Tuesday July 24, 2012
Research reveals future of world's coral reefs bleak
By TAN CHENG LI
Ocean acidification is another addition to the slew of problems battering coral reefs – pollution from the land, over-fishing, habitat destruction, sea level rise, warmer waters, destructive storms and coral bleaching.
THE difference in the two coral reefs could not have been more startling for marine scientist Dr Katharina Fabricius. At one site, she saw a resplendent reef with different types of corals vying for space in an interlocking mass of wondrous colours.
At another spot, the reef appeared to have bleached out. There were no colours, the corals were not as diverse, and dominating the reef were seagrasses and algae.
These reefs, located off D’Entrecastraux Islands in Milne Bay, Papua New Guinea, give an inkling of what is to come for coral reefs in a warmer world.
Fabricius, a researcher at the Australian Institute of Marine Science, had studied these sites in eastern PNG to see how ocean acidification is affecting marine life.
The oceans absorb up to a third of all the carbon dioxide spewed out by man, and that is changing sea water chemistry. Seas are turning acidic, and scientists foresee that triggering massive transformations in coral reef structures and communities.
In Milne Bay, there are areas where carbon dioxide naturally bubbles out from the seabed. This creates areas where the ph of the water is lower than the ambient condition, and offers Fabricius the perfect field setting for her research.
Presenting her findings at the recent International Coral Reef Symposium in Cairns, Australia, she shares that in areas where the sea water is more acidic, there is reduced coral diversity, especially in the types of corals with branches, lattices and plates, which are structurally complex reef builders.
There are also fewer soft corals, sea sponges and juvenile corals, but more algae and seagrass growths. At one site with the most intense CO2 vents and where the ph dropped to 7.7, practically nothing grew on the reef.
Fabricius says while some groups of organisms can live at high CO2 levels, at the same time, it is of great concern to see that coral reefs – home to many marine species – will lose their structural complexity.
“Our empirical data from this unique field setting confirm model predictions that ocean acidification, together with temperature stress, will probably lead to severely reduced diversity, structural complexity and resilience of Indo-Pacific coral reefs within this century.”
The work of another scientist, Shihori Inoue of University of Tokyo in Japan, also shows an ecosystem shift from a stony to soft coral population in the volcanically acidified water of Iwo-Tori-Shima Island off Okinawa.
He found stony corals only in areas with little or no influence of acidification, whereas areas with high CO2 was dominated by the soft coral Sarcophyton elegans. In areas with the highest CO2 levels, neither stony nor soft corals survived.
Many similar findings, showing how complex changes have already begun that can fundamentally change what reefs look like in the future, were shared at the symposium from July 9 to 13.
Many of the over 2,000 scientists attending the conference – held every four years to discuss advances in coral reef science – said that coral reefs, as we know them today, will disappear by the end of the century as the oceans turn more acidic due to global warming.
The scientists collectively warned of the dire state which the world’s coral reefs are in: a quarter of these habitats have been severely degraded by human activities such as land development, pollution and over-fishing.
They predicted that the current trajectory of human development will eliminate 90% of the world’s coral reefs by the end of the century.
There are sceptics who still deny the occurrence of climate change but Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, Australia, says such a debate is non-existent among marine scientists. “We’ve been measuring the impact of climate change on reefs since the 1980s. Many scientists have experiences where the reefs they had studied as PhD students have disappeared before their eyes.”
Jeremy Jackson, senior scientist at Smithsonian Institution in Washington DC, the United States, says “the huge decline in live coral cover is real and has been documented”.
He, too, finds that the reefs he used to study (in the Caribbean) are no longer there. He says in the early 1970s, corals dominated over half of the reefs there. Today, they form only 10% of it. And all is not well even in the world’s best-protected reef ecosystem, Australia’s Great Barrier Reef. Jackson says the marine park has seen its coral cover drop by half in the last 50 years, from 40% to 20%.
The world’s coral reefs are facing a double whammy: already suffering from the impact of human activities such as over-fishing, pollution and habitat destruction, they now have to grapple with the problems triggered by climate change.
“Tropical coral reef waters are already significantly warmer than they were and the rate of warming is accelerating,” says Janice Lough, senior principal research scientist at the Australian Institute of Marine Science. “With or without drastic curtailment of greenhouse gas emissions, we are facing, for the foreseeable future, changes in the physical environment of present-day coral reefs.”
She says over the past century, global temperatures have warmed by 0.7°C and those of the surface tropical oceans, by 0.5°C.
The warmer waters have led to widespread coral bleaching events and outbreaks of coral diseases. Reefs have also been battered by more frequent and intense tropical storms. The increased frequency means reefs have no time to recover.
Current projections indicate that the tropical oceans could be 1°C to 3°C warmer by the end of this century, and the ph drop from the present 8.1 to less than 7.9.
These changes threaten the formation of the reef structure, says Lough. She shares that at the Great Barrief Reef and in Thailand, scientists have found massive corals (the types which grow in mounds) growing slower than they used to.
Different corals respond differently to the changes, adds Lough. “The cold water reefs off Western Australia can keep up with warming so far but it is unlikely that this growth can be sustained given the setbacks in growth following coral bleaching.
