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To hot to handle.

Earlier this year, Australia's Great Barrier Reef suffered the worst mass bleaching event in recorded history. With such events becoming both more freguent and more severe, the future of the reef appears to be hanging in the balance, as Geordie Torr reports

We saw the soft corals just ... It was like they were melting away. I've never seen anything like it; it was like they had leprosy. Bits were falling off; you could see pieces of soft coral tumble down the slope. On the bottom, you could see all of the bits that had come off from colonies in the shallower water.' Lyle Vail is the director of the Australian Museum's research station on Lizard Island in the northern portion of the Great Barrier Reef (GBR), 270 kilometres north of Cairns. He and his wife Anne have lived on the island for more than 26 years. Earlier this year, Vail watched as the coral reefs in the waters around the island fluoresced and then turned white In what was to become the worst coral bleaching event on the GBR in recorded history.

'By early February we were starting to see some stress and low levels of bleaching,' Vail says. 'By March it had picked up quite dramatically, likewise in April, and then we started to see some mortality in April and May.'

Bleaching occurs when corals (both hard and soft) eject the symbiotic algae (zooxanthellae) that live inside their tissue. The photosynthetic zooxanthellae provide up to 90 per cent of the energy the corals need to grow and reproduce, while also giving them most of their colour. When corals are stressed, especially when the surrounding water gets too hot, they eject the algae. The tissues of the polyps then appear transparent, revealing the white skeleton beneath--hence 'bleaching'.

If conditions return to normal quickly enough, the zooxanthellae are taken back in and the corals will survive; if the stress continues for too long, however, the coral colonies will eventually die.

NORTH-SOUTH DIVIDE

'Corals are simple organisms,' says Professor Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland. 'In summer, they're all sitting at 1[degrees]C below the temperature that causes bleaching and mortality. So very small changes in sea temperature can have dramatic effects.'

But during the recent bleaching event, the changes in sea temperature were far from small. Around Lizard, water temperatures reached 33[degrees]C in the shallows. 'The corals around here are adapted to temperatures up to around 29.5[degrees]C,' says Vail. 'Once it starts getting over that for any length of weeks, they start stressing out. We were hitting those high temperatures in February and they stayed there into late April and May.'

Unsurprisingly, given the conditions, the damage was extensive. 'In the shallow waters in the lagoon, the acroporas and the staghorn corals, which are the ones that are really prone to bleaching, we've probably lost 80 or 90 per cent of the colonies,' says Vail. 'And the hot water went down to at least 20 metres; in some places it was down to 30 metres. So we were seeing high levels of stress in corals in deeper water. The hot water was fairly uniform, both across the shelf and with depth, so there are very few places where you can go and say, "Gee, luckily this area escaped the bleaching"--it didn't happen that way.

'We had quite a bit of warning,' Vail continues. 'Even last year, they were saying that an El Nino event was on its way and that often causes bleaching. It's not that the El Nino itself causes the bleaching, but it brings conditions such as clear skies. A bleaching event isn't just about hot water, it's also the amount of UV radiation hitting the corals.'

When it became clear that a major bleaching event was imminent, Professor Terry Hughes, director of the Australian Research Council Centre of Excellence for Coral Reef Studies at the James Cook University in Townsville, convened the National Coral Bleaching Taskforce, drawing together more than 300 scientists from ten research institutions around Australia to co-ordinate the research efforts into the event. The task force carried out extensive aerial surveys, scoring the bleaching severity at the peak of the event for about a third of the GBR--close to 1,200 reefs along the whole length and breadth of the reef, a process that Hughes describes as 'very confronting'.

'We found a very strong north-south pattern,' he says. 'The southern third is okay, the middle third was moderately bleached, at about the same intensity as the two previous mass bleaching events in 1998 and 2002, and then the northern 800 kilometres or so, from Port Douglas up, were really, really badly damaged.'

The task force also carried out in-water surveys of 145 reefs between Townsville and the Torres Strait. 'We did most of our surveys in shallow water, but we did dive to 40 metres and we found bleaching all the way down,' says Hughes. 'There was some lessening of bleaching below about 20 metres, but even at 20,30 and 40 metres we saw substantial bleaching.'

