BookPDF Available

Mesophotic coral ecosystems: A lifeboat for coral reefs?

Authors:

Abstract

Picture a coral reef — most people will probably imagine brightly coloured corals, fish and other animals swimming in well-lit shallow waters. In fact, the coral reefs that live close to the surface of the sea — the ones that we can swim, snorkel, or dive near and see from space — are only a small portion of the complete coral reef ecosystem. Light-dependent corals can live in much deeper water (up to a depth of 150 m in clear waters). The shallow coral reefs from the surface of the sea to 30–40 m below are more like the tip of an iceberg; they are the more visible part of an extensive coral ecosystem that reaches into depths far beyond where most people visit. These intermediate depth reefs, known as mesophotic coral ecosystems (MCEs), are the subject of this report. Although MCEs are widespread and diverse, they remain largely unexplored in most parts of the world, and there is little awareness of their importance among policy makers and resource managers. As a result, MCEs are for the most part not considered in conservation planning, marine zoning and other marine policy and management frameworks. The goal of this report is to raise awareness in policy makers and resource managers by providing an accessible summary on MCEs, including a discussion of the ecosystem services they provide, the threats they face, and the gaps in our understanding.
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
1
Mesophotic Coral Ecosystems
A lifeboat for coral reefs?
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
2
A Centre Collaborating with UNEP
Steering Committee
Dominic Andradi-Brown, University of Oxford, UK
Richard S. Appeldoorn, University of Puerto Rico at Mayagüez, USA
Elaine Baker, GRID-Arendal at the University of Sydney, Australia
omas C.L. Bridge, Australian Research Council Centre of Excellence for
Coral Reef Studies, James Cook University and Australian Institute of Marine
Science, Australia
Patrick L. Colin, Coral Reef Research Foundation, Palau
Peter T. Harris, GRID-Arendal, Norway
Kimberly A. Puglise, National Centers for Coastal Ocean Science, U.S.
National Oceanic and Atmospheric Administration (NOAA), USA
Jerker Tamelander, United Nations Environment Programme (UNEP), ailand
Editors
Elaine Baker, GRID-Arendal at the University of Sydney, Australia
Kimberly A. Puglise, National Centers for Coastal Ocean Science, U.S.
National Oceanic and Atmospheric Administration (NOAA), USA
Peter T. Harris, GRID-Arendal, Norway
Cartography
Kristina ygesen, GRID-Arendal, Norway
Production
GRID-Arendal
Authors (in alphabetical order)
Dominic Andradi-Brown, University of Oxford, UK
Richard S. Appeldoorn, University of Puerto Rico at Mayagüez, USA
Elaine Baker, GRID-Arendal at the University of Sydney, Australia
David Ballantine, National Museum of Natural History, Smithsonian
Institution and University of Puerto Rico at Mayagüez, USA
Ivonne Bejarano, University of Puerto Rico at Mayagüez, USA
omas C.L. Bridge, Australian Research Council Centre of Excellence
for Coral Reef Studies, James Cook University and Australian Institute
of Marine Science, Australia
Patrick L. Colin, Coral Reef Research Foundation, Palau
Gal Eyal, Tel Aviv University and e Interuniversity Institute for Marine
Sciences in Eilat, Israel
Peter T. Harris, GRID-Arendal, Norway
Daniel Holstein, University of the Virgin Islands, USA
Rachel Jones, Zoological Society of London, UK
Samuel E. Kahng, Hawai‘i Pacic University, USA
Jack Laverick, University of Oxford, UK
Yossi Loya, Tel Aviv University, Israel
Xavier Pochon, Cawthron Institute and University of Auckland, New Zealand
Shirley A. Pomponi, NOAA
Cooperative Institute for Ocean Exploration,
Research and Technology, Harbor Branch Oceanographic Institute —
Florida Atlantic University, USA
Kimberly A. Puglise, National Centers for Coastal Ocean Science, U.S.
National Oceanic and Atmospheric Administration (NOAA), USA
Richard L. Pyle, Bernice P. Bishop Museum, USA
Marjorie L. Reaka, University of Maryland, College Park, USA
John Reed, Harbor Branch Oceanographic Institute — Florida Atlantic
University, USA
John J. Rooney, Joint Institute for Marine and Atmospheric Research,
University of Hawai‘i at Mānoa and NOAA Pacic Islands Fisheries Science
Center, USA
Héctor Ruiz, University of Puerto Rico at Mayagüez, USA
Nancy Sealover,
University of Maryland, College Park, USA
Robert F. Semmler,
University of Maryland, College Park, USA
Nikolaos Schizas, University of Puerto Rico at Mayagüez, USA
Wilford Schmidt, University of Puerto Rico at Mayagüez, USA
Clark Sherman, University of Puerto Rico at Mayagüez, USA
Frederic Sinniger, University of the Ryukyus, Japan
Marc Slattery, University of Mississippi, USA
Heather L. Spalding, University of Hawai‘i at Mānoa, USA
Tyler B. Smith, University of the Virgin Islands, USA
Shaina G. Villalobos, University of Maryland, College Park, USA
Ernesto Weil, University of Puerto Rico at Mayagüez, USA
Elizabeth Wood, Marine Conservation Society, UK
Citation
Baker, E.K., Puglise, K.A. and Harris, P.T. (Eds.). (2016). Mesophotic coral
ecosystems — A lifeboat for coral reefs? e United Nations Environment
Programme and GRID-Arendal, Nairobi and Arendal, 98 p.
ISBN: 978-82-7701-150-9
Cover photo: Bright blue ascidians, known as sea squirts, are found thriving at
50 metres (164 feet) among corals, greenish brown algae (Lobophora) and red,
orange, and brown sponges o La Parguera, Puerto Rico (photo Héctor Ruiz).
