Miami - Low-oxygen areas of the ocean known as dead zones threaten hundreds of coral reefs worldwide, fragile ecosystems already struggling because of climate change and pollution, researchers said Monday.Although dead zones are not typically considered top killers of coral reefs, they may be far more common than previously thought, according to a study in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal."Ocean warming and acidification are recognised global threats to reefs and require large-scale solutions, whereas the newly recognised threats to coral reefs caused by dead zones are more localised," said co-author Andrew Altieri, staff scientist at the Smithsonian Tropical Research Institute."Fortunately dead zones can be reduced by controlling sewage and agricultural runoff into the ocean."Researchers became interested in the role of dead zones after seeing a widespread coral reef die-off on the Caribbean coast of Panama in 2010.The reefs in Almirante Bay, Bocas del Toro Province, turned white and died. Thick mats of bacterial slime covered the reefs, and dead crabs, sea urchins and sponges coated the ocean floor. In deeper water, the corals all died, but there was a clear line above which the reefs seemed healthy.So scientists measured the water quality and found extremely low oxygen levels in deeper waters, signalling a dead zone.High oxygen levels were measured in shallow waters where corals were still healthy, the report said.Researchers also found 20 other instances when dead zones were believed to play a role in mass coral die-offs worldwide."Hypoxia (low oxygen) isn't even mentioned in several of the most important academic reviews of threats to coral reefs and is rarely discussed at scientific meetings," Altieri said."Even worse, many coral-reef monitoring efforts do not include measurement of oxygen levels, making it nearly impossible to identify low oxygen as the cause of mass coral mortality after the fact." Researchers say their findings suggest that dead zones may be far more common than previously thought in the tropics."For every one dead zone in the tropics, there are probably 10 - nine of which have yet to be identified," said Nancy Knowlton, co-author and chair for Marine Science at the Smithsonian's National Museum of Natural History.