Mesophotic Coral Ecosystems

The deeper lagoon in the central GBR allows greater MCE development on the mid-shelf. The lower slopes of some reefs extend todepths of at least 50m(Chalker andDunlap1983), and are occupied by scleractinian or hard corals. Submerged banks and shoals are also abundant throughout the GBR (Pitcher et al. 2007) covering an area of about 25,600 km 2 (Harris et al. 2013). Three types of banks having a vertical relief exceeding 15 m were recognized: Type 1 (n = 1,145), with a mean depth of 27 m, have some portion of their surface covered by shallow coral reefs (and are thus co-located with shallow reefs); Type 2 (n = 251), with a mean depth of 27 m, are located landward of the shelf-edge barrier reef on the middle- to outer-shelf, with no shallow reefs superimposed; and Type 3 (n = 150), with a mean depth of 59 m, are located on the outer shelf, commonly seaward of the outer-shelf barrier reef (Harris et al. 2013). The shelf position of the different bank types is an important determinant of their ecological composition (Harris et al. 2013). Shallower shoals are dominated by hard corals, while deeper shoals are often colonized by gorgonians or calcareous algal species such as Halimeda (Hopley et al. 2007, Pitcher et al. 2007, Roberts et al. 2015). Interest in the biodiversity associated with MCEs in the GBR Marine Park has increased in recent years, although the majority of this research has focused on hard corals (Bridge and Guinotte 2012, Muir et al. 2015). Broad-scale patterns in community composition have been investigated primarily using an autonomous underwater vehicle (Williams et al. 2010). Several expeditions from 2011 to 2013 conducted extensive sampling of hard corals on lower reef slopes in the north and central GBR,

with most sampling occurring in the upper mesophotic (30– 40 m), although some specimens were collected from deeper than 100 m (Englebert et al. 2014). MCEs clearly support a considerable diversity of hard corals, including common shallow- water species such as Acropora (Muir et al. 2015). Considerable interest surrounds the question of whether MCEs are capable of providing refuges for shallow-water coral reef biodiversity. Quantitative, long-term data are currently unavailable for MCEs on the GBR, and understanding their potential vulnerability to disturbances is difficult. MCEs are well represented in no-take areas, aided by the robust and precautionary management approach taken in the 2003 rezoning process (Bridge et al. 2015), but severe tropical cyclones are currently the leading cause of coral decline on the GBR. Very severe storms, such as Tropical Cyclone Yasi in 2011, caused damage to depths of at least 70 m at Myrmidon Reef (Bongaerts et al. 2013a), although in general MCEs are less impacted by storms than shallower reefs (Roberts et al. 2015). There have been no observations of warm-water bleaching of MCEs in the GBR to date, although observations are limited. Sediment accumulation, due to the lack of wave energy in deeper waters, appears to be a significant factor limiting the growth of corals in mesophotic depths. Controlling sediment loads is therefore likely to be important for MCEs, particularly on submerged banks closer to shore. Lack of knowledge of the spatial location and extent of submerged banks may increase their incidental exposure to threats such as dumping of dredge spoil and ship anchoring (Kininmonth et al. 2014).

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Figure 3. Examples of MCEs on the Great Barrier Reef: (a) hard-coral dominated community at Mantis reef (photo Ed Roberts), (b) soft-coral dominated assemblage at Hydrographers Passage, (c and d) heterotrophic octocoral-dominated assemblages at Hydrographers Passage (photos Australian Centre for Field Robotics at the Unviersity of Sydney, figure from Bridge et al. 2012a).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 22

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