In memory of Dr. John J. Rooney (1960–2016)
and his dedication to exploring and
understanding mesophotic coral ecosystems.
UNEP promotes
environmentally sound practices
globally and in its own activities. This
publication is printed on fully recycled paper, FSC
certified, post-consumer waste and chlorine-
free. Inks are vegetable-based and coatings are water-
based. UNEP’s distribution policy aims to reduce its
carbon footprint.
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
3
Mesophotic Coral Ecosystems
A lifeboat for coral reefs?
Foreword
Summary and recommendations
1. Introduction
1.1. Coral reefs in peril
1.2. Mesophotic coral ecosystems — refuge for shallow-water coral ecosystems?
2. What are mesophotic coral ecosystems?
2.1. Introduction
2.2. Light reaching the mesophotic zone
2.3. Geomorphology of mesophotic coral ecosystems
2.4. Differences between shallow-water and mesophotic coral ecosystems
3. Mesophotic coral ecosystems examined
3.1. Introduction
3.2. The Great Barrier Reef, Australia
3.3. Pulley Ridge, Gulf of Mexico, USA
3.4. The United States Virgin Islands, USA
3.5. Eilat, Red Sea, Israel
3.6. Spotlight on Palau Island group
3.7. Gulf of Carpentaria, Australia
3.8. Hawaiian Archipelago, USA
3.9. Ryukyu Archipelago, Japan
3.10. La Parguera, Puerto Rico, USA
4. Biodiversity of mesophotic coral ecosystems
4.1. Introduction
4.2. Macroalgae
4.3. Sponges
4.4. Scleractinian corals
4.5. Symbionts
4.6. Fish
5. Ecosystem services provided by mesophotic coral ecosystems
5.1. Introduction
5.2. Essential habitat
5.3. Recovery source for shallow populations
5.4. Tourists exploring the mesophotic zone
5.5. A potential source of novel products
6. Threats to mesophotic coral ecosystems and management options
6.1. Introduction
6.2. Fisheries
6.3. Climate change
6.4. Sedimentation and pollution
6.5. Marine aquarium trade
6.6. Precious coral fishery
6.7. Invasive species
6.8. Management options
7. Understanding mesophotic coral ecosystems: knowledge gaps for management
7.1. Introduction
7.2. Where are mesophotic coral ecosystems located?
7.3. What controls where mesophotic coral ecosystems are found?
7.4. What ecological role do mesophotic coral ecosystems play and what organisims are found in them?
7.5. What are the impacts of natural and anthropogenic threats on mesophotic coral ecosystems ?
7.6. Are mesophotic coral ecosystems connected to shallower coral reef ecosystems and can they serve as refuges for
impacted shallow reef species?
References
Acknowledgements
4
5
9
9
9
11
11
13
17
19
20
20
21
23
26
28
31
37
39
43
45
50
50
51
54
55
57
58
63
63
64
66
66
66
67
67
68
70
75
76
78
78
82
83
83
84
84
84
85
85
86
98
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
4
Foreword
It should come as no surprise to you that coral reef ecosystems
are in trouble. Humans have le an indelible mark on these
ecosystems, resulting in almost 20 per cent of coral reefs
disappearing. Unless we change the status quo, another 35 per
cent are expected to be lost in the next 40 years.
Coral reefs provide both tangible and intangible benets to the
lives of millions of people. From providing food and income
to protecting our coasts from damaging storms, coral reefs
make an incalculable contribution to coastal communities, as
well as to the organisms that depend on them.
Is there something we can do to help improve their chances
of survival? In 2014, the United Nations Environment
Programme convened a workshop to examine whether there
were additional management strategies that we could employ
to increase the resilience and resistance of coral reef ecosystems
to arrest their decline. One of the recommendations of the
Scientic Workshop on Coral Reef Resilience in Planning
and Decision-support Frameworks was to develop knowledge
products on emerging issues, such as investigating the
role of little-known mesophotic coral reef ecosystems
(MCEs) in coral reef resilience. Could these intermediate
depth reefs serve as “lifeboats” for increasingly stressed coral
reef ecosystems?
is report aims to address this question by bringing together
thirty-ve MCE experts from around the globe to document
what is known about MCEs, the threats they face and the gaps
in our understanding. MCEs are one of the few remaining
ecosystems on earth that remain largely unexplored. While MCEs
are deeper and more remote than shallow coral ecosystems, they
are still subject to some of the same impacts such as bleaching
and habitat destruction. We are just beginning to understand
MCEs, but they have provided a glimmer of hope that, in some
locations, they may resist some of the most immediate impacts
of climate change, and may be able to help re-seed damaged or
destroyed surface reefs and sh populations. eir ability to do
this depends on how well we manage them.
I hope this report can help catalyze greater eorts to
understand and protect mesophotic deep reefs, as a key part
of eorts towards achieving the Sustainable Development
Agenda and in particular target 14 on oceans.
Achim Steiner
UNEP Executive Director and Under-Secretary-General of
the United Nations
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
5
Summary and recommendations
Picture a coral reef — most people will probably imagine
brightly coloured corals, sh and other animals swimming in
well-lit shallow waters. In fact, the coral reefs that live close to
the surface of the sea — the ones that we can swim, snorkel,
or dive near and see from space — are only a small portion
of the complete coral reef ecosystem. Light-dependent corals
can live in much deeper water (up to a depth of 150m in clear
waters). e shallow coral reefs from the surface of the sea
to 30–40m below are more like the tip of an iceberg; they
are the more visible part of an extensive coral ecosystem
that reaches into depths far beyond where most people visit.
ese intermediate depth reefs, known as mesophotic coral
ecosystems (MCEs), are the subject of this report.
