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(Credit: AIMS) |
The Great Barrier Reef is the largest coral reef system in the world. In fact, it is so large that astronauts can see it from outer space. But this World Heritage Site is facing a deadly enemy right here on Earth.
Corals are actually made of millions of tiny organisms called polyps. These shelter single-celled algae that capture energy from sunlight, becoming the basis of this ecosystem. The polyps secrete the limestone that forms the complex skeleton that becomes the coral reefs. This structure supports marine creatures ranging from microorganisms to sharks.
Coral reefs, while being the richest ecosystems in the world, are very fragile. They have to contend with several dangers. Between the effects of polluted runoff from land and damage from fishing activity, thousands of reefs have been dying.
However, the biggest and newest challenge facing the reefs is climate change. The rising temperatures of the ocean may be a strong proponent for the spread of disease among the reefs. The environmental changes can also destroy the very fabrics of this intricate ecosystem.
The beautiful colors that characterize corals are a result of the algae that exist in a symbiotic relationship with the corals. The corals themselves are actually white in color. Amidst the blue tranquility and cascade of vivid colors of the coral reefs, a disease called white syndrome is spreading in the Great Barrier Reef. The disease kills the polyps, leaving only the white color of the skeletons in its wake.
"In most cases, a recreational diver would hardly notice the impact of white syndrome on the Great Barrier Reef," said Cathie Page, a diver and ecologist at the Australian Institute of Marine Science (AIMS) Long-Term Monitoring Team. "But if you looked closely, you would see large tabulate corals partly dead and with bright white skeleton exposed by the disease amongst the live corals."
According to Page, white syndrome is killing off whole colonies of corals. So far, however, its effect on the entire reef is not very clear.
Unknown Epidemic
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| Casthie Page surveys coral reefs with white syndrome (Credit: AIMS) |
"The disease usually starts at the coral base and spread revealing bright white coral skeleton as it progresses killing the coral tissue," explained Page.
Researchers from the University of North Carolina at Chapel Hill launched a study to determine the factors that cause the spread of white syndrome. Dr. John Bruno, lead investigator in the study and assistant professor of marine ecology, was surprised by the results they found. Very small changes in the temperature caused a widespread infection of white syndrome.
The idea that temperature could be a cause of coral disease outbreaks is not new. According to Bruno, it has been around for at least a decade. "We have long suspected that climate change has played a role in causing the increase in the variety and severity of diseases we have observed in the ocean over the last 30 years," he said. "We just lacked the data to really test this idea, especially across such a large spatial scale."
The data needed has, however, become available. Bruno used records collected from satellite measurements to detect variations in the temperature of the water. The satellite detects and collects weekly sea surface temperature anomalies (WSSTAs). These are instances when the temperature is at least 1°C higher than normal. "These changes are naturally occurring but our very short-term temperature data (about 25 years or so) indicate that they are becoming more frequent and severe," explained Bruno.
Along with this data, the research team also collected information on the density of the reefs where the disease has been found to spread. This data was provided by the AIMS Long-Term Monitoring Program. The data covers reefs spanning over 1,500 km of the Great Barrier Reef and six years of monitoring.
Climate Change Shadow
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| A coral colony with white syndrome from the Great Barrier Reef (Credit: AIMS) |
Bruno discovered that higher temperatures has a profound effect on the spread of white syndrome. It was especially prevalent, however, in the denser, healthier coral populations. A healthy coral reef is one where there is a high percentage of living corals occupying the ocean floor. "If 50 percent were occupied, that would be a really healthy reef. If only 5 – 15 percent were occupied, that would be an unhealthy reef," explained Bruno.
"The temperature alleviation that caused the outbreaks was really modest, that was very surprising, just 1 or 2°C caused the outbreaks," said Bruno. He is worried that the temperature of the oceans might increase due to global warming, making the spread of diseases even faster. According to the Intergovernmental Panel on Climate Change (IPCC), some warming over the next century is inevitable. This is true even if all carbon emissions were to completely stop today.
White syndrome remains a mystery baffling many scientists. "It is a coral disease that is not completely understood," said Dr. Richard Aronson, a senior marine scientist at the Dauphin Island Sea Lab in Alabama and president of the International Society for Reef Studies. "The pathogen is probably bacterial but it has not yet been isolated."