The surveys assessed and quantified coral mortality at the peak of bleaching. 'We were already seeing corals in that northern region starting to die in April,' says Neal Cantin, a research scientist with the Australian Institute of Marine Science (AIMS) who co-ordinated the institute's surveys. 'Depending on the reef, depending on the bleaching severity, we were estimating at that point, with the in-water surveys, about 20-35 per cent of the total coral cover for the GBR would have been lost from this bleaching event.'

'The conventional thinking about loss of corals from bleaching is that a coral that's white is nutritionally compromised,' says Hughes. 'It has lost the microscopic algae that live inside its tissue. The algae photosynthesise and give energy to the coral host and if they don't come back quickly enough, the coral will slowly die. We're seeing that now, but in addition, we're seeing another kind of mortality that was much, much quicker. At the peak of the summer, the corals didn't just slowly die of starvation, they died directly from heat stress. When we censused those reefs in March, in the northern part of the reef, 20 per cent had already died.

'The concern is that this event was so severe in the north that we've witnessed severe bleaching and mortality of very old corals, corals that typically survived a cyclone or a bleaching,' Hughes continues. 'Scientists distinguish between winners and losers during a bleaching event. Bleaching is usually very selective, so there are susceptible losers and resistant, tougher winners. This time around, the disparity between winners and losers has almost disappeared on the northernmost reefs. We've seen very severe bleaching in corals that are normally resistant. And they won't come back in ten years. My conclusion is that the northern GBR will never again look the way that it did before March because the mix of species has changed. And we no longer have 50 years between bleaching events. They're not coming back.'

WEATHER PATTERNS

As terrible as the event was, it could have been much worse. 'At Cairns, Townsville, the central section and especially the southern section, even though they bleached, there was relatively low levels of mortality,' says Vail.

'The local weather is largely what saved the southern two thirds of the GBR,' Cantin says. 'The heat was in the system to bleach the entire reef the way it did the north, but ex-Cyclone Winston came through right at the peak of the warming in March and that provided extensive cloud cover from south of Cairns right through to Mackay. The cloud did two things: it slowed down the warming but it also shaded the reefs.' It also brought rain, which helped to further cool things. 'The temperature dropped by 2[degrees]C, so ex-Cyclone Winston rescued the southern half of the reef,' says Hughes.

The task force's surveys revealed that 93 per cent of the individual reefs on the GBR suffered from varying degrees of bleaching. Overall, almost a quarter, about 22 per cent, of the coral died. The north-south gradient was very strong, with about 85 per cent of the mortality occurring in the northernmost section, where about half of the coral died, compared with virtually no mortality in the southern section.

WATER QUALITY

The good news is that now the sea temperatures have returned to normal, there are some signs of recovery being observed. 'I'm amazed at how quickly corals start to bounce back,' Vail says. 'Obviously, we haven't had a spawning event since the bleaching, so it's not recruitment. It's the corals that didn't die. Often it seems to be the very little ones; perhaps they were shaded by a plate coral so they didn't get as much direct sunlight.'

Hughes is more circumspect. 'It's a bit disingenuous to talk about recovery,' he says. 'I was diving off Cairns last week and I went to a reef that in March was about 60 per cent bleached. Today it only has one or two per cent bleaching, but that's not what an ecologist like myself would refer to as recovery--that's regaining colour. If you lose a 50-year-old coral, the reef doesn't recover until a new 50-year-old coral grows and takes its place. We have good information on recovery of reefs from cyclones and we know that the fastest growing corals--the ones that are the earliest to come back still take ten to 15 years for those losses to be recouped.'

The background resilience of the reefs is important in determining how quickly they recover. 'If one reef is healthier than another it won't affect its tendency to bleach, but its ability to recover can be dramatically different,' says Professor Hoegh-Guldberg.

The most pressing concern, when it comes to the reef's resilience, is water quality. 'In the earlier bleaching events, some of the reefs that were damaged haven't recovered,' says Hughes. 'Recovery can be quick under the right conditions, in terms of recovery of 'weedier', faster-growing corals, but sometimes recovery just fails and often the reason is poor water quality.'

