Hard branching coral on consolidated substrate in very deep water

Long description

Subtidal mesophotic coral ecosystems (MCEs, i.e. low light coral ecosystems), dominated by branching hard coral on consolidated substrate in very deep water (below 30 metres depth). Branching hard corals are also known as staghorn corals. These ecosystems typically occur on old shorelines and/or in deeper paleochannels. Surveys of MCEs in the central Great Barrier Reef found that ‘reefal’ substrate was the best predictor of Structural macrobiota. These consolidated substrates consisted of sediment covered limestone or limestone Substrate composition, of ridge Terrain morphology, formed along the Great Barrier Reef shelf edge by shallow water coral communities during lower sea levels[2].

Dominant Structural macrobiota taxa of MCEs included hard corals, octocorallians* and sponges. There is a wide variation in structures and groups of taxa dominating different sites, depending on differences in Benthic depth, Terrain slope, Rugosity, Water clarity and System metabolism i.e. chlorophyll. 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)[3].

Branching hard corals (genera Acropora, Isopora) make up a substantial component of upper mesophotic communities (30 to 60 metres) and include deep water specialists exclusively below 40 metres depth[5]. However drilled cores document changing dominant genera on the shelf edge mesophotic reef terraces over the last 30,000 years, with five distinct reef sequences corresponding to different sea levels as the reef migrated back and forth in response[7].

Branching hard corals in MCEs can form large monospecific stands, e.g. extensive Acropora tenella colonies/stands cover over 200 square metres on the Great Detached Reef, far northern Great Barrier Reef, however this has been rarely observed (based on limited surveys[2][3][1][6][5][7]). These are comparable to some of the largest monospecific stands of branching hard corals in shallow waters. Other taxa include non-branching hard corals (also octocorals and sponges) occupying the upper mesophotic depths to around 60 metres depth and lower mesophotic zones to approximately 130 metres (based on surveys in the far northern Great Barrier Reef).

Like non-branching deep water corals, branching hard deep water corals are most likely to occur in the lower latitudes. These 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.). However deep water corals also occur in shaded microhabitats at higher latitudes, which indicates the low light tolerance of these corals[6].

*Octocorallia is a subclass of the class Anthozoa in the phylum Cnidaria, and include soft corals, gorgonians, sea whips, sea pens, sea fans and octocorals. Like some of the many other anthozoans, octocorallians are sessile polyp-bearing animals with a mobile larval phase. Octocorallians are distinguished by the eight (i.e. octo) tentacles in each polyp. Most octocorallians do not deposit a rigid calcium carbonate exoskeleton, and therefore tend to attach to reefs rather than contribute to reefal frameworks as per the reef building Scleractinian (hard) corals[4].

Special values

Mesophotic ecosystems are potential 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[6]. 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).

Hard coral taxa of MCEs in the northern Great Barrier Reef have high diversity and potentially can preserve evolutionary lineages of Indo-Pacific corals[6].

Diagnostic attributes

Inundation 'Subtidal'

Benthic depth 'Very deep (>30m)'

Structural macrobiota 'Hard coral – branching'

Consolidation 'Consolidated'

Qualifiers

The Naturalness qualifier is relevant to MCEs subject to disturbances such as anchoring and trawling. The Cover qualifier is relevant as percentage cover of branching hard corals is of relevance to coral ecosystem monitoring.

Distribution

A continuous line of submerged reefs extends along much of the shelf edge of the Great Barrier Reef in very deep waters[1]. In the central Great Barrier Reef, consolidated limestone framework from previous shallow reef growth during earlier sea levels provided the best substrate for Structural macrobiota of MCEs[2][3]. 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[6]. 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)[3].

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. Hard coral growth at depth requires very clear water to enable sufficient light to reach the corals (P. Muir, pers. comm.).

Other attributes include Terrain morphology, Slope and Water temperature[2][5][6]. Through bathymetric multi-beam surveys of deep water, likely MCEs were located by analysing terrain variables of Morphology, Slope etc., identifying consolidated substrates to survey for their Structural macrobiota and Benthic rugosity[3]. Energy source (current/upwelling) and Magnitude are also relevant[3].

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

References

1. ^ a b 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 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 c d e f Bridge, TCL, Done, TJ, Friedman, A, Beaman, RJ, Williams, SB, Pizarro, O & Webster, JM (2011a), 'Variability in mesophotic coral reef communities along the great barrier reef, Australia', Marine Ecology Progress Series, vol. 428, pp. 63-75.
4. ^ Fabricius, K (2010), 'Octocorallia', in Encyclopedia of Modern Coral Reefs, pp. Chapter-35.
5. ^ a b c 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].
6. ^ a b c d e f 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].
7. ^ a b Webster, JM, Braga, JC, Humblet, M, Potts, DC, Iryu, Y, Yokoyama, Y, Fujita, K, Bourillot, R, Esat, TM, Fallon, S, Thompson, WG, Thomas, AL, Kan, H, McGregor, HV, Hinestrosa, G, Obrochta, SP & Lougheed, BC (2018), 'Response of the Great Barrier Reef to sea-level and environmental changes over the past 30,000 years', Nature Geoscience. [online], vol. 11, no. 6, pp. 426-432. Available at: http://www.nature.com/articles/s41561-018-0127-3 [Accessed 25 March 2019].

Last updated: 12 July 2019