleaching events we documented were triggered by a global increase in temperature of 1⁰C above pre-industrial levels, whereas temperatures are predicted to rise by 1.5⁰C between 2030 and 2052.
In that scenario, the reefs that have bleached badly will unlikely have the capacity to fully recover, and mass bleaching will occur at the reefs that have so far escaped the worst impacts.
The future of WA’s coral reefs is uncertain, but until carbon emissions can be reduced, coral bleaching will continue to increase.
Surviving coral reef refuges must be protected
The extreme El Niño conditions in 2016 severely affected the northern reefs, and a similar pattern was seen in the long-term records.
The more southern reefs were affected by extreme La Niña conditions – most significantly by a heatwave in 2011 that caused coral bleaching, impacted fisheries and devastated other marine and terrestrial ecosystems.
Since 2010, all of WA’s reefs systems have bleached at least once.
Frequent bleaching and cyclone damage have stalled the recovery of reefs at Shark Bay, Ningaloo and at the Montebello and Barrow Islands. And coral cover at Scott Reef, Ashmore Reef and at Christmas Island is low following the 2016 mass bleaching.
In fact, average coral cover at most (75%) reef systems is at or near the lowest on record. But not all WA reefs have been affected equally.
In 2016 there was little (around 10%) bleaching recorded at the northern inshore Kimberley Reefs, at the Cocos Keeling Islands, and at the Rowley Shoals. Coral cover and diversity at these reefs remain high.
And during mass bleaching there were patches of reef that were less affected by heat stress.
These patches of reef will hopefully escape the worst impacts and retain moderate coral cover and diversity as the world warms, acting as refuges. There are also corals that have adapted to survive in parts of the reef where temperatures are naturally hotter.
Some reefs across WA will persist, thanks to these refuges from heat stress, their ability to adapt and to expand their range. These refuges must be protected from any additional stress, such as poor water quality and overfishing.
In any case, the longer it takes to curb carbon emissions and other pressures to coral reefs, the greater the loss will be.
Coral reefs support critical food stocks for fisheries around the world and provide a significant contribution to Australia’s Blue Economy, worth an estimated A$68.1 billion.
We are handing environmental uncertainty to the next generation of scientists, and we must better articulate to everyone that their dependence on nature is the most fundamental of all the scientific concepts we explore.
Off southern Tasmania, at depths between 700 and 1,500 metres, more than 100 undersea mountains provide rocky pedestals for deep-sea coral reefs.
Unlike shallow tropical corals, deep-sea corals live in a cold environment without sunlight or symbiotic algae. They feed on tiny organisms filtered from passing currents, and protect an assortment of other animals in their intricate structures.
Deep-sea corals are fragile and slow-growing, and vulnerable to human activities such as fishing, mining and climate-related changes in ocean temperatures and acidity.
This week we returned from a month-long research voyage on CSIRO vessel Investigator, part of Australia’s Marine National Facility. We criss-crossed many seamounts in and near the Huon and Tasman Fracture marine parks, which are home to both pristine and previously fished coral reefs. These two parks are part of a larger network of Australian Marine Parks that surround Australia’s coastline and protect our offshore marine environment.
The data we collected will answer our two key research questions: what grows where in these environments, and are corals regrowing after more than 20 years of protection?
Our eyes on the seafloor
Conducting research in rugged, remote deep-sea environments is expensive and technically challenging. It’s been a test of patience and ingenuity for the 40 ecologists, technicians and marine park managers on board, and the crew who provide electronics, computing and mechanical support.
But now, after four weeks of working around-the-clock shifts, we’re back in the port of Hobart. We have completed 147 transects covering more 200 kilometres in length and amassed more than 60,000 stereo images and some 300 hours of video for analysis.
A deep-tow camera system designed and built by CSIRO was our eye on the seafloor. This 350 kilogram system has four cameras, four lights and a control unit encased in high-strength aluminium housings.
An operations planner plots “flight-paths” down the seamounts, adding a one-kilometre run up for the vessel skipper to land the camera on each peak. The skipper navigates swell, wind and current to ensure a steady course for each one-hour transect.
An armoured fibre optic tow cable relays high-quality, real-time video back to the ship. This enables the camera “pilot” in the operations room to manoeuvre the camera system using a small joystick, and keep the view in focus, a mere two metres off the seafloor.
