The Great Barrier Reef was severely buffeted by Severe Tropical Cyclone Hamish, which roared down a substantial part of the reef system, causing widespread though variable damage and in some places reducing coral cover from 70 per cent to 10 per cent.
In a wild 16 hours or so between 8 and 9 March, Hamish made its presence felt on the GBR, leaving a legacy that could take up to 15 years to overcome. Bureau of Meteorology reef weather stations recorded wind speeds of over 200km per hour as the eye of the cyclone passed over about a quarter of the length of the GBR.
AIMS deployed two research teams soon after the cyclone hit to assess the damage caused. Preliminary results of those surveys are being made available publicly today.
While the GBR regularly experiences cyclones, TC Hamish was unusual in that it ran along the coast instead of crossing on to land quickly. This meant that about 25 per cent of the length of the GBR was directly in the path of the destructive force of a Category 5 cyclone.
Hamish moved from the Coral Sea on to the Great Barrier Reef on 8 March 2009 and tracked south parallel to the coast, downgrading from Category 5 to 4 during its passage along the reef. Most cyclones track fairly directly from the Coral Sea to the coast and so only relatively small areas of the GBR are affected.
By running parallel to the coast for about 500km, Hamish damaged much more of the GBR than most recent cyclones.
Destruction caused by TC Hamish. Image: Eric Matson
AIMS surveyed 24 reefs in late March and April. The teams found that many were damaged, but the extent of damage varied greatly between reefs and also within different parts of individual reefs.
"The damage was patchy," Dr Ray Berkelmans said. "The exposed faces of reefs lost most of their coral, but surprisingly delicate corals survived where a curve in the reef gave some shelter or where one reef was protected by a neighbouring reef. We found the greatest damage on reefs not sheltered by other reefs that were close to the eye of the cyclone."
"The type of damage also depended on the distance from the cyclone track and the type of coral community present," he said. "Reefs within 30km of the cyclone eye sustained most damage, with around half of the 500 reefs in this area exposed to destructive waves and suffering significant damage."
Damage ranged from "exfoliation", where the reef matrix was removed along with all that grew on it, leaving bare limestone, to "scouring" that essentially stripped all living tissue from living corals, to coral breakage in which massive coral heads as well as more delicate branching corals broke off.
"The most damaged areas of reef had forests of green slime, spectacular but ephemeral blooms of green algae, suggesting a pulse of nutrients had been released into the ecosystem," Dr Sweatman said. The source of these additional nutrients has now become an interesting research question.
Most damage was sustained from the surface down to eight metres, but on some reefs it extended down to 15m. Loss of coral cover was variable, dropping from 35-70 per cent to 10-15 per cent.
Based on past experience with similar reefs, the researchers estimate that coral should recover in eight to 15 years, if there are no further major disturbances such as cyclones or coral bleaching or declines in water quality which might reduce the resilience of these reefs.
Dr David Wachenfeld from the Great Barrier Reef Marine Park Authority said the summer of extreme weather earlier this year that brought a triple whammy of pressures to the Great Barrier Reef is a stark reminder of the potential impacts of climate change.
"We saw stifling heat, heavy rainfall causing significant flooding, and destructive winds from cyclone Hamish, all compounding to have a hard hitting impact on the Great Barrier Reef."
"The research results from the AIMS surveys continue to remind us of the remarkable nature of this extraordinary ecosystem and the importance of ensuring that it is as healthy as possible to withstand the impacts of more extreme weather events in the future."
"The Great Barrier Reef continues to provide one of the world's most outstanding visitor experiences for tourists and is highly valued and critically important to the industries and communities that depend on it for both their lifestyles and livelihoods," Dr Wachenfeld said.
The results of the AIMS investigation into the effect of TC Hamish on the Great Barrier Reef will be released today (Thursday 20 August) at a media conference at 10am at the Museum of Tropical Queensland.
Ms Sara Trenerry, GBRMPA, 0408 195 198
Ms Wendy Ellery, AIMS, 07 4753 4409, 0418 729 265, firstname.lastname@example.org