Researchers in Australia say the growth of coral on the country's iconic Great Barrier Reef has fallen since 1990 to its lowest rate in 400 years.
Scientists at the Australian Institute of Marine Science say their research is a troubling sign for the world's oceans.
They have found that the Great Barrier Reef, the world's largest expanse of coral, has been growing more slowly over the past two decades than at any time over four centuries.
Researchers say that could threaten a variety of marine ecosystems that rely on the reef. It also may signal similar problems for other environments around the world.
Glen De'ath from the Australian Institute of Marine Science says the reef is threatened by pollution and the effects of climate change.
"Certainly they are under severe stress and some reefs will eventually be lost," he said. "We believe the decline is due to two factors. One is the increasing ocean temperatures that are well documented and this stresses the corals and bleaches them. The other thing, though, is increasing ocean acidification."
The Australian team surveyed 328 large coral colonies and found their skeletal records showed that the deposit of calcium carbonate by these creatures has declined by more than 13 percent throughout the Barrier Reef since 1990.
Coral reefs are sensitive undersea structures, made by tiny animals called coral polyps. Reefs are important breeding grounds and shelters for fish and other sea life.
Theses delicate ecosystems also protect coastlines, are critical sources of food for millions of people, attract tourists and may house plant and animals that can be used to treat disease.
The biggest system is Australia's Great Barrier Reef, a collection of 2,900 reefs stretching along the northeast coast in a marine park the size of Germany.