Quirk protects reef from climate change

by Sue Emmett

PERTH: A quirk of nature may protect corals of Ningaloo Reef, off Western Australia, from excessive bleaching as climate change increases sea temperatures this century.

Researchers at the University of Western Australia (UWA) have found that winds blowing parallel to the reef coastline, during summer, produce an upwelling of cold water from the ocean depths to the surface. This colder water acts as an 'insulator' between the warm water of the tropical Leeuwin Current and Ningaloo Reef.

Fresh flush

The regular flushing of colder water across the entire reef system produces an immediate reduction in water temperature of between one and three degrees centigrade, ensuring the reef is kept cooler and protected from bleaching, the researchers said.

"Water flushes quickly into and out of the reef on the tide every five to eight hours with wind and waves moving comparatively fast due to the significant lagoon setup," said Charitha Pattiaratchi an environmental scientist and lead researcher behind the finding.

"Most coral reef systems around the world [already] suffer from warm water bleaching, but it has never been recorded on Ningaloo Reef coral which is strongly influenced by the Leeuwin Current," he said.

Pattiaratchi, from the UWA's School of Environmental Systems Engineering, has been studying how the currents are generated across Ningaloo Reef, how the system is flushed out and what processes are contributing to that system. He told marine scientists at a recent Western Australian Marine Science Institution symposium that the lack of coral bleaching had surprised researchers, who expected to see some damage.

Southerly winds

"We examined wind records over the past six or seven years and saw that the synoptic southern high pressure system consistently produced strong south westerly winds that blow parallel to the coastline in the Ningaloo region," said Pattiaratchi.

In 2006, the researchers found that the wind weakened over a few days and warm water was entrained onto the reef system. However, once the wind picked up strength again, it was only a few days before the wave height also increased and water temperatures dropped.

"For coral bleaching to occur it must be exposed to warm water for at least two to four weeks," he said. "Fortunately the wind system is such that the southerly winds weaken for only short periods, perhaps for only one or two weeks, so we will only see coral bleaching at Ningaloo Reef if there is a change in the dominant wind directions."