This is an often challenging job, as obstacles like large boulders or sheer rock walls loom out of the darkness with little warning. The greatest rapid ascent, a near-vertical cliff 45m in height, resulted in highly elevated blood pressure and one broken camera light!
Reaching into their world
Live imagery from the camera system was compelling. As well as the main reef-building stony coral Solenosmilia variabilis, we saw hundreds of other animals including feathery solitary soft corals, tulip-shaped glass sponges and crinoids. Their colours ranged from delicate creams and pinks to striking purples, bright yellows and golds.
To understand the make-up of coral communities glimpsed by our cameras, we also used a small net to sample the seafloor animals for identification. For several of the museum taxonomists onboard, this was their first contact with coral and mollusc species they had known, and even named, only from preserved specimens.
We found a raft of undescribed species, as expected in such remote environments. In many cases this is likely to be the only time these species are ever collected. We also found animals living among the corals, hinting at their complex interdependencies. This included brittlestars curled around corals, polychaete worms tunnelling inside corals, and corals growing on shells.
We used an oceanographic profiler to sample the chemical properties of the water to 2,000m. Although further analysis is required, our aim here is to see whether long-term climate change is impacting the living conditions at these depths.
A curious feature of one of the southern seamounts is that it hosts the world’s only known aggregation of deep-water eels. We have sampled these eels twice before and were keen to learn more about this rare phenomenon.
Using an electric big-game fishing rig we landed two egg-laden female eels from a depth of 1,100 metres: a possible first for the record books.
In a side-project, a team of observers recorded 42 seabird species and eight whale and dolphin species. They have one more set of data towards completing the first circum-Australia survey of marine birds and mammals.
More coral pedestals than we realise
An important finding was that living S. variabilis reefs extended between the seamounts on raised ridges down to about 1,450m. This means there is more of this important coral matrix in the Huon and Tasman Fracture marine parks than we previously realised.
In areas that were revisited to assess the regrowth of corals after two decades of protection from fishing, we saw no evidence that the coral communities are recovering. But there were signs that some individual species of corals, featherstars and urchins have re-established a foothold.
In coming months we will work through a sub-sample of our deep-sea image library to identify the number and type of organisms in certain areas. This will give us a clear, quantitative picture of where and at what depth different species and communities live in these marine parks, and a foundation for predicting their likely occurrence both in Australia and around the world.
The seamount corals survey involved 10 organisations: CSIRO, the National Environmental Science Program Marine Biodiversity Hub, Australian Museum, Museums Victoria, Tasmanian Museum and Art Gallery, NIWA (NZ), three Australian universities and Parks Australia.
“What happens if the water temperature rises by a few degrees?” is the 2018 International Year of the Reef leading question. While the ocean is the focus, urbanisation is the main reason for the rising temperatures and water pollution. Yet it receives little attention in this discussion.
In turn, rising temperatures increase downpours and urban floods, adding to the pressures on urban infrastructure.
Protecting the reef as Cairns grows
Cairns is an expanding Queensland city located between two World Heritage sites – the Great Barrier Reef and the Daintree Rainforest. While important research focuses on these sites themselves, not much is known about how the surrounding urban areas influence these natural environments. Similarly, little is known about how urban planning and design contribute to the health of the inner city and surrounding water bodies, including the ocean.
Cairns is a major Australian tourism destination with a unique coastal setting of rainforest and reef. This attracts growing numbers of visitors. One effect of this success is increased urbanisation to accommodate these tourists.
There are many opportunities to promote sustainable and socially acceptable growth in Cairns. Yet this growth is not without challenges. These include:
- impacts of climate change, including sea-level rise and ocean warming
- lack of comprehensive urban infrastructure strategy
- lack of comprehensive assessment of the benefits of integrated urban design to maximise coastal resilience and the health of streams and oceans.
As with most Australian cities, Cairns has an urban layout based on wide streets, mostly with little or no greenery. Rain gardens, for instance, are rare. Bioswales that slow and filter stormwater are present along highways, but seldom within the city.
The arguments for not adding greenery to the urban environment are familiar. These typically relate to costs of implementation and maintenance, but also to the speed with which water is taken out of streets during the tropical rainy season. This is because green stormwater solutions, if not well planned, can slow down the water flow, thus increasing floods.
However, cities can be designed in a way to imitate nature with solutions that are an integral part of the urban system. This can include dedicated areas of larger wetlands and parks, which capture water and filter pollution and undesired nutrients more efficiently, reducing polluted runoff to the reef.