Although MCEs are widespread and diverse, they remain
largely unexplored in most parts of the world, and there is
little awareness of their importance among policy makers
and resource managers. As a result, MCEs are for the most
part not considered in conservation planning, marine zoning
and other marine policy and management frameworks.
e goal of this report is to raise awareness in policy
makers and resource managers by providing an accessible
summary on MCEs, including a discussion of the ecosystem
services they provide, the threats they face, and the gaps in
our understanding.
Key questions addressed in this report include: can MCEs
provide a refuge for the many species in shallow water reef
ecosystems that are facing increasing threats from human
activities? If shallow reefs (< 30–40 m) continue to decline,
can MCEs provide the stock to re-populate them? e answer
is of course that it depends on the species involved. In some
situations, MCEs may provide this ecosystem service and
act as “lifeboats” for nearby, connected shallower reefs that
have been damaged. In other cases, however, MCEs may be
just as vulnerable as shallower reefs to the range of human
pressures exerted upon them.
Whether or not they are lifeboats for shallow reef species,
MCEs are worthy of protection, both for their inherent
biodiversity and for the wide range of ecosystem goods and
Table 1. Key differences between shallow and mesophotic coral ecosystems.
Mesophotic coral ecosystems (MCEs) are characterized by
light-dependent corals and associated communities typically
found at depths ranging from 30–40 m and extending to over
150 m in tropical and subtropical regions. They are populated
with organisms typically associated with shallow coral reefs,
such as corals, macroalgae, sponges, and fish, as well as
species unique to mesophotic depths or deeper.
0 to approx. 30–40 m.
Lower depth corresponds to a moderate
faunal transition.
Detectable in satellite images.
From approx. 30–40 m to deeper than 150 m.
Lower depth limit varies by location due to dierences in
light penetration and other abiotic factors.
Not detectable in satellite images.
Dominant species are plate-like and encrusting
zooxanthellate scleractinian corals, octocorals, antipatha-
ians, calcareous and foliose macroalgae and sponges.
Dominant species are zooxanthellate
scleractinian corals, octocorals, calcareous
and foliose macroalgae and sponges.
Depth range
Generally middle- to low-light environments.
Light levels
Generally stable thermal regime.
Shallow, stratified waters with high
residence time may be subject to extreme
thermal events causing coral bleaching.
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.
Thermal
regime
Subject to breaking waves and turbulence,
except in sheltered lagoons.
Wave-induced shear stress and mobilition
of seafloor sediments.
High residence times within lagoons.
Below the depth aected by breaking waves.
Seafloor generally unaected 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).
Hydrodynamic
regime
Dominant
habitat-
building taxa
Generally well-lit environments.
Shallow reefs can become light-limited in
turbid waters (e.g. near estuaries).
Shallow-water coral reef ecosystems Mesophotic coral ecosystems (MCEs)
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
6
services they provide. e biodiversity of MCEs is comparable
to that of shallow reefs, yet there are also unique species that
are found only in MCEs and/or deeper water. Table 1 shows
key dierences between MCEs and shallow reefs.
While buered from some of the natural and anthropogenic
threats faced by shallow reefs, MCEs are nevertheless
vulnerable to many of the same threats, such as shing,
pollution, thermal stress, diseases and tropical cyclones,
albeit to diering extents (Table 2). MCEs also face threats
from oil and gas exploration and cable and pipeline
laying,
which
are less common on shallow reefs. For light-dependent
mesophotic reef 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 an impact on their survival. In general, there
remains much to be discovered about the extent of impacts
from natural and anthropogenic threats on MCEs.
While some pressures on MCEs are global in origin, and
require a global response, many others are regional or local.
It is important that measures to protect an individual MCE
take an adaptive, ecosystem-based approach to address the
cumulative impacts, considering both global pressures and
specic local pressures. Most of the management tools used
to protect and conserve shallow coral reefs can also be used to
protect and conserve MCEs (Table 2).
Table 2. Summary of the major anthropogenic threats to MCEs and current and potential management actions that may help mitigate
these threats.
While this report primarily provides scientific background
information for policy makers and resource managers on MCEs
to improve their awareness of these ecosystems, we would be
remiss if we did not also provide some guidance on actions that
could be taken, based on our current knowledge. To this end, we
have identified the following actions that resource managers may
take to improve the conservation and management of MCEs.
1. Identify whether MCEs may exist within your jurisdiction.
2. Identify threats to the MCEs that exist in your area
and viable options for managing them (see Table 2 for
examples of management actions).
3. Determine whether existing marine managed areas for
shallow reefs needs to be extended to include nearby MCEs.
4. Expand shallow reef monitoring programmes to include
MCE habitats.
5. Introduce awareness-raising and education programmes for
the public and policy and decision-makers about MCEs and
the need for them to be included in marine spatial planning.
Guidance for resource managers
e main recommendations made in this report (see text
box on guidance for resource managers) relate to this lack of
awareness of MCEs, the anthropogenic threats facing them,
Fishing (overfishing, destructive fishing
with dynamite and poison, and damage
from lost fishing gear)
Thermal stress (bleaching) from ocean
warming
Diseases
Pollution (land-based)
Invasive species
Tourism and recreation
Anchor damage
Coral mining (for aggregate and lime)
Coastal development
Marine aquarium trade
Fishing (overfishing and damage from lost fishing gear)
Thermal stress (bleaching) reduced exposure to warm
water stress
Diseases
Pollution: reduced exposure to land-based sources;
exposed to deep-water sewage outfalls and dredging
spoils
Invasive species
Tourism and recreation (reduced exposure)
Anchor damage (reduced exposure)
Coral mining (reduced to negligible exposure)
Marine aquarium trade
Oil and gas exploration
Cable and pipelines
Fishing closures
MPAs (MCEs are not considered in most countries)
Wastewater treatment and management to reduce
pollution (potential)
Shipping industry guidelines to curb introduced species
(potential)
Shipping industry guidelines to restrict discharge of oil
(potential)
Ensure that international trade of mesophotic reef
species, their parts and products is sustainable (potential)
Placement of fixed mooring buoys to reduce anchor
damage (potential)
Diving guidelines to reduce reef damage (potential)
Guidelines for oil and gas exploration, alternative
energy, cable and pipelines (potential)
Fishing closures
Marine protected areas (MPAs)
Wastewater treatment and management
to reduce pollution
Shipping industry guidelines to curb
introduced species
Shipping industry guidelines to restrict
discharge of oil
Ensure that international trade of reef
species, their parts and products is
sustainable
Placement of fixed mooring buoys to
reduce anchor damage
Tourism guidelines to reduce reef damage
Coral reef rehabilitation for damaged areas
Public education and involvement
Major
anthropogenic
threats
Management
actions
(current and
potential)
Shallow-water coral reef ecosystems Mesophotic coral ecosystems (MCEs)
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
7
Table 3. Key management questions and their related research priorities that would enable policy makers and resource managers to
make informed decisions on MCE protection and conservation.
