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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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Worldwide, shallow coral reef ecosystems are facing an array
of natural and anthropogenic threats, including fishing,
pollution, invasive species, climate change and extreme
events (e.g. tropical cyclones), which are contributing to their
decline (Wilkinson 2008). MCEs face similar threats, albeit to
differing degrees. For light-dependent mesophotic organisms
living at low light levels (1 per cent of that found at the sea
surface), anything that inhibits light reaching the depths (e.g.
sedimentation, turbidity or pollution) has a marked impact
on their survival.
As we learned in Chapter 6, little is known or understood about
the extent of the impact fromnatural and anthropogenic threats
to MCEs. In many cases, our knowledge of these impacts is
incidental. For example, in Puerto Rico, a single colony of the
With the documented decline of shallow coral reefs, there has
been strong interest in determining the level of ecosystem
connectivity between shallow and mesophotic reefs.
Ecosystem connectivity in the broadest sense is the exchange
of materials (nutrients, organisms, and genes) between
ecosystems. Connectivity can be further broken down into
three types: genetic (exchange of genes and organisms),
ecological (exchange of individuals) and oceanographic
(water circulation patterns and material flow) connectivity.
The potential that MCEs may be ecologically or genetically
connected to shallow reefs, and may serve as refugia for
shallow reef species in decline from multiple natural and
anthropogenic stressors, has brought hope to resource
managers that all may not be lost. The ‘deep reef refugia’
hypothesis, first postulated in the mid-1990s, was based on
the premise that MCEs may serve as a refuge or population
source for replenishing shallow reef species being impacted
by thermal stress induced by climate change (Glynn 1996).
This hypothesis has since been expanded to also include
serving as a refuge from fishing, pollution and other threats.
The idea is that depth and distance from shore buffer or
protect MCEs from the direct impacts associated with these
threats, thereby allowing mesophotic populations to survive
through disturbances primarily affecting shallow-water reefs,
and reducing the likelihood that a species would be extirpated
from a region by a severe disturbance event. In addition to
serving as a refuge, a second premise of the hypothesis is that
surviving populations could assist the recovery of shallower
reefs by reseeding or replenishing shallower populations. Such
knobby cactus coral,
Mycetophyllia aliciae
, was documented
going from a healthy appearance to dead within five months.
We know this because it happened within a research study’s
photographic time-series, but we don’t know what caused it,
or whether it occurred in only this coral colony or was found
throughout colonies in the area. In general, the specific impacts
from climate change and increasing carbon dioxide levels,
fishing, pollution and invasive species and the effects of extreme
events (such as tropical cyclones, earthquakes and tsunamis) on
MCEs require documentation and study if resource managers
are to address them in a meaningful way.
Research Need:
Determine the anthropogenic and natural
threats to MCEs and assess the ecological impacts and their
subsequent recovery, if any, from them.
replenishment is dependent on a number of factors, including
whether the same species are present at both depths, the
extent of species adaptation at particular depths, and whether
there is oceanographic connectivity between them.
Data on connectivity between shallow and mesophotic reefs
is limited (Bongaerts et al. 2010a, Kahng et al. 2014). With
the exception of a few studies, the validity of the deep reef
refugia hypothesis can only be evaluated on known species
distributions. Considering this, there is potential that many
fish species are connected between shallow and mesophotic
habitats, as has been shown for the threespot damselfish,
Chromis verater
, in the Hawaiian Islands (Tenggardjaja et al.
2014) using genetics, and for commercially-important snappers
and groupers in the Caribbean (Bejarano et al. 2014). However,
for coral species, the possibility of connectedness only exists for
those living in the upper mesophotic zone (30–50 m) to mid-
mesophotic zone (50–70 m) in clear waters, because the deeper
mesophotic zone tends to be populated by coral species that are
not found in shallow waters (Bongaerts et al. 2010a, Pochon et
al. 2015). Determining the degree of connectivity of MCEs with
shallow reefs and other MCEs for key sessile andmobile species
is crucial to ensuring that effective management measures, such
as marine protected areas (Lesser et al. 2009), are implemented.
Research Need:
Understand the genetic, ecological and
oceanographic connectivity of MCEs with shallow reefs and
other MCEs.
Research Need:
Determine whether MCEs can serve as
refugia and reseed shallow reefs (or vice versa).
7.5.
What are the impacts of natural and anthropogenic
threats on mesophotic coral ecosystems?
7.6.
Aremesophotic coral ecosystems connected to shallow reefs
and can they serve as refuges for impacted shallow species?