Integrated urban design
Integrated urban design is an aspect of city planning and design that could be further developed to ensure the whole system works more efficiently. This involves integrating the three elements that make up urban infrastructure:
- the green – parks, residential gardens, rain gardens, green roofs and walls, bioswales, etc
- the grey – built drains, footpaths, buildings, underground vacuum
- the blue – streams, stormwater systems, etc.
Urban infrastructure, therefore, can and should be planned and designed to provide multiple services, including coastal resilience and healthier water streams and oceans. To achieve this, a neighbourhood or city-wide strategy needs to be implemented, instead of intermittent and ad hoc urban design solutions. Importantly, each element should coordinate with the others to avoid overlaps, gaps and pitfalls.
This is what integrated urban design is about. So why don’t we implement it more often?
Challenges and opportunities
Research has shown that planning, designing and creating climate-resilient cities that are energy-optimised, revitalise urban landscapes and restore and support ecosystem services is a major challenge at the planning scale. To generate an urban environment that promotes urban protection and resilience while minimising urbanisation impacts and restoring natural systems, we need to better anticipate the risks and have the means to take actions. In other words, it is a two-way system: well planned and designed green and blue infrastructures not only deliver better urbanised areas but will also protect the ocean from pollution. Additionally, it helps to manage future risks of severe weather.
The uncertainties of green infrastructure capacity and costs of maintenance, combined with inflexible finance schemes, are obstacles to integrated urban solutions. Furthermore, the lack of inter- and transdisciplinary approaches results in disciplinary barriers in research and policymaking to long-term planning of the sort that generates urban green infrastructure and its desired outcomes.
On the bright side, there is also strong evidence to suggest sound policy can help overcome these barriers through technical guides based on scientific research, standards and financial incentives.
Collaborative partnerships are promising, too. Partnerships between academia and industry tend to be more powerful than streamlined industry project developments.
Finally, and very promisingly, Australia has its own successful green infrastructure examples. Melbourne’s urban forest strategy has been internationally acclaimed. Examples like these provide valuable insights into local green infrastructure governance.
Cairns has stepped up with some stunning blue infrastructure on the Esplanade which raises awareness of both locals and visitors about the protection of our oceans.
This is only the start. Together academics, local authorities, industry stakeholders and communities can lead the way to resilient cities and healthier oceans.
The Great Barrier Reef is better able to heal itself than we previously imagined, according to new research that identifies 112 individual reefs that can help drive the entire system towards recovery.
The back-to-back bleaching events in 2016 and 2017 that killed many corals on the Great Barrier Reef have led many researchers to ask whether and how it can recover. Conventionally, we tend to focus on what controls recovery on individual reefs – for example, whether they are fouled by seaweed or sediments.
But in our study, published in PLoS Biology, my colleagues and I stepped back to view the entire Great Barrier Reef as a whole entity and ask how it can potentially repair itself.
We began by asking whether some reefs are exceptionally important for kick-starting widespread recovery after damage. To do this we set three criteria.
First, we looked for reefs that are major sources of coral larvae – the ultimate source of recovery. Every year corals engage in one of nature’s greatest spectacles, their mass reproduction during a November full moon. Fertilised eggs (larvae) travel on ocean currents for days or weeks in search of a new home.
With our partners at the CSIRO we’ve been able to model where these larvae go, and therefore the “connectivity” of the reef. By using this modelling (the Great Barrier Reef is far too large to observe this directly), we looked for reefs that strongly and consistently supply larvae to many other reefs.
Healthy reefs supply far more larvae than damaged ones, so our second criterion was that reefs should have a relatively low risk of being impacted by coral bleaching. Using satellite records of sea temperature dating back to 1985, we identified reefs that have not yet experienced the kind of temperature that causes mass coral loss. That doesn’t mean these reefs will never experience bleaching, but it does mean they have a relatively good chance of surviving at least for the foreseeable future.
Our final criterion was that reefs should supply coral larvae but not pests. Here we focused on the coral-eating crown-of-thorns starfish, whose larvae also travel on ocean currents. We know that outbreaks of these starfish tend to begin north of Cairns, and from that we can predict which reefs are most likely to become infested over time.
Fortunately, many good sources of coral larvae are relatively safe from crown-of-thorns starfish, particularly those reefs that are far offshore and bathed in oceanic water from the Coral Sea rather than the currents that flow past Cairns. Indeed, the access to deep – and often cooling – ocean water helps moderate temperature extremes in these outer reefs, which also reduces the risk of bleaching in some areas.
