Habitats and sessile benthic megafaunal communities in the mesophotic zone of the Great Barrier Reef World Heritage Area, Australia

The Great Barrier Reef (GBR) is the world's largest coral reef ecosystem, extending for over 2300 km along Australia's north-east coast. The Great Barrier Reef World Heritage Area (GBRWHA), one of the world's largest Marine Protected Areas, covers a total area of ~348 000 km², approximately 7% of which is currently classified as coral reef habitat. The vast majority of this habitat occurs in shallow waters <30 m depth. These iconic shallow-water reefs are well known, however a lesser-known but extensive series of submerged reefs also occurs on the shoulder of the continental shelf to depths of ~130 m. These reefs provide a vast potential habitat for mesophotic coral reef ecosystems (MCEs), tropical coral reef communities that occur in the middle to lower photic zone. Although there has been a significant increase in research interest on MCEs in recent years, very few studies have focused on the Indo-west Pacific region, the epicentre of coral reef biodiversity, and the ecology of MCEs in the GBR remains virtually unknown. This thesis provides the first quantitative analysis of the ecology of mesophotic coral reef ecosystems in the GBRWHA.\ud \ud Data were collected on a 3-week expedition on board the RV Southern Surveyor in September-October 2007 at four sites along the GBR outer-shelf: the Ribbon Reefs (approximately 15°S), Noggin Pass (17°S), Viper Reef (19°S) and Hydrographers Passage (20°S). The surveys combined high-resolution multibeam bathymetry with over 57 000 Autonomous Underwater Vehicle (AUV) images, and samples collected by dredging from 50 to 150 m water depth. The specific scientific objectives of the study are to (1) examine the diversity of sessile benthic megafauna (SBM) occurring on MCEs in the GBRWHA and compare them to adjacent shallow-water reefs; (2) identify how SBM communities change along a depth gradient and to identify geophysical variables which explain the observed variation; (3) identify how SBM communities change along the length of the GBR outer-shelf in response to a range of environmental variables; and (4) to develop models to predict the distribution of SBM taxa and communities and use them to estimate the total amount of mesophotic reef habitat in the GBRWHA.\ud \ud In general, the diversity of phototrophic (zooxanthellate) taxa is highest in shallower waters