The source of the reef's water quality problems is run-off from rivers and farms. 'Run-off is a big Issue, particularly in the central and southern parts of the GBR,' says Dr Katharina Fabricius, a senior principal research scientist at AIMS. 'In some places, we're seeing half the water clarity that we would expect.

'It's mostly the inshore reefs that are directly affected,' she continues. 'In areas with high levels of sediment and nutrients, coral reefs grow more slowly and recover more slowly from disturbance. They have lower diversity and higher levels of seaweed.'

The impact of sediment on the reef is two-pronged: it's an important carrier of nutrients and it also cuts out light and can smother corals when it settles, Fabricius explains. 'The very fine clays and silts travel for tens to hundreds of kilometres and they get re-suspended over and over again," she says. 'When it's calm, the particles settle on the organisms, which then need to spend energy to clean themselves. Then, when it gets windy, they get picked up from the sea floor and re-suspended, cutting out light again. After a big flood, that fine material is still being re-suspended up to half a year later."

The impact of increased nutrient levels can be even more damaging. 'There's very strong evidence that elevated levels of nutrients increase the availability of food for the larvae of the crown-of-thorns starfish (COTS), contributing to population outbreaks,' Fabricius explains. 'And at this stage, crown-of-thorns starfish are the biggest source of coral mortality on the GBR.' The larvae feed on microalgae, which form blooms when there are high levels of nutrients in the water. 'Lab experiments show very clearly that the larvae need relatively high levels of microalgae in the water to survive and establish themselves,' Fabricius says. 'COTS are incredibly fertile, so if the survival probability of these larvae increases substantially, you're more likely to get an outbreak. Outbreaks can happen naturally, but research suggests that the frequency of outbreaks is increased in areas where nutrient levels are high.'

EROSION SOURCES

It has long been assumed that the main source of these nutrients was agricultural fertiliser from intensive agriculture: cane farms, banana farms. But according to pilot studies carried out by Andrew Brooks, a senior research fellow at the Australian Rivers Institute at Griffith University, gully erosion may be an even bigger source. 'Previously, it was assumed that because these soils are fairly low In nutrients we don't need to worry about them,' he says. 'But because of the total load that's coming out of these gullies, erosion might be as large a source of nitrogen as intensive agriculture, if not greater. Our preliminary data show that per unit area, some of the gullies we've looked at have double the rates of nitrogen delivery as a cane paddock and up to ten times the concentrations of phosphorous.'

Since 2009, Brooks has been carrying out detailed research into erosion in the area of the Normandy River on Cape York--the fourth-largest catchment draining into the reef. When he began his research, our understanding of erosion and sediment and nutrient production was based almost entirely on computer models. 'For all intents and purposes, the whole of Cape York was just based on modelled data,' he says.

The models stated that 90 per cent of the erosion was so-called hill-slope erosion, which is general erosion off all of the slopes in the catchment. The other ten per cent was purported to come from erosion along rivers and stream banks, and some gully erosion. Brooks and his team used a number of methods, including radionuclide analysis and LIDAR, to measure the rates of erosion and to quantify the amount of sediment being produced by different elements of the landscape. 'What we found in Normandy was that it was somewhere between 80 and 90 per cent subsoil erosion, so the complete opposite of what the model predicted,' he says. 'We've found similar degrees of disagreement with the model wherever we've looked.'

The initial cause of the erosion is cattle grazing. 'These were essentially unstable landscapes and once we Introduced cattle, they started to destabilise them,' Brooks explains. 'In the late dry season, they're grazing up on the floodplain and then walking down to the rivers to drink. They create cattle pads or tracks down the river banks that incise through the surface soil and expose these unstable subsoils.

'You have a cap of stable soil on the surface, which is where all of the organic material is, but it's literally only 10,20 or 30 centimetres deep,' he continues. 'Once you get below that--and that can happen just by the cattle repeatedly walking over the same location--you get into what are known as weathered dispersible sodic subsoils, which literally dissolve when you add water to them.'

The new understanding of where the sediment is coming from means that we need to change the way we manage erosion in these catchments. 'For the past decade or so there has been a really active water quality programme for the rivers running into the GBR lagoon,' Brooks says. 'It has focused on dealing with surface erosion on grazing land--managing grazing regimes to try to improve ground cover and thereby reduce overall erosion rates. But that doesn't really address these other sources.'