Using these three criteria, we pinpointed 112 reefs that are likely to be important in driving reef recovery for the wider system. They represent only 3% of the reefs of the Great Barrier Reef, but are so widely connected that their larvae can reach 47% of all the reefs within a single summer spawning season.
Unfortunately, their distribution across the reef is patchy. Relatively few are in the north (see map) so this area is relatively vulnerable.
Our study shows that reefs vary hugely, both in their exposure to damage and in their ability to contribute to the recovery of corals elsewhere. Where these patterns are pretty consistent over time – as is the case for the reefs we identified – it makes sense to factor this information into management planning.
It would be sensible to improve surveillance of these particular reefs, to check that crown-of-thorns starfish do not reach them, and to eradicate the starfish if they do.
To be clear, these are not the only reefs that should be managed. The Great Barrier Reef already has more than 30% of its area under protection from fishing, and many of its other individual reefs are important for tourism, fisheries and cultural benefits.
But the point here is that some reefs are far more important for ecosystem recovery than others. Factoring these patterns into tactical management – such as how best to respond in the aftermath of a cyclone strike – is the next step. It’s a need that has been articulated repeatedly by the Great Barrier Reef Marine Park Authority.
Taking the long-term view, the greatest threats to the reef are rising sea temperatures and ocean acidification caused by elevated carbon dioxide levels. This is clearly a challenge for humanity and one that requires consistent policies across governments.
But local protection is vital in order to maintain the reef in the best state possible given the global environment. Actions include improvements to the quality of the water emerging from rivers, controlling crown-of-thorns starfish, and maintaining healthy fish populations.
This is an expensive process and resources need to be deployed as effectively as possible. Our results help target management effectively by revealing the underlying mechanisms of repair on the reef. If management can help protect and facilitate corals’ natural processes of recovery, this might go a long way towards sustaining the Great Barrier Reef in an already challenging environment.
Regional variations in sea surface temperature, related to seasons and El Niño, could be crucial for the survival of coral reefs, according to our new research. This suggests that we should be able to identify the reefs most at risk of mass bleaching, and those that are more likely to survive unscathed.
But global warming, coupled with other pressures such as nutrient and sediment input, changes in sea level, waves, storms, ventilation, hydrodynamics, and ocean acidification, could lead to the end of the world’s coral reefs in a couple of decades.
Climate warming is the major cause of stress for corals. The world just witnessed an event described as the “longest global coral die-off on record”, and scientists have been raising the alarm about coral bleaching for decades.
The first global-scale mass bleaching event happened in 1998, destroying 16% of the world coral reefs. Unless greenhouse emissions are drastically reduced, the question is no longer if coral bleaching will happen again, but when and how often?
To help protect coral reefs and their ecosystems, effective management and conservation strategies are crucial. Our research shows that understanding the relationship between natural variations of sea temperature and human-driven ocean warming will help us identify the areas that are most at risk, and also those that are best placed to provide safe haven.
A recurrent threat
Bleaching happens when sea temperatures are unusually high, causing the corals to expel the coloured algae that live within their tissues. Without these algae, corals are unable to reproduce or to build their skeletons properly, and can ultimately die.
Certain types of coral can also acclimatise to rising sea temperatures. But as our planet warms, periods of bleaching risk will become more frequent and more severe. As a consequence, corals will have less and less time to recover between bleaching events.
We are already witnessing a decline in coral reefs. Global populations have declined by 1-2% per year in response to repeated bleaching events. Closer to home, the Great Barrier Reef lost 50% of its coral cover between 1985 and 2012.
A non-uniform response to warming
While the future of worldwide coral reefs looks dim, not all reefs will be at risk of recurrent bleaching at the same time. In particular, reefs located south of 15ºS (including the Great Barrier Reef, as well as islands in south Polynesia and Melanesia) are likely to be the last regions to be affected by harmful recurrent bleaching.
We used to think that Micronesia’s reefs would be among the first to die off, because the climate is warming faster there than in many other places. But our research, published today in Nature Climate Change, shows that the overall increase in temperature is not the only factor that affects coral bleaching response.
In fact, the key determinant of recurrent bleaching is the natural variability of ocean temperature. Under warming, temperature variations associated with seasons and climate processes like El Niño influence the pace of recurrent bleaching, and explain why some reefs will experience bleaching risk sooner than others in the future.
