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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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While MCEs are viewed as extensions of shallow-water coral
reef ecosystems, there are some notable differences between
them (Table 2.1). It is important to note that the transition from
shallow-water reefs to MCEs does not occur at a specific depth;
rather the depth of transition varies between locations depending
on water clarity (depth of light penetration), temperature,
substrate type and other factors. In general, in tropical and
subtropical areas, coral reefs shallower than approximately 30–
40 m are considered to be shallow-water reefs. For example, in
the Great Barrier Reef, shallow reefs cover an area of 20,680 km
2
and have a mean depth of 14.9 ± 15.4 m (Harris et al. 2013).
MCEs generally occur below a depth of approximately 30–40 m
and may extend to over 150 m in clear waters. There is no specific
lower depth limit of MCEs because this also varies by location.
Shallow reefs may occur adjacent to land, as in the case of
fringing reefs, or they may be located a distance offshore, such
Depth range
Dominant habitat-
building taxa
Light levels
Thermal regime
Hydrodynamic
regime
Shallow-water coral reef ecosystems
• 0 to approx. 30–40 m.
• Lower depth corresponds to a moderate faunal
transition.
• Detectable in satellite images.
• Dominant species are zooxanthellate scleractinian
corals, octocorals, calcareous and foliose
macroalgae and sponges.
• Generally well-lit environments.
• Shallow reefs can become light-limited in turbid
waters (e.g. near estuaries).
• Generally stable thermal regime.
• Shallow, stratified waters with high residence time
may be subject to extreme thermal events causing
coral bleaching.
• Subject to breaking waves and turbulence, except
in sheltered lagoons.
• Wave-induced shear stress and mobilization of
seafloor sediments.
• High residence times within lagoons.
Mesophotic coral ecosystems (MCEs)
• From approx. 30–40 m to deeper than 150 m.
• Lower depth limit varies by location due to
differences in light penetration and other abiotic
factors.
• Not detectable in satellite images.
• Dominant species are plate-like and encrusting
zooxanthellate scleractinian corals, octocorals,
antipatharians, calcareous and foliose macroalgae
and sponges.
• Generally middle- to low-light environments.
• Generally temperatures are cooler and naturally
more variable on MCEs than on shallower reefs,
especially those located on the continental slope,
which are subject to internal waves.
• Deeper water column may protect MCEs from
extreme (warm) thermal events.
• Below the depth affected by breaking waves.
• Seafloor generally unaffected by wave motion.
Powerful storms can directly and indirectly
impact MCEs (resuspend sediment or cause
a debris avalanche), especially in the upper
mesophotic zone (30–50 m).
as in the case of platform reefs, shelf-edge barrier reefs and
atolls. MCEs may be located close to shore in areas with steep
bathymetry, but are also found a distance from land, either
independently or as deep-water extensions of shallow reefs.
Overall, distance from land is not a reliable predictor of reef
occurrence for either shallow coral reefs or MCEs.
The hydrodynamic environment of surface coral reefs is
quite different from that of MCEs. Breaking waves over
surface reefs induce flow and circulation within the reef
(Gourlay and Colleter 2005). Surface reefs may locally
amplify tidal currents such that they are accelerated
through narrow, inter-reef channels, a process which
controls their geomorphic evolution (Hopley 2006). Finally,
shallow lagoon waters may become thermally stratified (e.g.
Andrews et al. 1984). These processes are much reduced or
non-existent on MCEs.
2.4.
Differences between shallow-water and mesophotic coral
ecosystems
Table 2.1.
General differences between shallow-water coral reef ecosystems and MCEs.