Researchers said that the low-oxygen areas of the ocean, known as dead zones, threaten hundreds of coral reefs around the world, fragile ecosystems already struggling with climate change and pollution.
According to a National Academy of Sciences study, although dead zones are not usually considered the main killers of coral reefs, they may be much more common than what experts thought.
Warming oceans and acidification are global threats to reefs and require large-scale solutions, while newly recognized threats to coral reefs caused by dead zones are more localized,
said co-author Andrew Altieri, a member of the Smithsonian Tropical Research Institute.
Fortunately, dead zones can be reduced by controlling sewage and agricultural effluents in the ocean,
he added.
The researchers were interested in the impact of dead zones, after seeing coral reef mortality on the Caribbean coast of Panama in 2010.
The reefs of the Admiral Bay, in the province of Bocas del Toro, turned white and died. A thick layer of bacterial slime covered the reefs, while dead crabs, sea urchins and sponges covered the seabed.
In deeper waters, all corals died, but there was a clear line above in which the reefs were healthy.
Scientists measured water quality and found extremely low levels of oxygen in deeper water, which was a dead-zone sign.
High oxygen levels were measured in shallow waters where corals remained healthy, as stated in the report.
The researchers also found another 20 cases in which dead zones were believed to play a significant role in massive coral death worldwide.