Mesophotic Coral Ecosystems

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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 17
MCE habitats may be broadly characterized as either
platforms or slopes (Locker et al. 2010). Low-gradient
platform MCE habitats include outer continental and
insular shelves, relic terraces and isolated banks with
relatively flat tops. Slope habitats include the steep margins
of continental and insular shelves and banks that extend
from the platform break to the adjacent basin. MCEs are
often extensions of shallow coral ecosystems, located
directly below shallow reefs. However, not all MCEs have
a shallow-water counterpart, for example Pulley Ridge and
Gulf of Carpentaria MCEs, described in Chapter 3, are not
adjacent to shallow reefs and are located offshore.
2.3.1. Platform habitats
Platform habitats that dip gently into the mesophotic zone
can include relict ridges, terraces and banks that formed
during periods of lower sea level (Harris and Davies 1989,
Macintyre et al. 1991, Beaman et al. 2008, Harris et al. 2008;
see text box). These features may be the result of erosional
processes (e.g. wave cut platforms), constructional processes
(i.e., relict reefs) or a combination of the two. Importantly,
they are hard substrates that are topographically high or
prominent slope breaks that are conducive to colonization
by MCEs. Examples include extensive areas (> 25,000 km2)
of submerged banks in the Great Barrier Reef (Harris et al.
2013), submerged ridges off the south coast of Barbados,
and relict terraces on many Pacific Islands (Bare et al. 2010).
Often, a series of terraces can be found off a given stretch
of coastline (e.g. Barbados), with the terraces at different
mesophotic depths being colonized by different species and
growth forms of corals (Rooney et al. 2010).
2.3.2. Slope habitats
MCEs in slope habitats are influenced by slope gradient and
geomorphology (Sherman et al. 2010). Optimal slope habitats
for MCEs are stable, rocky protrusions affording access to
light and away from gullies and submarine canyons in which
sediment and debris are transported downslope (Sherman
et al. 2010). In the Caribbean, many islands and banks have
steep outer slopes within the mesophotic zone, and in the
tropical Indian and Pacific Oceans, both barrier and fringing
reefs may have MCEs on their lower slopes.
2.3. Geomorphology of mesophotic coral ecosystems
Figure 2.12. MCEs established under rising sea level.
Holocene reef
growth initiated
Sea level
-50 metres
Reef
growth
stalled
Sea level
-30 metres
Reef
growth
initiated
Shallow
reef
Present
Sea level
MCEs
Reef
growth
continues
Reef
growth
stalled
Sea level
-120 metres
Relict reef limestone Pleistocene
reef
slope
atoll
shelf
Shallow
reef
18000 years before present 12000 years before present 10500 years before present 6500 years to present
Mesophotic coral ecosystems (MCEs) established under rising sea levels
Source: GRID-Arendal
All MCE habitats were established under rising global sea
levels after the last ice age (Figure 2.12). Sea level was 120 m
below its present position at around 18,000 years before
present (BP) when Pleistocene reefs lived on the continental
slope. Sea level rose to 50 m by around 12,000 years BP and
corals colonized relict limestone platforms and other rocky
surfaces on the outer shelf (or on atoll rims), leaving the
Pleistocene reefs stranded below rising sea levels on the slope.
MCEs established after the last ice age
Sea level rose rapidly to 30 m by around 10,500 years BP. Some
reefs were able to keep up with sea level rise but others, for
reasons that are not fully understood, were not (Montaggioni
2005, Harris et al. 2008, Woodroffe and Webster 2014). By
the time sea level reached its present position around 6,500
years BP, only some reefs had kept pace with rising sea levels;
those that had not are sites of many of today’s MCEs (sensu
Macintyre 1972).