Mesophotic Coral Ecosystems

3.2. The Great Barrier Reef, Australia Thomas C.L. Bridge , Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University & Australian Institute of Marine Science, Australia

The Great Barrier Reef (GBR) Marine Park contains over 2,900 individual shallow reefs and covers an area of 344,000 km 2 , of which approximately 7 per cent (20,679 km 2 ) is occupied by shallow-water coral reefs, mapped using aerial photography and satellite imagery (GBR Marine Park Authority, http:// www.gbrmpa.gov.au/resources-and-publications/spatial- data-information-services). MCEs are common within the GBR Marine Park and occur on the deeper flanks of shallow reefs and on submerged banks, both along the shelf edge and inside the GBR lagoon (Bridge et al. 2012a, Harris et al. 2013). The morphology of the GBR shelf changes significantly with latitude, being narrower and steeper in the north than in the south. These changes affect reef morphology, influencing both the amount and nature of habitats available for MCE development. The northernmost 800 km of the GBR is characterized by a relatively narrow continental shelf with a shallow lagoon (approximately 30 m), and long, narrow ribbon reefs separated by narrow passages occurring along the shelf edge (Figure 1). The seaward slope of the reefs drop steeply into very deep water, leaving limited room for the development of submerged reefs along the shelf edge. However, MCEs inhabited by diverse scleractinian and octocoral assemblages are known to occur along narrow submerged reefs seaward of the Ribbon Reefs at depths of approximately 50 to 70 m (Hopley et al. 2007, Beaman et al. 2008, Bridge et al. 2012b). South of Cairns, the shelf widens and shallow reefs are set back from the shelf edge. The more gently sloping seafloor has resulted in a series of submerged reefs and terraces occurring along the shelf edge at depths of 50 to 130 m (Figure 2). Ecological communities inhabiting these MCEs have been examined at Noggin Pass, Viper Reef and Hydrographers Passage (Bridge et al. 2011a, b). In general, phototrophic taxa including hard and soft corals, phototrophic sponges and macroalgae are the dominant habitat-forming benthos at depths shallower than 65 m (Figure 3). In some regions, inter- reef terraces are occupied by dense fields of the macroalgae

Figure 1. Great Barrier Reef.

Halimeda (Bridge et al. 2011b). Below 65 m, hard substratum is increasingly dominated by heterotrophic filter-feeders, particularly octocorals, with very large benthic foraminifera (particularly Cycloclypeus carpenter ) occurring on soft sediments (Bridge et al. 2011a). Given that submerged shelf-edge reefs appear to be consistent features of the GBR shelf edge over hundreds of kilometres, it is likely that MCEs also occur more or less continuously along the GBR shelf edge to at least the southernmost extent of the Swain Reefs at 23°S (Figure 1).

Figure 2. Bathymetry of the GBR outer-shelf at Hydrographers Passage, showing submerged shelf-edge reefs (from Bridge et al. 2011a).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 21

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