Our results suggest that El Niño events will continue to be the major drivers of mass bleaching events in the central Pacific. As average ocean temperatures rise, even mild El Niño events will have the potential to trigger widespread bleaching, meaning that these regions could face severe bleaching every three to five years within just a few decades. In contrast, only the strongest El Niño events will cause mass bleaching in the South Pacific.
In the future, the risk of recurrent bleaching will be more seasonally driven in the South Pacific. Once the global warming signal pushes summer temperatures to dangerously warm levels, the coral reefs will experience bleaching events every summers. In the western Pacific, the absence of natural variations of temperatures initially protects the coral reefs, but only a small warming increase can rapidly transition the coral reefs from a safe haven to a permanent bleaching situation.
One consequence is that, for future projections of coral bleaching risk, the global warming rate is important but the details of the regional warming are not so much. The absence of consensus about regional patterns of warming across climate models is therefore less of an obstacle than previously thought, because globally averaged warming provided by climate models combined with locally observed sea temperature variations will give us better projections anyway.
Understanding the regional differences can help reef managers identify the reef areas that are at high risk of recurring bleaching events, and which ones are potential temporary safe havens. This can buy us valuable time in the battle to protect the world’s corals.
Clothilde Emilie Langlais, research scientist at CSIRO Oceans and Atmosphere, CSIRO; Andrew Lenton, Senior Research Scientist, Oceans and Atmosphere, CSIRO, and Scott Heron, Physical Scientist, National Oceanic and Atmospheric Administration
The world’s coral reefs are undoubtedly in deep trouble. But as we and our colleagues argue in a review published today in Nature, we shouldn’t give up hope for coral reefs, despite the pervasive doom and gloom.
Instead, we have to accept that coral reefs around the world are transforming rapidly into a newly emerging ecosystem unlike anything humans have experienced before. Realistically, we can no longer expect to conserve, maintain, preserve or restore coral reefs as they used to be.
This is a confronting message. But it also focuses attention on what we need to do to secure a realistic future for reefs, and to retain the food security and other benefits they provide to society.
The past three years have been the warmest on record, and many coral reefs throughout the tropics have suffered one or more bouts of bleaching during prolonged underwater heatwaves.
A bleached coral doesn’t necessarily die. But in 2016, two-thirds of corals on the northern Great Barrier Reef did die in just six months, as a result of unprecedented heat stress. This year the bleaching happened again, this time mainly on the middle section of the reef.
In both years, the southern third of the reef escaped with little or no bleaching, because it was cooler. So bleaching is patchy and it varies in severity, depending partly on where the water is hottest each summer, and on regional differences in the rate of warming. Consequently some regions, reefs, or even local sites within reefs, can escape damage even during a global heatwave.
Moderate bleaching events are also highly selective, affecting some coral species and individual colonies more than others, creating winners and losers. Coral species also differ in their capacity to reproduce, disperse as larvae, and to rebound afterwards.
This natural variability offers hope for the future, and represents different sources of resilience. Surviving corals will continue to produce billions of larvae each year, and their genetic makeup will evolve under intense natural selection.
In response to fishing, coastal development, pollution and four bouts of bleaching in 1998, 2002, 2016 and 2017, the Great Barrier Reef is already a highly altered ecosystem, and it will change even more in the coming decades. Although reefs will be different in future, they could still be perfectly functional in centuries to come – capable of sustaining ecological processes and regenerating themselves. But this will only be possible if we act quickly to curb climate change.
The Paris climate agreement provides the key framework for avoiding very dangerous levels of global warming. Its 1.5℃ and 2℃ targets refer to increases in global average land and sea temperatures, relative to pre-industrial times. For most shallow tropical oceans, where temperatures are rising more slowly than the global average, that translates to 0.5℃ of further warming by the end of this century – slightly less than the amount of warming that coral reefs have already experienced since industrialisation began.
If we can improve the management of reefs to help them run this climate gauntlet, then reefs should survive. Reefs of the future will have a different mix of species, but they should nonetheless retain their aesthetic values, and support tourism and fishing. However, this cautious optimism is entirely contingent on steering global greenhouse emissions away from their current trajectory, which could see annual bleaching of corals occurring in most tropical locations by 2050. There is no time to lose before this narrowing window of opportunity closes.
