Background

The reefs of the Great Barrier Reef are very dynamic ecosystems. They are periodically affected by major disturbances such as cyclones and the intense winds generated can create massive waves that break upon coral reefs. These waves, and the debris carried by them, can completely remove all the standing corals and break up the physical structure that forms the reef platform.

The impact of cyclones on coral reefs is often ‘patchy’ (Puotinen et al. 1997). In the southern hemisphere, wind and wave fields are stronger on the southern side of the cyclone’s path and may result in greater damage on reefs situated to the south of a cyclone’s track compared to reefs located to the north. Similarly, some reefs may be sheltered from cyclone-generated waves by other reefs closer to the cyclone’s track. Damage may also vary across a single reef with the more sheltered ‘lee’ side of a reef receiving less wave energy than the more exposed reef front.

The amount of damage caused may also vary according to the characteristics of the reef community including:

• strength of the coral attachment to the substrate
• nature of the substrate (loose rubble vs consolidated limestone)
• amount and type of coral present (fragile branching or plate corals vs large ‘boulder’ corals)
• size of the corals present (medium sized corals are more easily dislodged by waves (Puotinen et al, 1997

Recovery from cyclone events

In the absence of further disturbances or impacts, coral reefs usually recover in subsequent years. In 1986 Cyclone Manu crossed the coast near Cooktown and generated winds of up to 100km/h. Live coral cover was reduced by 25 percent on fringing coral reefs at Cape Tribulation. While recovery was hindered by a coral bleaching event in 1987, reef recovery was rapid. Subsequent surveys showed that recovery was well underway and driven by fast growing Acropora ‘staghorn’ and ‘table’ corals that had survived these events (Chin & Ayling, 2000). However, the rate of recovery is thought to depend on a number of factors including:

• amount of damage caused
• amount and type of coral remaining (fast growing species hasten recovery)
• additional factors such as subsequent storms, coral bleaching, crown-of-thorns starfish outbreaks and water quality
• the variety of topography whereby refuge from cyclone damage is offered by overhangs, ridges and outcrops, thus enabling the surviving corals to quickly re-establish the coral community

Summary

Tropical Cyclone Larry (TC Larry) crossed the Queensland coast on 20 March 2006, close to the town of Innisfail. As a severe Category 5 cyclone, it generated a 50km wide band of ‘very destructive’ winds between Cairns and Tully. To the north ‘destructive’ winds were experienced in a band between Cairns and Cape Tribulation. To the south ‘destructive’ winds were experienced between Tully and the Hinchinbrook/Cardwell region.

To assess the impact of TC Larry, the Research and Monitoring Coordination Unit inspected eight reefs between Townsville and Cairns (between 17º and 19º South). This included surveying six reefs primarily for cyclone impacts, and an additional two reefs as part of the on-going Coral Bleaching Response Plan. These reefs lie perpendicular to TC Larry’s track and represent exposure to a variety of wind strengths.

Reefs were surveyed using the ‘manta tow’ method, and where time and conditions permitted, detailed assessments were conducted by snorkel or SCUBA. While all six ‘target’ reefs were surveyed for cyclone damage, poor weather conditions prevented assessment of all parts of each reef.

The extent and type of impact varied with proximity to the path of the cyclone. The most heavily damaged reefs were Feather Reef, Ellison Reef and Taylor Reef. These reefs are situated between Tully and Innisfail where the most destructive winds occurred. Reefs further to the north and south were less affected. Hedley Reef appeared to be the least affected of the mid-shelf reefs.

Normanby Island reef (part of the Frankland Islands group) was the only inshore reef surveyed and thus comparisons with other inshore reefs in the region are not possible. Anecdotal information from tourism operators involved in the “Eye on the Reef” Programme indicated that reefs offshore from Cairns have suffered only minor damage.

The most common signs of impact included damage to the underlying reef structure, broken and dislodged corals and movement of coral rubble and debris. The majority of the reefs surveyed had experienced previous damage through crown-of-thorns starfish outbreaks. Consequently, coral cover was at low levels before TC Larry affected these reefs and most of the observed impact was damage to the reef structure rather than damage to living corals.

Of the living coral present, coral breakage was most commonly seen on branching and table Acropora corals. Coral breakage ranged between

5 percent and 30 percent of the corals present, with most reefs showing breakage in approximately 20 percent of the corals present. Dislodged corals were mainly tabulate Acropora and Porites bommies. More sheltered back-reef habitats also exhibited coral damage as they had a higher proportion of fragile corals such as branching or tabulate Acropora species.

A rapid assessment of island flora and fauna was also undertaken at Russell Island (south of Cairns and part of the Frankland Islands group). Damage to vegetation and beaches varied in different parts of the island. The north-western beach of Russell Island suffered the greatest erosion while large amounts of coral rubble and debris were deposited on the western beach. Vegetation damage varied depending on the trees present and their location on the island. Most trees suffered some damage such as broken branches and stripped leaves, but most of the vegetation remained intact. However, vegetation on the northern and north-western aspects suffered severe damage with trees broken and completely stripped of all foliage. Camping infrastructure on the islands was damaged although three Marine Parks mooring buoys were still present.

Observations of damage to island vegetation among the Frankland Island group were also recorded while sailing past Round Island, Mabel Island, Normanby Island and Little Fitzroy Island. The most extreme vegetation damage was again located on the northern and north-western aspects of these islands.

Conclusion

Overall, the impacts observed are consistent with those resulting from previous cyclone events. In the absence of further pressures or perturbations, the affected reefs should fully recover. Future surveys of these reefs by the Australian Institute of Marine Science Long-term Monitoring Programme will enable their recovery to be assessed. The Environmental effects of Tropical Cyclone Larry - full report is also available.