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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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learned that MCEs are not protected from all threats, and that
oceanographic processes are perhaps more important than
depth in terms of buffering corals from the impact of thermal
stress (Neal et al. 2014, Nir et al. 2014).
6.3.1.
Coral bleaching
MCEs are not immune to thermal stress (Neal et al. 2014)
and coral bleaching. Coral bleaching, as evidenced by coral
skeletons that are white or bleached in appearance, occurs
when zooxanthellate corals become stressed and lose or
expel their symbiotic algae (Figure 6.4). Corals can regain
their algae and recover from coral bleaching, but prolonged
stressful conditions may cause corals to die from starvation.
Coral bleaching often results from an interaction between
high water temperatures and high light irradiance (Baird et al.
2009). The thresholds at which bleaching occurs vary among
individual species, populations and geographic locations
(Baker et al. 2008). In some cases, previous exposure to
stressors is also a factor affecting resilience. For example, corals
that inhabit areas with fluctuating environmental conditions
are often more tolerant than those from stable environments
(Oliver and Palumbi 2011). The mechanisms influencing
thermal tolerance include photo-protective pigments, a
natural coral sunscreen (Salih et al. 2000), and switching of
algal symbionts to more light-tolerant strains (Bongaerts et
al. 2015b). However, since corals are thermally adapted or
acclimated to their local environment, their ultimate response
to fluctuating temperatures will be dictated by how quickly the
external environment changes relative to their recent history
(Oliver and Palumbi 2011). Such dependence is evident in
the observed seasonal bleaching in mesophotic corals in
the Red Sea as summer months bring higher light levels and
temperatures (Nir et al. 2014). Consequently, as observed in
the U.S. Virgin Islands (Smith et al. 2015), mesophotic corals
may exhibit lower bleaching thresholds than corals in shallower
habitats. However, other mesophotic corals occur in dynamic
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Jan Feb Mar
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Temperature at 57 metres water depth
Water temperature
o
c
Source: Colin 2010
“El Nino” like
Date photographs taken
“La Nina” like
Temperature at 57 m depth recorded
every 30 minutes. Location, short drop o , Palau
Figure 6.5.
Temperature at 57 m in depth recorded every 30 minutes during 2010 off Palau. The rapid shift from a highly stratified El Niño
type water column to a uni-thermal (at near 30
o
C) water column, with associated coral bleaching seen on the mesophotic reef (photos
below graph). The bleached corals are platy agariciid corals within 30 m of the thermograph station. These and other similar corals did
not survive the bleaching event of 2010 (photos Patrick L. Colin).