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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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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).
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).
(a)
(c)
(b)
(d)