Locate where MCEs exist, with a priority
in the equatorial regions of the
Indo-West Pacific region, eastern Atlantic
Ocean, and the Pacific coasts of Mexico,
Central America and South America.
Detailed maps showing the distribution of MCEs.
Models and maps showing predicted MCE habitat.Understand the geological and physical
processes that control MCE distribution
to enable us to predict where MCEs
occur.
Where are MCEs located?
Determine whether MCEs can serve as
refugia and reseed shallow reefs (or vice
versa).
Maps of larval dispersal pathways for key mesophotic
species under different oceanographic scenarios.
Are MCEs connected to
shallower coral ecosys-
tems and can they serve
as refuges for impacted
shallow reef species?
Characterize community structure,
including patterns of distribution and
abundance.
Inventory of species associated with MCEs.
What organisms are
found in MCEs?
Understand the role of MCEs in support-
ing various life stages of living marine
resources and the processes that regulate
these ecosystems.
Descriptions of trophic structures and food web models.
What ecological role do
MCEs play?
Determine the anthropogenic and
natural threats to MCEs and assess the
ecological impacts and their subsequent
recovery, if any, from them.
Maps depicting the distribution and intensity of human
activities in areas known to contain MCEs.
What are the impacts
from natural and
anthropogenic threats on
MCEs?
What controls where
MCEs are found?
Distribution and abundance estimates for key
mesophotic species.
Information on mesophotic species taxonomy, life
history, and responses to environmental conditions
(including tolerance limits) that are useful for modelling
impacts to climate change and other disturbances.
Understand the genetic, ecological and
oceanographic connectivity of MCEs
with shallow reefs and other MCEs.
Population connectivity information for key mesophotic
species.
Characterize MCE biodiversity to better
understand, protect and conserve MCEs.
Descriptions of the range of habitat types and their
distribution, how they are utilized and how these
relationships change over time.
Technologies or methods designed to reduce interac-
tions between harmful activities (such as fishing gear)
and MCEs.
Areas recommended for protection as a marine
protected area.
Management
questions
Research priority
Anticipated management
focused products
High priorityPriority
and the immediate actions that can be taken, at the local and
regional levels, to protect and conserve them.
Although the study of MCEs has increased exponentially in
the past 30 years, there are still large gaps in our scientic
knowledge of them, especially in comparison with shallow
reefs. e best way to close these information gaps is to focus
research eorts on answering questions that are critical to
enabling resource managers to make informed decisions
about MCE protection and conservation. For MCEs, the most
crucial information is what scientists would call “baseline
information, including information on their location,
biological and physical characteristics, threats, condition
and the causes and consequences of that condition. e key
questions for resource managers and the corresponding
research priorities to address them are detailed in Table 3.
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
8
(Photo Sonia J. Rowley)
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
9
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 from 30–40 m and extending
to over 150 m 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 shing practices (Hughes et al. 2003, Pandol
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 sucient to induce severe bleaching every
year by the 2050s.
Coral reefs most likely to survive the twenty-rst century
include those that sustain low impact from terrestrial runo
and that occur in locations safeguarded from extreme
sea surface temperatures. ese 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
buered from some human and natural disturbances that
negatively aect 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.
is report aims to raise awareness of the importance of MCEs
in order to improve their protection and catalyze appropriate
policy, management and research responses. e 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,
oers a glimmer of hope that MCEs may aid in the recovery
of degraded shallow reefs. is 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.
e 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 aecting
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 aected by storm waves (Death
et al. 2012). Similarly, warm-water coral bleaching, resulting
from overheating of the upper few metres of surface waters
(in calm, stratied water columns) and a synergistic eect
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,
oshore locations, such as along the edge of the continental
shelf or on remote, submerged patch reefs. ese isolated
MCEs are less exposed to many stressors commonly aecting
shallower reefs, such as terrestrial runo. MCEs may also oer
a refuge from shing pressure, particularly for commercially-
important species (Bejarano et al. 2014, Lindeld et al. 2014).
e concept of ecological refugia as a potential option for
mitigating biodiversity loss under climate change has been
increasingly debated in the scientic literature of recent
years (Ashcro 2010, Keppel et al. 2012), including dening
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 eects 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
aected by a particular disturbance, such as a cyclone or a
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
10
bleaching event, than adjacent shallow reefs (Bongaerts et al.
2010a, Bridge et al. 2014). MCEs may have the potential to
act as refugia over longer timescales in some circumstances,
particularly to provide lineage continuation for key coral reef
taxa (Muir et al. 2015).
