Mesophotic Coral Ecosystems

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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 21
The Great Barrier Reef (GBR) Marine Park contains over 2,900
individual shallow reefs and covers an area of 344,000 km2, of
which approximately 7 per cent (20,679 km2) is occupied by
shallow-water coral reefs, mapped using aerial photography
and satellite imagery (GBR Marine Park Authority, http://
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
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).
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
Figure 1. Great Barrier Reef.
Figure 2. Bathymetry of the GBR outer-shelf at Hydrographers Passage, showing submerged shelf-edge reefs (from Bridge et al. 2011a).