Coral growth since 1990 in Australia's Great Barrier Reef has fallen to its lowest rate for 400 years, in a troubling sign for the world's oceans, researchers said on Thursday.
This could threaten a variety of marine ecosystems that rely on the reef and signal similar problems for other similar organisms worldwide, Glen De'ath and colleagues at the Australian Institute of Marine Science said.
The Great Barrier Reef is the world's largest coral expanse, and like similar reefs worldwide is threatened by climate change and pollution.
"These organisms are central to the formation and function of ecosystems and food webs, and precipitous changes in the biodiversity and productivity of the world's oceans may be imminent," the researchers wrote in the journal Science.
Coral reefs, delicate undersea structures resembling rocky gardens made by tiny animals called coral polyps, are important nurseries and shelters for fish and other sea life.
They also protect coastlines, provide a critical source of food for millions of people, attract tourists and are potential storehouses of medicines for cancer and other diseases.
De'ath and his team studied 328 large coral colonies from 69 reefs and found the skeletal records indicate that calcification -- or the deposit of calcium carbonate -- by these creatures has declined by 13.3 percent throughout the Barrier Reef since 1990.
The researchers blamed a combination of global warming, ocean acidity level and decreasing carbonate content in seawater for the decline, unprecedented over the past 400 years.
"Verification of the causes of this decline should be made a high priority," the researchers said.
Coral covers about 400,000 square km (154,000 sq miles) of tropical ocean floor, but needs sustained sunlight, warmer waters and high levels of carbonate to flourish.
The biggest is Australia's Great Barrier Reef, a collection of 2,900 reefs along 2,100 km (1,300 miles) of Australia's northeast coast in a marine park the size of Germany.