A crisis of governance
Reef governance is failing because it is largely set up to manage local threats, such as overfishing and pollution. In Australia, when the Great Barrier Reef Marine Park Authority was set up in 1976, the objective of managing threats at the scale of (almost) the entire Great Barrier Reef was revolutionary. But today, the scale of threats is global: market pressures for Australian reef fish now come from overseas; port dredging and shipping across the reef are spurred on by fossil fuel exports to Asia; a housing crisis in the United States can batter reef tourism half a world away; and record breaking marine heatwaves due to global warming can kill even the most highly protected and remote corals.
Increasingly, coral reef researchers are turning to the social sciences, not just biology, in search of solutions. We need better governance that addresses both local and larger-scale threats to coral reef degradation, rather than band-aid measures such as culling starfish that eat corals.
In many tropical countries, the root causes of reef degradation include poverty, increasing market pressures from globalisation, and of course the extra impacts of global warming. Yet these global issues desperately need more attention at just the time when some governments are reducing foreign aid, failing to address global climate change, and in the case of Australia and the US, trying to resuscitate the dying fossil fuel industry with subsidies for economically unviable projects.
Effective reef governance will not only require increased cooperation among nations to tackle global issues, as in the case of the Paris climate deal, but will also require policy coordination at the national level to ensure that domestic action matches and supports these larger-scale goals.
Quite simply, we can’t expect to have thriving coral reefs in the future as well as new coal mines – policies to promote both are incompatible.
Terry Hughes, Distinguished Professor, James Cook University, James Cook University and Joshua Cinner, Professor & ARC Future Fellow, ARC Centre of Excellence, Coral Reef Studies, James Cook University
Images of this year’s coral bleaching on the Great Barrier Reef shocked the world. Some tour operators expressed concern that the extensive and sometimes simplistic media coverage would hurt their businesses.
The reef was a hot-button issue during the federal election, with both major parties pledging funding for programs to enhance water quality. Some politicians and tour operators expressed optimism about the reef’s ability to recover.
It was the culmination of the longest, most extensive and most severe mass coral bleaching event ever recorded – an event that began in the North Pacific in mid-2014. The Great Barrier Reef was not spared, this year experiencing its hottest sea surface temperatures since records began – 29.1℃ in February (1.1℃ above the 1961-90 average), 29.1℃ (1.3℃ above average) in March and 27.8℃ (1.0℃ above average) in April.
Evidence of bleaching was found on 93% of the more than 900 individual reefs surveyed that month, with the most severe impacts on the most pristine and isolated reefs of the far north. A preliminary estimate is that 22% of coral has now died, with 85% of these deaths occurring between Cape York and just north of Lizard Island.
At the height of the bleaching, the Climate Council’s chief executive, Amanda McKenzie, and councillor Tim Flannery visited a reef off Port Douglas that local tour operators have long regarded as one of the best – the quintessential underwater wonderland.
Several months later, the public’s shock and outrage has largely dissipated, but the question remains: are there signs that the reef’s hoped-for recovery is actually happening?
This week, I joined Tim and Amanda in revisiting the site that had so dismayed them back in April. We were guided by the passionate conservationist John Rumney, who has been diving here for more than 40 years. John’s son-in-law Dean Miller was our videographer, above and below the water.
It was a beautiful day on the reef – calm and sunny. We noted that it was three years to the day since the incoming Abbott government sacked the Climate Commission – which in turn led to the establishment of the ongoing Climate Council.
Wet-suited, we slipped into the water and paddled towards the coral, our progress monitored by a drone buzzing overhead like some giant, demented mosquito.
So what did we find? Structurally, the reef appears intact, but the whole landscape is, well, subdued. While pockets of brilliant blue staghorn remain, much of the coral that bleached earlier this year is dead, the white skeletons filmed over by greenish-brown filamentous algae.
The fish community has also changed. Algae-eating species such as surgeon fish are doing well, but coral-feeders are hardly to be seen – I spotted only a single parrot fish in an hour of snorkelling. Meanwhile, the corals themselves seem to be showing symptoms of white spot and white band diseases, conditions associated with their diminished immune systems after the stress of bleaching.
The mood on the boat after the snorkelling was also subdued. The locals had not visited this particular reef since the height of the bleaching. Having now seen the extent of the coral death that has resulted, they fear this will eventually weaken the structural integrity of the reef, making it susceptible to future damage from storms.