Currently, few long-term datasets exist to enable quantitative
evaluation of the deep reef refugia hypothesis, particularly
over longer temporal scales (years to decades), primarily due
to the logistical diculties involved in monitoring mesophotic
habitats. ere is evidence that mesophotic reef populations
can mitigate against local extinction following disturbance
(e.g. Sinniger et al. 2013, Smith et al. 2014). However, it
is also clear that MCEs are not immune from natural and
human threats, such as coral bleaching and tropical storms
(see Chapter 6), and should not be considered as a panacea
to addressing the threats faced by coral reef ecosystems. For
example, bleaching of MCEs is known to occur where internal
waves or vertical mixing brings over-heated surface waters or
cooler deep waters into contact with mesophotic corals (Bak
et al. 2005, Smith et al. 2015).
In addition to serving as a refuge, a second premise of the
deep reef refugia hypothesis is whether MCEs can provide a
source of larvae to repopulate adjacent shallow reefs following
a disturbance on ecologically signicant timescales. e
viability of MCEs to serve as a source to reseed or replenish
shallow reef species is dependent on several factors, including
Figure 1.1. Impacts of human and natural disturbances tend to decrease with depth and distance from the coast, making shallow reefs
generally more vulnerable than MCEs.
whether the same species are present at both depths, the extent
of species adaptation at particular depths, and whether there
is oceanographic connectivity between the reefs. Studies
addressing this question for coral species have, to date,
generally looked at genetic connectivity between mesophotic
and shallow populations, and have revealed complex patterns.
In general, deeper mesophotic coral populations (> 60–70m
in depth) appear to be isolated from shallower populations
(Bongaerts et al. 2015b). In contrast, coral connectivity
between populations shallower than 60–70 m appears to
be both species and location-specic and dependent on
oceanographic connectivity (van Oppen et al. 2011, Serrano
et al. 2014). For sh species, connectivity has been evaluated
using genetics and ecology (presence of the same species at
both depths). In the case of the common coral reef damselsh,
Chromis verater, no genetic dierences were found among
shallow and mesophotic populations (Tenggardjaja et al.
2014), meaning they constitute a single population and should
be managed as such. Meanwhile, ecological connectivity has
been shown for sh species between shallow reefs and MCEs
o La Parguera in southwest Puerto Rico. ese MCEs serve as
a refuge, particularly for exploited large groupers and snappers,
and 76 per cent of species present at mesophotic depths
were common inhabitants of shallow reefs, indicating that
connectivity exists between shallow reefs and MCEs (Bejarano
et al. 2014). Irrespective of their potential to repopulate
shallow-water reefs, MCEs support unique biodiversity and
warrant appropriate attention from managers.
Interconnection between land and shallow-water and mesophotic reefs
- the impacts of human and natural disturbances on coral reefs tend to diminish with depth and distance from shore
Sedimentation (e.g. from rivers,
coastal development) and
shing pressure diminish with
distance from shore
Storms diminish
with depth
0m
60m
Sediment plume
S
me
t
t
t
me
um
plum
en
t p
dim
me
e
m
m
n
p
p
Source: Adapted from Bridge et al. 2013
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
11
30 m
90 m
150 m
60 m
180 m
120 m
Shallow surface reefs
“Uppermesophotic coral ecosystems
“Lowermesophotic coral ecosystems
Approx. limit of most recreational scuba diving
Shallow reefs dominated by scleractinian corals
Decrease in light intensity
Lower range for most research diving with mixed-gas equipment
Upper range for most research using deep-diving vehicles
Depth in metres
The mesophotic coral ecosystem
Source: Richard Pyle, unpublished data.
What are mesophotic coral
ecosystems?
2.1. Introduction
Chapter 2.
Elaine Baker, GRID-Arendal at the University of Sydney, Australia
Kimberly A. Puglise, National Centers for Coastal Ocean Science, U.S. National Oceanic and Atmospheric Administration (NOAA), USA
Patrick L. Colin, Coral Reef Research Foundation, Palau
Peter T. Harris, GRID-Arendal, Norway
Samuel E. Kahng, Hawai‘i Pacific University, USA
John J. Rooney, Joint Institute for Marine and Atmospheric Research, University of Hawai‘i at Mānoa and NOAA Pacific Islands Fisheries Science Center, USA
Clark Sherman, University of Puerto Rico at Mayagüez, USA
Marc Slattery, University of Mississippi, USA
Heather L. Spalding, University of Hawai‘i at Mānoa, USA
MCEs are dominated by light-dependent coral, sponge and/
or algal communities that live in the middle light (‘meso’ =
middle and ‘photic’ = light) zone. MCEs have oen been
referred to as the coral reef ‘twilight zone’ because they
represent the transition between the brightly lit surface
waters and the perpetually dark deeper depths. ey are
Figure 2.1. MCEs can form on high-angle continental and insular slopes as illustrated here, or on low-angle outer insular shelves and on
the tops of submerged banks. Decreased light penetration rather than reduced temperature appears to be the primary limiting factor
controlling the depth distribution of MCEs at most locations (Kahng et al. 2010).
30 m
90 m
150 m
60 m
180 m
120 m
Shallow surface reefs
“Uppermesophotic coral ecosystems
“Lowermesophotic coral ecosystems
Approx. limit of most recreational scuba diving
Shallow reefs dominated by scleractinian corals
Decrease in light intensity
Lower range for most research diving with mixed-gas equipment
Upper range for most research using deep-diving vehicles
Depth in metres
The mesophotic coral ecosystem
Source: Richard Pyle, unpublished data.
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
12
typically found at depths from 30–40 m and extending to
depths of over 150 m in tropical and subtropical waters
(Hinderstein et al. 2010; Figure 2.1). e occurrence of MCEs
is dependent not only on light availability, but also on water
temperature and quality, substrate and geomorphology.
MCEs are defined by the presence of corals that have
zooxanthellae and to some extent are light-dependent.
Some corals that live in the mesophotic depth range, such
as black corals and octocorals, are azooxanthellate and not
dependent on light.