Some of the most active gullies that Brooks has been monitoring are eroding at rates of ten to 20 metres of retreat per year. One gully system in the Bowen River catchment is producing 25,000 tonnes of sediment a year. 'If we want to deal with something like that, we need a very different approach,' he says. 'We need to direct much more focused resources at treating individual gullies. We need to spend millions of dollars at that one site, which is the opposite of the strategies that have been used in the past, where it's all about doing things across the whole landscape.'

Brooks has mapped all of the gullies in the Normandy catchment. 'We know where they are, we now know how much sediment they're producing, so we can really start to pinpoint where we should be directing resources,' he says.

While a major part of the solution is fencing around the gullies to keep the livestock out, stopping the erosion is more labour intensive--and costly. 'Effectively you have to re-grade the gully and then re-engineer a stable soil on top of the unstable subsoils,' Brooks explains. 'You stabilise the soil by adding gypsum and then, by adding compost and mulch, you re-establish a medium where you can actually get grass to grow again.' The results can be very effective. 'You can see the results within 12 months and from these trials, we've found that we can reduce the pre-treatment erosion rates by about 80 per cent," Brooks says.

But it isn't a cheap exercise. 'Based on our initial plot-scale trials, it's costing something like AU$30,000 (17,500 [pounds sterling]) a hectare,' Brooks says. 'In the worst areas, we're talking something in the order of a few thousand hectares. So it starts to add up to some pretty large numbers. But they're not inconceivable numbers.'

He estimates that for the Normandy catchment, the cost would come to about AU$60million (35million [pounds sterling]). For the larger catchments, it would be more like a few hundred million dollars. 'You will have heard numbers bandied around suggesting that it's going to cost billions of dollars to do all of this catchment-restoration stuff,' Brooks says. 'I think those numbers are probably over inflated.'

ALL AT SEA

Another source of sediment in the reef lagoon is dredging for coal ports. 'In the past few years, we've been concerned about proposals for major port development up and down the coast, including large dredging proposals where a lot of the dredge spoil was going to be dumped at sea,' says Louise Matthiesson, the GBR coastal campaign manager for WWF. 'A lot of those port proposals have now been withdrawn, so it's less of an urgent issue, but it hasn't gone away completely.'

Of particular concern is the expansion of Abbott Point Port, which has most of the necessary approvals in place. 'It's located just north of the Whitsunday islands and will entail more than a million cubic metres of dredging,' says Cherry Muddle, the GBR community campaigner for the Australian Marine Conservation Society. Initially, the dredge spoil from this project was going to be dumped on the reef, but when the new Labour government recently came to power in Queensland, it legislated to stop the dumping at sea of capital dredge spoil, which is the material generated by digging a new trench (as opposed to maintenance dredge spoil, which is generated by clearing out an existing trench).

An expansion of this and other ports will bring an expansion in shipping across the reef, which brings with It a number of threats of its own. There are chronic impacts from anchor damage and pollution, and the risk of accidents. 'There's a case going through the courts here in Brisbane at the moment regarding the Chinese coal carrier Shen Neng One, which ran aground on Douglas Shoal on 3 April, 2010,' says Matthiesson. 'And we know that there are a lot of near misses on the reef. We have a pretty good safety system in place, but it can't prevent human error. We've been very lucky not to have a major shipping accident on the GBR.'

TIME TO ACT

Unfortunately, the future for the reef looks bleak. A study released earlier this year predicted that if emissions continued on their current trajectory, the unusually high sea-surface temperatures that caused this year's mass bleaching would be the norm by 2034. 'Improving water quality is a very good idea,' says the Coral Bleaching Taskforce's Professor Hughes. 'The reef now needs all the help it can get to try to bounce back, but the elephant in the room remains climate change. We can't climate-proof reefs by making them more resilient. Bleaching can happen anywhere --even the most remote and protected areas. In the north, we saw the most pristine part of the reef severely damaged, despite the fact that there's low fishing pressure and very low amounts of run-off.