Bruno thinks that the reason the disease was more widespread in healthy coral populations is likely due to their close proximity to one another. This facilitates the transfer of disease between them. "Another likely explanation is that when the corals are close to one another, they fight intensely for space at night. They digest one another's tissue with their tentacles. This causes wounds that could allow infection by bacterial or viral pathogens."
Acidic Threat
| Click here to listen to an interview with Bruno about the coral reefs and climate change. |
While white syndrome risks leaving a barren wasteland where coral reefs once thrived, it is not the only danger climate change poses to the bustling ecosystem.
"Ocean acidification will be a problem because as the amount of carbon dioxide increases in the atmosphere, more of it dissolves in the ocean." Aronson pointed out. "The excess dissolved CO2 in the ocean makes it more acidic."
These acidic conditions inhibit the deposition of limestone that makes up the skeletons of corals. Aronson said this could have a negative impact on the continued growth of reefs as geological structures.
He explained the problem the corals will face through a simple household example. "If you have a calcium carbonate (limestone) crust on your water tap from hard water, you use vinegar (an acid) to dissolve it away. With corals, more acidic conditions will make it harder for them to deposit their skeletons."
While acidification is not a problem right now, Aronson thinks it will likely become a critical issue for coral reefs within the next 50 years.
"We're really concerned about ocean acidification," Bruno said. "That is going to be a big problem over the next century. Coral growth may slow down and we may completely lose those creatures that create the infrastructure on which all those fish live. That is the effect of climate change," said Bruno sadly.
Saving an Underwater Paradise
Addressing climate change is high on the agenda to save the coral reefs. Aronson pointed out that curbing climate change "would go a long way toward controlling white syndrome as well as a host of other environmental problems." However, while politicians barter with climate change for their political careers, the coral reefs may not have enough time.
Bruno, on the other hand, thinks that a good start to protect coral reefs would be to try to reduce some of the other stressors acting on them. "There are some things we can do locally and more immediately," he explained. "We certainly can manage fisheries better, we can control our land-use practices to reduce sediment and nutrient pollution. Those are things that could be more easily implemented locally."
Apart from the environmental value of the coral reefs, they have strong socioeconomic effects. For many coastal cities, especially in the developing world, most people depend on the coral reefs directly for survival. They are among the richest fisheries in the world. "If you don't have the reef, you won't have the fish," pointed out Bruno.
The reefs also attract millions of tourists every year, drawn to the diverse ecosystem and spectacular colors. The Great Barrier Reef alone generates over US$1 billion each year from tourism.
Coral reefs also protect shorelines against storms. They act as a buffer, breaking the severity of the event. Losing the reef structure could cause much more devastation from the different types of storms.
The reefs are disappearing at an alarming rate, however. According to Bruno, nearly half of the world's reef-building corals have been lost already. They are disappearing at a rate of about one percent every year, which is about twice the rate of rainforest loss. This is very worrying, since less than one percent of the ocean is covered by reefs.
Not Lost
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| A reefscape from the Marshall Islands (Credit: M. Sussman) |
But Bruno thinks all is not lost. "A lot of reefs in the Great Barrier Reef were really devastated by predator outbreaks in the 1970s and 1980s and they've really bounced back," he said hopefully.
Page, ecologist from AIMS, also believes there are some positive signs. "All plants and animals experience disease, so it is not surprising that corals too also can become diseased."
"While it is sad to see high levels of disease, these reefs [the Great Barrier Reef] still have lots of corals and are beautiful places to visit."
"You know they still have the ability to bounce back," said Bruno. "The trick is just not making the stress so frequent that they never have the chance to recover."
But this will require some commitment from humans towards the reefs, which are sometimes called "the rainforests of the ocean." Even from a sheer practical business viewpoint, the loss of the coral reefs would be an economic disaster to millions of people.
"It's like an ATM that is in the ocean," explained Bruno. "You just press a button and you get cash from it. We don't have to do anything really. We just take money out of it — as long as we can keep it working."
Sources:
Bruno, John et al. Thermal Stress and Coral Cover as Drivers of Coral Disease Outbreaks. 2007. Accessed 21 May 2007.
Bruno, John.The Bruno Lab. Accessed 21 May 2007.
Intergovernmental Panel on Climate Change. Accessed 21 May 2007.
Interviews conducted by the writer.
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