MCEs are populated with organisms typically associated
with shallow coral reefs: macroalgae, scleractinian corals,
octocorals, antipatharians, sponges, a wide assortment
of other sessile
invertebrates and families of fish common
on shallow reefs (Figure
2.2), as well as species unique to
mesophotic depths or deeper.
Dominant communities providing structural habitat include
macroalgae, sponges and corals.
MCEs are defined by their ecology, not their absolute depth range.
Few of the world’s known MCEs have been mapped or studied.
The more we look, the more we find (Figure 2.3).
Key facts about MCEs
?
?
?
?
?
?
?
?
?
?
?
?
?
Primary MCE study areas
Preliminary MCE surveys
Almost nothing known
Current extent of MCE studies
Source: Adapted from Richard Pyle, unpublished data
Figure 2.3. Extent of MCE investigations to date (adapted from Richard Pyle unpublished data). At least 80 countries (those with
documented shallow reefs; Spalding et al. 2001) have potential MCEs. Countries that do not have surface reefs, but potentially have
MCEs, include those on the west coasts of Africa and South America.
Figure 2.2. Many MCEs are dominated by macroalgae, gorgonian
and antipatharian corals, sponges and other invertebrates as
illustrated in this image from 130 m in Pohnpei, Federated States
of Micronesia (photo Sonia J. Rowley).
However, there is little understanding of the degree
to which these factors (and potentially others, such as
nutrient levels, currents and competition) control the
distribution and community structure of MCEs (Puglise
et al. 2009).
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
13
Light attenuation in the ocean rapidly reduces both the
amount and quality of visible light with depth, so that only
a portion of the light spectrum is available at mesophotic
depths. Attenuation is due to absorption and scattering of
light by seawater, dissolved constituents and suspended
particles. Long wavelength colours such as red, orange and
yellow are most quickly absorbed, so that by the time the light
reaches the mesophotic zone, only the blue wavelengths of the
spectrum remain (Figure 2.4). is zone of light penetration
in the water column is referred to as the euphotic zone, and it
extends to the depth where light diminishes to approximately
1 per cent of its surface value. e depth of the euphotic zone
depends on the concentration of dissolved and suspended
light-absorbing and light-scattering materials in the water
column. In the clearest ocean water, zooxanthellate (light-
dependent) scleractinian corals have been documented at
depths as great as 165m at Johnston Atoll in the Pacic Ocean
(Maragos and Jokiel 1986; Figure 2.5).
2.2. Light reaching the mesophotic zone
Figure 2.5. The depth range of zooxanthellate mesophotic scleractinian corals is location-dependent due to differences in light
penetration and other abiotic factors.
Figure 2.4. Conceptual model of light penetration in the ocean. Blue light dominates the photic zone below 30 m, but the actual depth
of light penetration is site-specific and dependent on a variety of physical factors, such as suspended particulate matter.
300 400 500 600 700 800
Wavelength (nm)
100
200
300
400
500
0
Depth
in
metres
Source: GRID-Arendal
Reef environment and light reaching the mesophotic zone
Mesophotic zone
Deep-sea or cold-water coral ecosystems
Fringing
reef
Shallow-water coral reefs
Patch reef
Mesophotic coral ecosystems
Barrier reef
Fringing reef on atoll
Johnston
Atoll
Hawai‘i
Gulf of Aqaba
Bahamas
Marshall
Islands
Belize
Jamaica
Puerto
Rico
American
Samoa
Okinawa
Barbados
Northern
Gulf of
Mexico
Bermuda
Curaçao
West
Florida
Shelf
South Pacic Ocean MCEs
Red Sea MCEs
Caribbean Sea MCEs
Marianas
Islands
Great Barrier
Reef
North Pacic Ocean MCEs
North Atlantic Ocean MCEs
Gulf of Mexico MCEs
0
50
100
150
200
Depth in metres
Deepest observations of zooxanthellate scleractinian coral
Source: Table 4 in Kahng et al. 2010 and references therein, Blythe-Skyrme et al. 2013 and Englebert et al. 2014.
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
14
The dominant habitat-forming communities in the mesophotic
zone can be comprised of coral, sponge and macroalgal species
(Figures 2.6–2.8).
MCEs, similar to shallow-water reefs, include habitat-forming
scleractinian corals that exploit a symbiotic relationship with
zooxanthellae (genus Symbiodinium), a type of microscopic algae
(see also section 4.5). This single-celled organism lives within the
cells of the coral’s gastrodermis. The coral provides a safe home
and essential compounds for the algae, and in return the algae
supply the coral with nutrients from photosynthesis (hence the
need for light). The algae are generous guests, and on shallow
reefs can provide as much as 100 per cent of the organic material
needed by the host’s coral tissue (Muscatine 1990). However,
mesophotic coral zooxanthellae often cannot produce enough
energy given the light limitations, thus mesophotic corals may
also rely on planktonic food captured by their tentacles (Davies
1977, Lesser et al. 2010).
As coral and algal cover decline with decreasing light at depth,
the benthic communities of MCEs may shift towards communities
dominated by particle-capturing species, such as sponges and
gorgonians (e.g. Bridge et al. 2012b, Slattery and Lesser 2012).
Ecological work in the Caribbean has shown that mesophotic
sponges rely less on photosymbionts, and more on plankton
feeding. In some Caribbean MCEs, sponge biodiversity and biomass
exceed that of shallow reefs by almost ten to one (Slattery and
Figure 2.6. A Leptoseris coral-dominated MCE in the Auau Channel, offshore of Maui, Hawaii, depth of 70 m (photo NOAAs Hawaii
Undersea Research Laboratory).
Figure 2.7. A 0.25 m
2
mosaic of a Caribbean mesophotic reef (depth
60 m). Note the high coverage and diversity of sponges in the quadrat,
which is typical of many Atlantic MCEs (photo Marc Slattery).