'The Australian government has something called the Reef 2050 Plan, which is essentially a water quality plan for the southern two thirds of the GBR,' he continues. 'The response by both sides of politics to the bleaching was to announce extra funding for the plan, which is very seriously under funded, but that doesn't deal with the issue at hand, which is global warming causing coral bleaching.

'The appropriate response to the bleaching would have been things like cancelling the permits for new coal mines and increasing Australia's renewable energy target, which was cut by 30 per cent last year and which was already very low compared to other OECD countries,' he concludes. 'Australia isn't exactly leading the pack when it comes to dealing with emissions. Both sides of the government still aspire to growing Australia's coal and coal-seam-gas export industries--through the marine park, as it happens.'

There is still a glimmer of hope, however. 'The GBR is, thankfully, a very diverse ecosystem, with 250 to 300 different coral species,' says Cantin. 'That lends itself to what's known as functional redundancy--when you have more species in the system, you potentially have more species that can cope with a changing environment.

'Some of the reefs are still looking pretty spectacular,' he continues. 'The reefs off Townsville, some of them bleached and some of them didn't bleach very much; some of them still had pretty high coral cover. To me, the fact that you do still have diverse, healthy systems after this event is a positive. But we're definitely seeing more frequent bleaching events, more severe bleaching events. They're only getting worse, so the future for coral reefs doesn't look promising, but it's a diverse system that can recover if it's given the time.'

The question remains, will it be given the time? 'It's sobering to think that we've had three events, this last one really severe, with just 1[degrees]C of global warming,' says Hughes. 'The COP21 target of 1.5[degrees]C is probably not achievable, so the question is, can we limit warming to 2[degrees]C? But even 2[degrees]C of warming won't be a safe place for coral reefs. We'll still have the weedy corals and the tougher ones, and It will come down to a roll of the dice in terms of the gaps between recurrent bleachings. If we look at the bleaching records from around the world, we can see that the number is trending upwards and time between them is shrinking--it's now far less than a decade, which is, on average, the minimum return time for the weedier corals. We're rapidly getting to a point where there won't be enough time between them for anything like a full recovery.'

REEF HISTORY

Stretching for more than 2,600km and covering an area about the size of Germany, the GBR is made up of more than 2,900 individual reefs and more than 900 islands. It's the world's biggest structure made by living organisms and supports more than 11,000 different species, including 400 species of hard and soft coral, about 1,600 fish species, including 134 species of shark and ray, and about 4,000 species of mollusk.

It's also one of Australia's tourism icons, attracting about two million visitors a year. Studies suggest that reef-based tourism supports as many as 70,000 jobs and brings in about AU$6billion in annual revenue. Fishing on and around the reef generates a further AU$1billion annually and employs about 2,000 people.

The reef is protected by the Great Barrier Reef Marine Park. With an area of 344,400 square kilometres, it was the world's largest marine park when it was established in 1975. Six years later, the GBR was inscribed on the World Heritage List.

In the past, the primary threats to the reef's integrity were tropical cyclones and outbreaks of crown-of-thorns starfish, which eat the coral polyps. In 2012, the Australian Institute of Marine Science published a report that blamed these two threats, as well as coral bleaching, for the loss of half the coral cover on the reef since 1985. Two thirds of that loss took place after 1998, when the GBR experienced its first recorded mass bleaching.

Further mass bleaching events, caused by unusually warm sea surface temperatures during the summer, took place in 2002, 2006 and 2016. There were also serious bleaching events in 2008 and 2011 caused by an influx of freshwater after a period of heavy rainfall and flooding.

Data

69 per cent of tourist surveyed were motivated to visit the GBR by a desire to see it before it's too late

80 percent of Queenslanders work in service industries; 1 per cent work in mining

85 per cent of the mortality in the recent mass bleaching was in the region north of Lizard Island, where about half of the coral died

5 per cent of the coral in reefs south of Cairns died in the bleaching event

Observed coral mortality as at 13 June 2016

Note: since surveys were last conducted around Lizard Island, further mortality has been reported on reefs at that location. Initial estimates indicate mortality levels are likely to have increased

Source: Great Barrier Reef Marine Park Authority, Australian Government. Date based on latest surveys conducted between I March 2016 and 13 June 2016.

