Impacts of Cyclone Yasi on nearshore, terrigenous sediment-dominated reefs of the central Great Barrier Reef, Australia

Abstract

Tropical Cyclone (TC) Yasi (Category 5) was a large (~700km across) cyclone that crossed Australia's Queensland coast on the 3rd of February 2011. TC Yasi was one of the region's most powerful recorded cyclones, with winds gusting to 290km/h and wave heights exceeding 7m. Here we describe the impacts of TC Yasi on a number of nearshore, turbid-zone coral reefs, that include several in the immediate vicinity of the cyclone's landfall path (King Reef, Lugger Shoal and Dunk Island), as well as a more distally located reef (Paluma Shoals) ~150km to the south in Halifax Bay. These reefs were the focus of recent (between 2006 and 2009) pre-Yasi studies into their geomorphology, sedimentology and community structure, and here we discuss data from a recent (August 2011) post-Yasi re-assessment. This provided a unique opportunity to identify and describe the impacts of an intense tropical cyclone on nearshore reefs, which are often assumed to be vulnerable to physical disturbance and reworking due to their poorly lithified framework. Observed impacts of TC Yasi were site specific and spatially highly heterogeneous, but appear to have been strongly influenced by the contemporary evolutionary stage and ecological make-up of the individual reefs, with site setting (i.e. exposure to prevailing wave action) apparently more important than proximity to the landfall path. The most significant ecological impacts occurred at King Reef (probably a result of freshwater bleaching) and at Paluma Shoals, where widespread physical destruction of branched Acropora occurred. New coral recruits are, however, common at all sites and colony re-growth clearly evident at King Reef. Only localised geomorphic change was evident, mainly in the form of coral fracturing, rubble deposition, and sediment movement, but again these impacts were highly site specific. The dominant impact at Paluma Shoals was localised storm ridge/shingle sheet deposition, at Lugger Shoal major offshore fine sediment flushing, and at Dunk Island major onshore coarse sand deposition. There was little geomorphic change evident at King Reef. Thus whilst small-scale and taxa specific impacts from Cyclone Yasi are clearly evident, geomorphological changes appear minor and ecological impacts highly variable between sites, and there is no observed evidence for major reef structural change. The study suggests that the vulnerability of reefs to major physical disturbance events can be extremely site specific and determined by interacting factors of location relative to storm path and pre-event geomorphology and ecology.