This November’s spawning will hopefully reseed the reef, but our companions on the trip acknowledged that any repeated bleaching within the next few years will greatly reduce the chances of recovery.
As our boat pulled away from the reef, another took its place, full of tourists donning their snorkelling gear. I found myself hoping that most of them were first-timers – unencumbered by memories of the reef’s former glory.
Back in the real world, Australia’s greenhouse emissions continue to rise (by 1.1% in 2014-15) and the government’s current target of reducing emissions 26-28% below 2005 levels by 2030 is manifestly inadequate, even if achieved.
The continued burning of coal, oil and gas is estimated to have made the bleaching on the Great Barrier Reef this year at least 175 times more likely. At present rates of climate change, this level of bleaching could occur every two years by the 2030s. That would make recovery between events virtually impossible.
The forlorn, diminished state of Australia’s greatest natural treasure must continue to serve as a visible warning of what we stand to lose. The new normal is a very sad place to be.
Lesley Hughes, Professor, Department of Biological Sciences
Coral reefs around the world are facing a whole spectrum of human-induced disturbances that are affecting their ability to grow, reproduce and survive. These range from local pressures such as overfishing and sedimentation, to global ones such as ocean acidification and warming. With the third global coral bleaching event underway, we now more than ever, need to understand how coral responds to these stressors.
Our new research, published in Science Advances, now shows that young corals develop deformed and porous skeletons when they grow in more acidified waters, potentially making it more difficult for them to establish themselves on the reef and survive to adulthood.
Corals vary in their responses to stress, not only between species and location, but also among different stages of their life cycle. Juvenile corals are extremely important to the health of a reef, as they help to replenish the reef’s coral population and also help it recover from severe disturbances such as bleaching and storms.
However, newly settled young corals are small (typically about 1 mm across) and therefore very vulnerable to things like overgrowth and predation. To survive into adulthood they need to grow quickly out of this vulnerable size class. To do that they need to build a robust skeleton that can maintain its structural integrity during growth.
Two major factors that affect coral skeletal growth are ocean temperature and carbon dioxide concentration. Both are on the rise as we continue to emit huge amounts of CO₂ into the atmosphere. Generally with adult corals, increased temperature and CO₂ both reduce growth rates. But this varies considerably depending on the species and the environmental conditions to which the coral has been exposed.
Much less is known about the impacts of these factors on juvenile corals. This is mainly because their small size makes them more difficult to study, and they are only usually around once a year during the annual coral spawn. The corals we studied spawn for just a couple of hours, on one night of the year, meaning that our study hinged on taking samples during a crucial one-hour window.
When collecting the samples, at Western Australia’s Basile Island in the Houtman Abrolhos archipelago in March 2013, we watched the adult spawners each night waiting to see if they would spawn and, when they did, we worked all night fertilising the eggs to collect our juvenile samples.
Having collected our elusive coral samples, we cultured and grew newly settled coral recruits under temperature and CO₂ conditions that are expected to occur by the end of the century if no action is taken to curb the current trajectory of CO₂ emissions.
We then used three-dimensional X-ray microscopy to look at how these conditions affect the structure of the skeleton. This technique involves taking many X-ray projection images of the sample (in this case around 3,200) and then reconstructing them into a 3D image.
Deformed and porous skeletons
Corals grown under high-CO₂ conditions not only showed reduced skeletal growth overall, but developed a range of skeletal deformities.
These included reduced overall size, gaps, over- and under-sized structures, and in some cases, large sections of skeleton completely missing. We also saw deep pitting and fractures in the skeletons of corals grown under high CO₂, typical of skeletal dissolution and structural fragility.
Surprisingly, increased temperature did not have a negative impact on skeletal growth and for some measures even appeared to help to offset the negative impacts of high CO₂ – a response we think may be unique to sub-tropical juveniles.
Nevertheless, our study highlights the vulnerability of juvenile corals to ocean acidification.
Under the current CO₂ emissions trajectory, our findings indicate that young corals will not be able to effectively build their skeletons. This could have wider implications for coral reef health, because without healthy new recruits, reefs will not replenish and will be less able to bounce back from disturbances.
The effect of temperature in this study however, was both a surprising and welcome finding. There is a lot of variation even between species, but it is possible that subtropical organisms have more plasticity due to their natural exposure to a wider range of conditions. This could indicate that subtropical juveniles may have an unexpected edge when it comes to ocean warming.