Hard branching coral on unconsolidated substrate in very deep

Long description

Subtidal mesophotic coral ecosystems (MCEs, i.e. low light coral ecosystems) dominated by branching hard coral growing on unconsolidated or intermediate substrates in very deep water (below 30 metres depth). Hard corals if present are more likely to occur on boulders and gravel (e.g. coral rubble), as based on limited inventory of MCEs to date, macrobiota other than macroalgae is largely absent on mud and sand[2].

Mesophotic branching hard corals (genera Acropora and Isopora) below 40m depth are morphologically different from those in shallow water, having light and fragile skeletons, increased spacing between branches and corallites, forming laterally flattened branches. These changes may enhance their ability to capture light and cope with sediment that rains down from above. Some species are found in shallow, deep and very deep water and some are specialist mesophotic species only found in very deep water. They can form large monospecific stands and colonies, comparable to similar stands of branching coral in shallow waters[3], however this has been rarely observed (based on limited surveys).

Mesophotic hard corals are also more likely to be heterotrophs, i.e. feeding on other biota rather than through photosynthesis[5].

Special values

Mesophotic ecosystems are potentially refugia from disturbance for coral species also found on shallow water ecosystems and include species tolerant to low light levels. Forty-five per cent of shallow reef coral species also occur in the mesophotic[4]. These ecosystems can provide protection from coral bleaching, however the mechanism for reduced bleaching at depth is complicated and appears to be related to Water temperature and/or light levels. In general, MCEs are also a refuge from the physical damage associated with storms and cyclones (Energy magnitude), apart from very intense ones.

Coral reefs and communities (including MCEs) are highly valued for their diverse flora and fauna, fish habitat values, as commercial fisheries. The values of Queensland’s coral reefs are internationally recognised in the World Heritage (Great Barrier Reef) and many MCEs are found within its boundaries. The Outstanding Universal Value of the Great Barrier Reef World Heritage area is based on four criteria (vii), (viii), (ix), (x).

Mesophotic branching hard coral ecosystems are diverse and have high species richness, with the potential to preserve evolutionary lineages of Indo-Pacific corals[4].

Diagnostic attributes

Inundation 'Subtidal'

Benthic depth 'Very deep (>30m)'

Structural macrobiota 'Hard coral – branching'

Consolidation 'Unconsolidated', 'Intermediate', 'Unknown'

Qualifiers

Potentially Naturalness qualifiers are relevant. Being able to be shifted by mechanical action means that these ecosystems can be potentially modified by trawling or dredging.

Distribution

A continuous line of submerged reefs extends along much of the shelf edge of the Great Barrier Reef in very deep waters[1]. Branching hard corals are more likely to occur on ‘reefal’ substrates (i.e. consolidated and unconsolidated boulders and gravels) based on limited inventory to date[2]. There is very little data for MCEs south of Townsville due to limited surveys.

Deep water corals are most likely to occur in the lower latitudes. Lower latitude waters tend to have higher light levels than higher latitudes (everything else equal) due to the angle of the light and reduced path length through the water column (P. Muir, pers. comm.). Deep water corals may also occur in shaded microhabitats at higher latitudes, which also indicates the low light tolerance of these corals[4]. The flat tops of the ridges provided the best light for corals deriving their food from photosynthesis (phototrophic) and the steep slopes and sides of the ridges were dominated by corals capable of capturing planktonic prey (heterotrophic)[2].

The following relates to distribution of this ecosystem type within the Central Queensland mapping area:

• In Central Queensland the extent of branching coral on unconsolidated substrate in very deep water is unknown.

Comments

Very deep water depth is associated with low light levels. Very clear water (Water clarity) is required for hard corals to grow in very deep water to enable sufficient light to reach the corals (P. Muir, pers. comm.).

Other attributes include Terrain morphology, Terrain slope and Water temperature[2][3][4], Structural macrobiota, Benthic rugosity[2], Energy source (current/upwelling) and Energy magnitude[2].

Additional Information

What are Mesophotic Coral Ecosystems? - National Oceanic and Atmospheric Administration

Mesophotic.org

Corals of the World

Mapping the life mesophotic - Marine Biodiversity Hub

Exploration of relict faunas at Osprey and Shark Reefs - Deep Down Under Expedition

Precious coral and rock sponge gardens on the deep aphotic fore-reef of Osprey Reef (Coral Sea, Australia) - Deep Down Under Expedition

References

1. ^ Bridge, T & Guinotte, J (2013), Mesophotic coral reef ecosystems in the Great Barrier Reef World Heritage area: their potential distribution and possible role as refugia from disturbance. [online] Available at: http://hdl.handle.net/10462/pdf/1382 [Accessed 21 March 2019].
2. ^ a b c d e f Bridge, TCL, Done, TJ, Beaman, RJ, Friedman, A, Williams, SB, Pizarro, O & Webster, JM (2011), 'Topography, substratum and benthic macrofaunal relationships on a tropical mesophotic shelf margin, central Great Barrier Reef, Australia', Coral Reefs, vol. 30, no. 1, pp. 143-153.
3. ^ a b Muir, P, Wallace, C, Bridge, TCL & Bongaerts, P (25 February 2015), 'Diverse Staghorn Coral Fauna on the Mesophotic Reefs of North-East Australia', PLOS ONE. [online], vol. 10, no. 2, p. e0117933, ed. S C A Ferse. Available at: http://dx.plos.org/10.1371/journal.pone.0117933 [Accessed 19 March 2019].
4. ^ a b c d Muir, PR, Wallace, CC, Pichon, M & Bongaerts, P (19 December 2018), 'High species richness and lineage diversity of reef corals in the mesophotic zone', Proceedings of the Royal Society B: Biological Sciences. [online], vol. 285, no. 1893, p. 20181987. Available at: http://www.royalsocietypublishing.org/doi/10.1098/rspb.2018.1987 [Accessed 19 March 2019].
5. ^ Turner, JA, Babcock, RC, Hovey, R & Kendrick, GA (26 May 2017), 'Deep thinking: a systematic review of mesophotic coral ecosystems', ICES Journal of Marine Science. [online] Available at: https://academic.oup.com/icesjms/article-lookup/doi/10.1093/icesjms/fsx085 [Accessed 22 March 2019].

Last updated: 12 July 2019