Sue Emmett, ScienceNetwork WA 23 Sep 08;
A quirk of nature may protect Ningaloo Reef corals from excessive bleaching as climate change increases sea temperatures this century.
University of Western Australia School of Environmental Systems Engineeering Professor Charitha Pattiaratchi says winds blowing parallel to the Ningaloo Reef coastline in summer produce upwelling of cold water from the ocean depths to the surface.
The regular flushing of this colder water across the entire reef system produces an immediate reduction in water temperature of between one and three degress celsius, ensuring the reef is kept cooler and protected from coral bleaching.
“Most coral reef systems around the world suffer from warm water bleaching, but it has never been recorded on Ningaloo Reef coral which is strongly influenced by the Leeuwin Current,” says Professor Pattiaratchi.
“At Ningaloo, currents across the reef system are correlated to offshore wave height and push water across the reef into the lagoons. This water exits through channels between the reefs back into the open sea.
“The circulation is dominantly wave-driven and light winds produce smaller waves and less flow, so balance is essential.
“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.
“Although this movement is modulated with the tide, the tide is not driving the current, but simply increasing the water depth above the reef so there is more water transported inshore. It is not a direct movement by the current.”
Professor Pattiaratchi, from the School of Environmental Systems Engineeering, 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 a group of more than 200 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 coral damage.
“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,” he said.
According to Prof Pattiaratchi, in 2006 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.”