Figure 2.8. A green algal-dominated MCE in the Auau Channel,
offshore of Maui, Hawaii, of Halimeda distorta, 75 m depth (photo
NOAAs Hawaii Undersea Research Laboratory).
Habitat-forming organisms
Lesser 2012), and growth rates are higher (Lesser and Slattery 2013).
Thus, faster growth and enhanced competitive strategies may allow
mesophotic sponges to thrive while coral reefs worldwide are on
the decline (Slattery et al. 2011). This may not be the case outside
the Caribbean, such as in the Pacific Ocean (Pawlik et al. 2015a, b,
see Slattery and Lesser 2015). In addition, the different selective
pressures (e.g. predation) between shallow and mesophotic reefs
have resulted in significant phenotypic differences in sponges with
increasing depth (Slattery et al. 2015).
Macroalgae, or seaweed, can also form vast beds and meadows
over rocky or sandy substrate in the mesophotic zone, or grow
intermixed with mesophotic corals. Although some native
macroalgae, such as the brown alga Lobophora, can be invasive —
overgrowing corals in areas where native herbivores are removed
(Lesser and Slattery 2011, Slattery and Lesser 2014) — luxuriant
stands of native macroalgae also occur naturally and are important
ecologically. For example, species such as the mesh-shaped alga
Microdictyon create bottom complexity, which forms significant
habitat for reef fish (Abbott and Huisman 2004, Huisman et
al. 2007). Calcified green algae, such as the meadow-forming
Halimeda spp., can live for several years and are important sand
producers (Spalding 2012). Thirteen different dominant macroalgal
mesophotic communities have been documented in the Hawaiian
Archipelago alone, suggesting that rich and diverse assemblages of
macroalgal species may exist at mesophotic depths, and many are
distinct from shallow-water populations (Spalding 2012).
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
14
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
15
2.2.1. Living in the shade
Corals existing in the low-light environment of the mesophotic
zone, like the plants in the understory of a rainforest, can have
specialized morphology and physiological traits (Kuhlmann
1983, Kahng et al. 2014) that enable capture and ecient use
of as much light as possible. For example, in shallow water,
the Caribbean coral Montastraea cavernosa normally has a
boulder-like shape (Figure 2.9a), while at mesophotic depths,
it exhibits a attened phenotype, which enhances light capture
(Figure 2.9b; Lesser et al. 2010). Moreover, deep (> 50 m)
mesophotic corals can have unique zooxanthellae clades that
are adapted to low light and not found in shallower depths
(Lesser et al. 2010, Bongaerts et al. 2011a, 2013b, Nir et al.
2011, Pochon et al. 2015).
In shallow water, adaptation to high light irradiance
dominates coral photophysiology (e.g. photo-protective
proteins, antioxidant enzyme capacity and self-shading
morphologies; Falkowski and Raven 2007). However,
because light attenuates exponentially with increasing depth,
photosynthetic organisms eventually become light-limited
(Kirk 1994). Corals (and algae) transplanted to lower light
regimes oen increase photosynthetic pigment concentrations
per unit area to maximize utilization of ambient light. While
potentially advantageous at intermediate depths, this form
of shade adaptation becomes self-limiting with increasing
depth, as the incremental gain in photosynthetic production
per unit pigment diminishes (Falkowski et al. 1990, Stambler
and Dubinsky 2007). erefore at lower mesophotic depths,
zooxanthellate corals employ multiple adaptation and
Figure 2.9. (a) In shallow waters, the Caribbean coral Montastraea cavernosa exhibits a boulder-like morphology, shown at 5 m (photo
John Reed), and (b) in mesophotic waters, a flattened morphology, shown at 75 m (photo Mike Echevarria).
(a)
(b)
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
16
Source: Enríquez et al. 2005, Kahng et al. 2012a, Kahng 2014
Porite s structure
Leptoseris structure
Flat skeletonLeaf
Eect of morphology on light harvesting
Water column Tissue Coral skeletonSunlight
acclimatization strategies (both ecological and biological).
ese include the following (reviewed in Kahng et al. 2010,
2014):
Minimizing self-shading and maximizing surface area at
a colony morphology level (e.g. horizontally attened or
encrusting colony morphologies), at a cellular level (e.g.
monolayered zooxanthellate), and possibly at a subcellular
level.
Reducing the amount of tissue biomass, surface area and
respiratory demand to increase growth eciency.
Reducing skeletal mass per unit colony area to reduce
energy requirements.
Optimizing skeletal light-scattering properties (Figure
2.10).
e reective properties of calcium carbonate play an
important role in increasing the light-harvesting eciency of
mesophotic corals (Enríquez et al. 2005, Kahng et al. 2012a,
Kahng 2014) and may also occur in other organisms, such
as calcareous green algae and coralline red algae. For a plant
leaf (or non-calcareous macroalgae), light passes through the
tissue only once and, unless absorbed by pigments, is lost. In
contrast, the skeleton of a coral can reect light back through
the tissue, thereby increasing the probability of absorption.
Light-harvesting eciency is not only inuenced by skeletal
composition, but can also be aected by the light-scattering
properties of skeletal micromorphology. Internal scattering
can increase the probability of light absorption, independent of
pigment concentration, by increasing the photon path length
within the coral tissue (Figure 2.10).
Location can also aect the amount of ambient light available
for mesophotic corals and algae. On at or gently sloping
areas, sessile organisms can be exposed to diuse low light
throughout the day, but on a steep slope, light is limited
because the slope obstructs the light for a portion of the day
(Brakel 1979). us, an MCE in clear water may have ample
light at a given depth in areas with at open seaoor, but may
Figure 2.11. A near-vertical mesophotic reef slope on the western
side of Tobi (Hatohobei) Island, Palau at 55 m in depth. This area
is heavily shaded during morning periods when the sun is in the
east, casting a shadow across the area (photo Patrick L. Colin).
become light-limited on a slope that is shaded for much of the
day (Figure 2.11).
