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Since 1990 the growth of coral in Australia’s Great Barrier Reef has slowed its lowest rate in at least 400 years as a result of warming waters and ocean acidification, report researchers writing in Science. The finding portends a bleak near-term future for the giant reef ecosystem as well as calcifying marine organisms around the world.
Studying 328 colonies of Porites corals from 69 reefs of the Great Barrier Reef in Australia, Glenn De’ath, Janice M. Lough, and Katharina E. Fabricius measured a 13.3 percent drop in calcification since 1990, an unprecedented decline in at least the past 400 years. The researchers write that while the cause of the decline is still unknown, “this study suggests that increasing temperature stress and a declining saturation state of seawater aragonite may be diminishing the ability of Great Barrier Reef corals to deposit calcium carbonate.”
De’ath and colleagues selected Porites coral for its longevity and tendency to form annual density bands that can be used to date the coral. Constructing a record of sea surface temperatures (SSTs) and calcification rates, the researchers found that the positive relationship between SST and calcification rates appears to no longer be holding, possibly as a result of extreme temperature events.
 Great Barrier Reef in Australia |
“The recent increase in heat stress episodes is likely to have contributed to declining coral calcification in the period 1990–2005,” they write. “Our results may suggest that, after a period of a slight increase in extension and prolonged decline in density, a tipping point was reached in
the late 20th century. The nonlinear and delayed responses may reflect synergistic effects of several forms of environmental stress.”
Ocean acidification is increasingly caused by the absorption of carbon dioxide from the atmosphere. Because atmospheric concentrations of CO2 are rising, oceans are becoming more acidic. Acidification is important because it reduces the availability of free carbonate ions in sea water, making more difficult for marine organisms to extract calcium carbonate to build the aragonite and calcite shells and skeletons they need to survive. Some of the most affected creatures are tiny polyps that build coral reefs. As the calcification rates slow, so does the growth of coral reefs, leaving them vulnerable to weathering from wave action and other stress. Other marine life is affected too, including microorganisms that form the basis of the oceanic food chain.
Coral reefs are also affected by elevated sea temperatures which can trigger ‘bleaching’ events. Bleaching causes coral to expel symbiotic zooxanthellae algae living in their tissues — algae that provide corals with nourishment. Corals can recover from short-term bleaching, but prolonged bleaching (over a week) can cause irreversible damage and subsequent death. Since 1979 there have been at least seven mass-bleaching events, each of which has been progressively more frequent and severe. Some researchers believe that the Great Barrier Reef could lose 95 percent of its living coral by 2050 should ocean temperatures increase by the 1.5 degrees Celsius projected by climate scientists.
The decline of corals on the Great Barrier Reef could have significant ecological and economic impacts. The reef provides a habitat for over 1,500 species of fish and 400 species of coral, while buffering coastal areas from storm damage and erosion. Further, each year more than 1.8 million tourists visit the reef, spending more than AU$4 billion on reef-related industries from diving to boat rental to island resort stays. Revenue from tourism dwarfs the commercial and recreational fishing industries which generate AU$360 million.
G. De’ath; J.M. Lough; K.E. Fabricius. Declining Coral Calcification on the Great Barrier Reef. SCIENCE 2 JAN 2008.
Increasingly acidic oceans harm marine life July 5, 2006
Carbon dioxide emissions are altering ocean chemistry and putting sea life at risk according to a new report released today.
The report, “Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers,” summarizes known effects of increased atmospheric carbon dioxide on marine organisms that produce calcium carbonate skeletal structures, such as coral
Coral reefs decimated by 2050, Great Barrier Reef’s coral 95% dead November 17, 2005
Australia’s Great Barrier Reef could lose 95 percent of its living coral by 2050 should ocean temperatures increase by the 1.5 degrees Celsius projected by climate scientists. The startling and controversial prediction, made last year in a report commissioned by the World Worldwide Fund for Nature (WWF) and the Queensland government, is just one of the dire scenarios forecast for reefs in the near future. The degradation and possible disappearance of these ecosystems would have profound socioeconomic ramifications as well as ecological impacts says Ove Hoegh-Guldberg, head of the University of Queensland’s Center for Marine Studies.