ACID TEST

With attention focused on the bleaching, the other impact directly related to our C[O.sub.2] emissions--acidification--has taken a back seat. But it hasn't gone away, it's just playing the long game. When C[O.sub.2] dissolves in seawater, it interacts with the water to form carbonic acid, lowering the water's pH. 'Research on the ecological effects of acidification really only started a few years ago,' says Katerina Fabricius of AIMS. 'In the past, we did a lot of work in the laboratory, exposing organisms to short-term conditions of elevated levels of C[O.sub.2]. The results were often significant--you see changes in the calcification of corals, they grow less quickly, fish show different behaviours and so on--but that is at levels of C[O.sub.2] that we're not expecting to see until later this century.'

But that doesn't mean that there aren't effects already. 'Seawater is already 30 per cent more acidic than it used to be and sensitive organisms such as crustose coralline algae are likely to be responding to those changes,' Fabricius says.

Coralline algae are essential for reef building as they are the preferred substrate for free-swimming coral larvae to settle on and start building their limestone skeletons. 'We're finding that even with small changes in pH, the number of coralline algae declines substantially,' Fabricius says. 'Acidification is a slow and creeping issue. It's not as acute as bleaching, it doesn't kill corals at the level that we're seeing, but it's building. And it's irreversible at human timescales and probably irreversible at a timescale of thousands of years. The C[O.sub.2] that's in the water now can't easily be removed, so the changes that are happening now will remain with us for a long time.'

THE UNESCO FIASCO

In May, it emerged that the Australian government had put pressure on UNESCO to remove references to the GBR from a report on the threat of climate change to World Heritage sites. It had argued that the inclusion of the reef in the report could 'cause confusion' and harm tourism.

The news came less than a year after the government successfully lobbied UNESCO to leave the reef off its list of endangered World Heritage sites (an exercise that was reported to have cost the government AU$400million). UNESCO's decision not to place the reef on the list was largely down to the Reef 2050 Plan, a 'framework for protecting the GBR until 2050'.

How long it can keep it off the list is uncertain. 'Australia has to report back to UNESCO about the implementation and adequate funding of the Reef 2050 Plan,' says Terry Hughes. 'The Queensland minister for the environment and the GBR put out a costing for the 2050 Plan. The number he came up with was AU$8.2billion over ten years. By my calculation, only eight per cent of that money has been made available.'

There's also the matter of the widespread damage caused by the recent bleaching event. 'In the southern two thirds of the reef, we've lost about half the coral cover in the past 30 or 40 years,' says Hughes. 'The northern third, before the bleaching, was considered to be in pristine condition and the federal government argued against UNESCO putting the GBR on the endangered list because of this. That argument is no longer tenable because in three months we lost as much coral in the northern third as we've already lost in the southern two thirds over the past 30 years.'

GBR DATA

The conditions that caused the bleaching are 175 times more likely as a result of anthropogenic carbon emissions.

Since 1982, the average proportion of the GBR exposed to temperatures where bleaching is likely to occur has increased from 11 per cent per year to 27 per cent.

By the mid-2030s, the conditions that caused this year's mass bleaching will be the average.

Within 10 years, on average, an event similar to this year's bleaching will occur every second year.

Average global sea surface temperatures have increased by 1[degrees]C in the past years.

A 2[degrees]C rise in global surface temperatures will result in the loss of per cent of the world's coral. A 1.5[degrees]C rise will cause the loss of 90 per cent. A 1.2[degrees]C rise means that half of the world's coral should survive.

Compared to before the turn of the 20th century, three times as much sediment and twice as much fertiliser is flowing onto the reef, as well as 17,000kg of herbicide.

Caption: 'Bleaching isn't just about hot water, it's also the amount of UV radiation hitting the corals'.

Caption: 'The future for coral reefs doesn't look promising, but it's a diverse system that can recover if it's given the time'
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Title Annotation:DOSSIER: Great Barrier Reef
Comment:To hot to handle.(DOSSIER: Great Barrier Reef)
Author:Torr, Geordie
Publication:Geographical
Date:Jan 1, 2017
Words:5097
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