Bleaching occurred for the first time in this cold water reef last year.” (In coral bleaching, the colourful algae that live within coral tissues and provide food for the host, die off due to warmer waters, leaving behind white coral skeletons.)
Robert Richmond, research professor at University of Hawaii in the US, says in 1998 which saw massive coral bleaching worldwide, a third of reefs in the Palau archipelago in the Pacific was lost. “Thousands of square kilometres were affected and recovery is not happening as algae have taken over the reef.”
He describes the situation as an “Irish potato famine of reefs”, where there will be massive losses of species and genetic diversity, such as those seen in the 1845-1849 potato crop failure.
Richmond says unhealthy reefs will suffer problems with reproduction and repopulation. “If the decline is not arrested, by 2070, reefs worldwide will be totally not functional as only 10% will be left.”
Increased atmospheric carbon dioxide has raised the acidity in the world’s oceans by 25%, according to Stephen Palumbi, director of Hopkins Marine Station in Stanford University, California. “Acidity is important for calcium formation but that calcium dissolves in high acidity, affecting the corals’ ability to build skeletons. Corals can grow and keep up with sea level rise if they are developing at normal rates but they are not, and they will not be able to handle the increased acidity of oceans if the trend continues.”
The skeleton of hard corals is the backbone of tropical coral reefs – they are what builds the reef. Without these skeletons, reefs just cannot form. A change in the coral community can have wide-ranging impact, says James Cook University professor, Philip L. Munday.
He says without the reef, other marine species will not have a habitat. Fish, for instance, seek shelter in the nooks and crevices of the reef.
Munday’s research found that the growth and reproduction rates of fish drop when water temperature rises. He also discovered that elevated CO2 levels interfere with the sensory activities of fish and cause abnormal behaviours.
For instance, he found small fish moving further out to sea, making them vulnerable to predators. An impaired sense of smell made larval fish attracted to scents they normally avoid, such as those from predators and unfavourable habitats.
Meanwhile, research by Florida International University in the US found that warmer waters can thin the layer of protective mucus around hard corals. This mucus plays a key role in coral immunity, acting as a barrier against UV light, sediment deposition and bacteria.
In a laboratory study, scientist Zoe Pratte found three Caribbean coral species, when exposed to higher temperatures over seven weeks, to have a significant decrease in their mucus thickness.
“The steady increase in sea water temperature associated with global warming may have major implications for coral health and their ability to withstand biotic and abiotic stressors,” she says.
There will be variability in responses among species to temperature change and ocean acidification; some species might survive while others could go extinct.
More importantly, says John M. Pandolfi, director at the centre for marine science at University of Queensland, Australia, coral reefs that are already degraded from human pressures will be much less likely to handle the increase in temperature and ocean acidity.
“There will be winners and losers in climate change and ocean acidification, but reefs will demonstrably change and, for most people’s idea of what reefs are, not for the better,” he says.
In his research tracing the story of the world’s reefs over the past 50 million years, he found evidence that global warming and acidification had played a role in reef destruction and the reefs have bounced back. “Corals are remarkably resilient but while they have withstood hot climates and high CO2 in the past, we have so far been unable to identify any period in Earth’s history when CO2 levels rose as rapidly as today.”
To avert the decline in coral reefs, he points out three things requiring action: aggressively reduce CO2 emissions as slowing the rate of climate change will diminish the impact and maximise the chance of corals to recover; reduce local threats such as pollution, over-fishing and habitat destruction; and expand marine protected areas.
“What we want is to focus less on what is going to happen and when it will happen, and instead, focus on the triage. Studies have shown that corals have a great capacity to bounce back if you take these pressures off them. And this means we still have a window of opportunity to act. But we need to act immediately.”
Jackson, from the Smithsonian, agrees: “Local protection matters. In places where action is taken, where there are no runoffs from land, no over-fishing, corals are in better shape.”
With coral reefs around the world in rapid decline, it is imperative to make every effort to save what is left.
At the conference, the scientists released their Consensus Statement on Climate Change and Coral Reefs, urging governments to ensure the future of coral reefs by reducing emissions of greenhouse gases and improving protection of these rich, underwater ecosystems.
Over 2,800 scientists worldwide have signed the document so far. The Statement calls for action to head off the escalating damage caused by rising sea temperatures, ocean acidification, over-fishing and pollution from the land.
The measures to help coral grow and remain strong include: rebuilding fish stocks to restore key ecosystem functions; reducing runoffs and pollutants from the land; reducing destruction of mangroves, seagrass and coral reef habitats; protecting key ecosystems by establishing marine protected areas; rebuilding populations of large animals such as dugongs and turtles; promoting reef tourism and sustainable fishing rather than destructive industries; and using aquaculture, without increasing pollution and runoff, to reduce pressure on wild fish stocks.
Stanford University’s Palumbi says that while governments must make a stronger commitment to reduce greenhouse gas emissions, it is crucial at the same time to reduce the local threats posed by pollution, over-fishing and habitat destruction as healthy reefs would be more resilient to the effects of climate change and recover faster from bleaching events.
“Local action buys us time to deal with the bigger issue of climate change,” he asserts.
Reefs at risk