Disturbance gradients on inshore and offshore coral reefs caused by a severe tropical cyclone

Tropical storms (cyclones, hurricanes, or typhoons) are the most severe form of mechanical disturbance of coral reefs. In 2005, severe tropical cyclone Ingrid crossed the far northern Great Barrier Reef, a region that had not been affected by a major disturbance for several decades, and where benthic data had been collected before the cyclone crossed. This storm provided a unique opportunity to improve understanding of the extent and type of damage inflicted on inshore and offshore coral reefs along a gradient of wind speeds. Modeled maximum wind speeds ranged from 46 m s21 (equivalent to category 4) near the path to 22 m s21 (category 1) ,70 km to either side of the path. Surveys of 82 sites on 32 reefs along the wind gradient showed that the types and intensity of disturbance were well explained by local maximum wind speed, and by spatial and biotic factors. While offshore reefs had the deepest depth of damage, inshore reefs had the greatest rates of coral breakage and dislodgement. On a severely affected inshore reef, hard coral cover decreased about 800%, taxonomic richness decreased 250%, the density of coral recruits decreased by 30%, while massive coral cover remained unaltered. Maximum winds ,28 m s21 for ,12 h inflicted only minor damage on any reef, but winds .33 m s21 and .40 m s21 caused catastrophic damage on inshore and offshore reefs, respectively. Observations from this cyclone were used to predict potential changes in storm-related coral loss under altered cyclone-intensity scenarios. Severe tropical storms frequently occur at latitudes 10– 30u on both sides of the equator. Known as tropical cyclones (TC) in the Southern Hemisphere, typhoons in the northwest Pacific, and hurricanes in North America, they cause significant perturbations in marine ecosystems including coral reefs. Extreme wave and current forces entrain reef sand, gravel, and rubble, break and dislodge corals, strip off the superficial reef framework, and deposit loosened material onto beaches or cays above sea level, or propel them into deeper subreefal environments (Done 1992; Scoffin and Walton Smith 1993). These impacts, and the resulting redistribution of reef materials, are significant aspects in the geomorphology and evolution of coral reefs. The ecological effects of cyclones on coral reefs have been reviewed by Harmelin-Vivien (1994). A number of studies have documented the extent of direct mortality caused by storms at local or regional scales (Done 1992; Gardner et al. 2005). Other studies have shown that the abundances of fish and other coral-associated organisms that depend on this structurally complex habitat also decline where reef structures are flattened; such indirect mortality may manifest soon after the storm or years to decades later (Woodley et al. 1981; Harmelin-Vivien 1994; Wilson et al. 2006). All studies agree that there is a significant level of variability in the type and intensity of storm effects, and several studies have aimed at identifying the best predictors for storm damage (e.g., Done 1992; Gardner et al. 2005; Puotinen 2007). The ecological effects of storms on coral reefs can have legacies of years to centuries (Connell 1997), so is important to further improve our understanding of the factors that determine differences in storm effects between reef locations and among coral community types. The vulnerability of coral reefs to storm damage is likely related to the robustness and fragility of reefs, which varies according to (1) location, (2) coral community type, and (3) successional stage of coral development. On the Great Barrier Reef (GBR), the main spatial factors determining vulnerability of a particular locale are its position across the continental shelf and its location within a reef. One the one hand, outer-shelf reefs are more exposed to prevailing southeasterly waves than inshore reefs, which are sheltered by outer reefs. On the other hand, the framework of offshore reefs is substantially stronger than that of inshore 1 Corresponding author ([email protected]). 2 Present address: School of Integrative Biology, University of Queensland, Brisbane, QLD 4072, Australia. Acknowledgments We thank Stephen Neale for processing the video tapes, and Peter Otto and Jeff Callaghan at the Bureau of Meteorology for providing preliminary estimates of the path and wind speeds of tropical cyclone Ingrid. We also thank Christopher Shelbourn, Shamimara Begum, and the crew of the RV Cape Ferguson for their field assistance, and an anonymous reviewer for helping to improve the manuscript. The study was funded by the Australian Institute of Marine Science. Limnol. Oceanogr., 53(2), 2008, 690–704 E 2008, by the American Society of Limnology and Oceanography, Inc.