Climate Blamed for Slow Growth of Great Barrier Reef Corals

"Evidence is strong that the decline has been caused by a combination of rising sea surface temperatures and ocean acidification," say the scientists. Both phenomena are effects of a warming climate.

As oceans absorb more of the greenhouse gas carbon dioxide, they become more acidic, affecting the rate at which corals absorb calcium from seawater to build their hard skeletons.

The study shows that the biggest and most robust Porites corals on Australia's Great Barrier Reef have slowed their growth by more than 14 percent since the "tipping point" year of 1990.

Written by researchers from the Australian Institute of Marine Science, AIMS, in Townsville, the study is featured in the January 2 in the journal "Science," a publication of the American Association for the Advancement of Science.

Giant Porites coral on the Great Barrier Reef (Photo by Eric Matson courtesy AIMS)

"It is cause for extreme concern that such changes are already evident, with the relatively modest climate changes observed to date, in the world's best protected and managed coral reef ecosystem," said co-author Dr. Janice Lough.

Dr. Lough and co-authors Dr. Glenn De'ath and Dr. Katharina Fabricius say theirs is the most comprehensive study to date on calcification rates of Great Barrier Reef corals.

"The data suggest that this severe and sudden decline in calcification is unprecedented in at least 400 years," said Dr. De'ath, the study's principal author.

"The causes of this sharp decline remain unknown, but our study suggests that the combination of increasing temperature stress and ocean acidification may be diminishing the ability of Great Barrier Reef corals to deposit calcium carbonate," he said.

This research adds to the growing weight of evidence that global climate change is already affecting the reef system. Protected as a national park, the reef extends for 2,600 kilometers (1,600 miles) along the coast of Queensland in northeast Australia.

Russell Reichelt, who chairs the Great Barrier Reef Marine Park Authority, said he "recognizes the seriousness" of this new evidence of a widespread slowing of coral growth rates throughout the reef system.

"The Great Barrier Reef's ability to survive the global phenomenon of climate change is largely up to us," he said Wednesday. "It is imperative we act now to minimize all stresses to the reef and in this way help build its resilience to the effects of global warming and changing ocean acidity."

"Coral reefs can bounce back from major disturbance if the natural ecosystem is operating normally," said Reichelt. "Human-induced climate change is a global issue which must, ultimately, be solved at a global level."

An aerial view of the Great Barrier Reef (Photo by Pietro Borensteino)

"We can help the reef by ensuring it is not exposed to local pressures that are readily avoidable," he said, citing poor water quality as the most significant local issue.

More acidic oceans will affect many sea creatures, not just coral, explain the AIMS scientists. Calcifying organisms that are central to the function of marine ecosystems and food webs will be affected, and "precipitous changes in the biodiversity and productivity of the world's oceans may be imminent," they warn.

Calcification is how much skeleton the coral puts down each year. Reef corals create their hard skeletons from materials dissolved in seawater. When large amounts of atmospheric carbon dioxide enter seawater, the resulting chemical changes effectively reduce the ability of marine organisms to form skeletons.

The findings reported in the study are based on rigorous statistical analyses of annual growth bands from 328 Porites corals from 69 reefs across the length and breadth of the Great Barrier Reef, and extending 400 years back in time. Porites are the major reef-building coral species.

The data are from the Coral Core Archive at the Australian Institute of Marine Science, the most extensive such collection in the world.

Up to the tipping point in 1990, there were modest fluctuations in calcification, with an annual decline rate recorded that year of 0.3 percent.

However, by 2005, growth was declining by 1.5 percent per year. On current trends, the corals would stop growing altogether by 2050.

"Coral skeletons form the backbone of reef ecosystems. Their complexity provides the habitat for the tens of thousands of plant and animal species associated with the reef," said Dr. Fabricius. "Skeleton formation also offsets natural erosion and breakage."

"Previous laboratory experiments and models have predicted that calcification will decline in response to acidification, but here we have shown for the first time that corals are already affected in their natural environment throughout the Great Barrier Reef," she said.

More carbon dioxide in the oceans causes the oceans' alkaline-acid balance, or pH, to shift towards acidic.

Oceanic pH has already dropped by 0.1 because the oceans are absorbing about a third of the extra carbon dioxide that humans have put into the atmosphere.

The oceans' pH level could decrease by 0.4 by the end of this century, the scientsts estimate.

If projections of a 0.4 decline in pH are correct, this would be "well outside the realms of anything organisms have experienced over hundreds of thousands of years," said Dr. Lough.

She says the rate, as well as the magnitude, of changes in ocean chemistry and temperature are of great concern for the future of marine ecosystems.

Carbon dioxide is the most prevalent greenhouse gas emitted by human activities such as burning coal, oil and natural gas to generate electricity, run factories and power motor vehicles.

Copyright Environment News Service (ENS) 2009. All rights reserved.