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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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Introduction
1.1.
Coral reefs in peril
1.2.
Mesophotic coral ecosystems — a refuge for shallow-
water coral reefs?
Chapter 1.
Peter T. Harris
, GRID-Arendal, Norway
Thomas C.L. Bridge
, Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University & Australian Institute of Marine Science,
Australia
Mesophotic coral ecosystems
are characterized by the
presence of light-dependent corals and associated communities
typically found at depths ranging from30–40mand extending
to over 150m in tropical and subtropical regions.The dominant
communities providing structural habitat in the mesophotic
zone can be comprised of coral, sponge, and algal species
(Puglise
et al.
2009, Hinderstein
et al.
2010).
Globally, coral reefs are deteriorating rapidly due to elevated
sea surface temperatures, coastal development, pollution and
unsustainable fishing practices (Hughes et al. 2003, Pandolfi
et al. 2003). About 19 per cent of coral reefs have already
been lost, with a further 35 per cent expected to disappear
in the next 40 years (Wilkinson 2008). Unless something
changes, almost all shallow-water coral reefs will experience
thermal stress sufficient to induce severe bleaching every
year by the 2050s.
Coral reefs most likely to survive the twenty-first century
include those that sustain low impact from terrestrial runoff
and that occur in locations safeguarded from extreme
sea surface temperatures. These include large areas of
intermediate depth reefs, also known as mesophotic coral
ecosystems (MCEs; Glynn 1996, Riegl and Piller 2003).
Occurring at depths greater than 30–40 m, MCEs may be
buffered from some human and natural disturbances that
negatively affect shallow-water reefs (Bongaerts et al. 2010a,
Bridge et al. 2013), but not all stressors (Stokes et al. 2010,
Lesser and Slattery 2011).
Science has shown that MCEs are far more widespread and
diverse than previously thought (Locker et al. 2010, Harris
et al. 2013). However, they remain largely understudied in
most parts of the world and there is little awareness of their
importance among policy makers and resource managers
(Bridge et al. 2013, Madin and Madin 2015). Consequently,
they are for the most part not considered in conservation
planning, marine zoning and other marine policy and
management frameworks.
This report aims to raise awareness of the importance of MCEs
in order to improve their protection and catalyze appropriate
policy, management and research responses. The potential
that MCEs may act as “refugia” and a source of replenishment
for some shallow reef species (Glynn 1996, Riegl and Piller
2003, Bongaerts et al. 2010a) or, in other words, “lifeboats”,
offers a glimmer of hope that MCEs may aid in the recovery
of degraded shallow reefs. This report provides an accessible
summary on MCEs, including a discussion of the ecosystem
services they provide, the threats they face, and gaps in our
understanding, as well as addressing the question of whether
MCEs can serve as lifeboats for coral reefs.
The notion that MCEs could provide a refuge for coral reef
biodiversity from natural and human impacts has been
formalized in the ‘deep reef refugia hypothesis’ (Glynn
1996, Bongaerts et al. 2010a). Some disturbances affecting
coral reefs are most acute in shallow waters (Figure 1.1):
for example, wave energy attenuates with increasing depth,
making MCEs less likely to be affected by storm waves (De’ath
et al. 2012). Similarly, warm-water coral bleaching, resulting
from overheating of the upper few metres of surface waters
(in calm, stratified water columns) and a synergistic effect
between heat and light, has less of an impact on MCEs located
in deeper water (> 30–40 m to over 150 m) and receiving
lower irradiance. In addition, many MCEs occur in remote,
offshore locations, such as along the edge of the continental
shelf or on remote, submerged patch reefs. These isolated
MCEs are less exposed to many stressors commonly affecting
shallower reefs, such as terrestrial runoff. MCEs may also offer
a refuge from fishing pressure, particularly for commercially-
important species (Bejarano et al. 2014, Lindfield et al. 2014).
The concept of ecological refugia as a potential option for
mitigating biodiversity loss under climate change has been
increasingly debated in the scientific literature of recent
years (Ashcroft 2010, Keppel et al. 2012), including defining
the spatial and temporal scales of what is termed a refugium
(Keppel et al. 2012). It is now accepted that the term ‘refuge’
refers to short timescales (e.g. a particular MCE may be a
refuge from the effects of a tropical cyclone), whereas ‘refugia’
operate on longer temporal scales. Most studies addressing
refugia in relation to MCEs are actually referring to their role
as a refuge; that is, whether mesophotic habitats were less
affected by a particular disturbance, such as a cyclone or a