United States government scientists have found a dramatic impact from the continuing decline of coral reefs: The sea floor around them is eroding and sinking, deepening coastal waters and exposing nearby communities to damaging waves that reefs used to weaken.

The new study, conducted by researchers with the US Geological Survey, examined reefs in Hawaii, the Florida Keys, and the US Virgin Islands, finding sea floor drops in all three locations. Near Maui, where the largest changes were observed, the researchers found that the sea floor had lost so much sand that, by volume, it would be the equivalent of 81 Empire State Buildings.

"We knew that coral reefs were degrading, but we didn't really know how much until we did this study," said USGS oceanographer Kimberly Yates, the lead study author. "We didn't really realise until now that they're degrading enough that it's actually affecting the rest of the sea floor as well."

The work was published yesterday in the journal Biogeosciences.

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Coral reefs naturally generate sand as hard coral skeletons die, and their calcium carbonate bodies become the next layer of the sea floor.

Meanwhile, the living tops of coral columns grow taller and taller, which allows them to keep pace in eras of rising seas.

But as corals are subjected to more and more assaults from a combination of global climate change, local pollution, and direct human-caused damage, this natural dynamic appears to have been undermined, and sea floor accretion has swung to erosion.

"When corals stop growing fast enough, and when they stop making those big skeletons, you also lose that supply of sand to the rest of the seafloor, and you lose that supply of sand to the beaches," said Yates.

"Erosion of coral reefs and sea floor is happening much more and much faster than what was previously known or expected, enough so that it's affecting those local sea level rises," said Yates. "Enough so that it increases the risk to the coastlines from coastal hazards, storm waves, every day persistent waves, tsunamis and those kinds of things."

The authors caution that these findings only apply to their study areas. Globally, similar processes may well also be afoot - reefs across the world are generally threatened.