Mesophotic corals exhibit several adaptations relative to
dependence on low light at depth, one of which is the switch
from autotrophic (i.e., energy from light) to heterotrophic
(i.e., energy from consumed foods) nutrition. is has been
demonstrated using stable isotope techniques in scleractinian
corals, Montastraea cavernosa (Lesser et al. 2010) and in
a facultative zooxanthellate gorgonian from a temperate
ecosystem (Gori et al. 2012). Specically, planktonic
resources, which are oen higher on mesophotic reefs (e.g.
Lesser and Slattery 2013) due to upwelled nutrients (Leichter
and Genovese 2006, Leichter et al. 2007), are captured by the
corals tentacles, thereby osetting the lmss of energy from
phototrophic sources.
Figure 2.10. The absorption of light is influenced by the micromorphology of coral and algal skeletons.
Source: Enríquez et al. 2005, Kahng et al. 2012a, Kahng 2014
Porites structure
Leptoseris structure
Flat skeletonLeaf
Eect of morphology on light harvesting
Water column Tissue Coral skeletonSunlight
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 at 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
oen 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 oshore.
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). ese 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 km
2
)
of submerged banks in the Great Barrier Reef (Harris et al.
2013), submerged ridges o the south coast of Barbados,
and relict terraces on many Pacic Islands (Bare et al. 2010).
Oen, a series of terraces can be found o a given stretch
of coastline (e.g. Barbados), with the terraces at dierent
mesophotic depths being colonized by dierent species and
growth forms of corals (Rooney et al. 2010).
2.3.2. Slope habitats
MCEs in slope habitats are inuenced by slope gradient and
geomorphology (Sherman et al. 2010). Optimal slope habitats
for MCEs are stable, rocky protrusions aording 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 Pacic 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).
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
18
Scientific knowledge of mesophotic reefs and their resident
species largely began in the Age of Exploration in the
eighteenth and nineteenth centuries, when dredging and
trawling revealed new mesophotic reef species. Pioneering
ichthyologists, such as Felipe Poey in Cuba and Pieter Bleeker
in Indonesia, produced surprisingly thorough surveys,
unsurpassed until recent times. In the early and mid-twentieth
century, knowledge of the geology and origin of coral reefs —
and by inference, MCEs — grew rapidly. Geologic investigations
into submerged reefs focused on the back-stepping of reefs,
some of which developed into MCEs, under rising global sea
levels at the end of the last ice age (Macintyre 1967, Harris and
Davies 1989; Fig 2.12).
After World War II, open-circuit scuba diving was adopted
by scientists, and by the 1960s and 1970s collections were
being made using compressed air at mesophotic depths
down to approximately 70–75 m. In the Western Atlantic, early
investigators were exploring Jamaican reefs (Goreau and Goreau
1973, Goreau and Land 1974, Lang et al. 1975) and documenting
the carbonate framework producing sclerosponges (Hartmann
1969, Hartmann and Goreau 1970) and a diverse variety of
deeper water Caribbean corals (Wells 1973). Work in the Indo-
West Pacific also brought new deep-water species to the
attention of scientists. Much of the work on the ecology of MCEs
in Hawai‘i was undertaken to understand antipatharians (Grigg
1965) and other precious corals (Grigg 1984). In the Indo-Pacific
and Caribbean, scientists also discovered that species diversity
at depths below 40 m were similar between the two regions
(Kuhlmann 1983).
Some early coral reef field guides for the Western Atlantic
region also included mesophotic fauna (Randall 1968, Bohlke
and Chaplin 1968, Colin 1978) and today many mesophotic
reef organisms, both fish and invertebrates, are in field guides
with excellent in situ photographs (e.g. Veron 2000, Fabricius
and Alderslade 2001, Allen and Erdmann 2014). Much of the
interest in MCEs was inspired by the underwater photographers
who first penetrated these depths, including Douglas Faulkner
(Faulkner and Chesher 1979). Photographic documentation
techniques have since become a mainstay of MCE research.
The potential for nitrogen narcosis (and the risks of
decompression sickness”) and the need for decompression
were recognized quickly in the early days of open-circuit scuba
diving, but it was not until the advent of mixed-gas diving
that depth and time limits could be extended, making MCEs
more readily accessible. The ability to monitor and control
the oxygen content of a breathing gas mixture resulted in the
development of mixed-gas rebreathers — first for the military
and later for civilian applications. Walter A. Starck II and John
Kanwisher developed the first practical closed-circuit mixed-
gas rebreather, the Electrolung, in the later 1960s (Starck 1969,
Starck and Starck 1972). At the upper depths of the mesophotic
History of mesophotic reef investigation
Patrick L. Colin, Coral Reef Research Foundation, Palau
zone (30–40 m), the introduction of Nitrox (enriched oxygen
air) diving in 1977 allowed increased bottom times compared
with compressed air diving. In the last decade, use of mixed-
gas rebreathers with galvanic oxygen sensors and computer
technology for gas control and decompression computation
has become increasingly common for scientific research
(Pyle 1996b), and has made diving to the lower depths of the
mesophotic zone (90–100 m) practical.
Small research submersibles (Figure 2.13) have been used on
many occasions to document mesophotic environments. The
first notable reef projects were carried out in Hawai‘i in the late
1960s (Strasburg et al. 1968), and later in Belize (James and
Ginsburg 1979) and Jamaica using the Nekton submersible
in the 1970s. In the Pacific, a fishery resource study in 1967
provided the first report of dense mesophotic scleractinian
coral communities in Japan (Yamazato 1972). In the Red Sea,
submersibles allowed for the first studies on the ecophysiology
of mesophotic corals and their distribution (Fricke and
Schumacher 1983, Fricke and Knauer 1986).
Other technological advances have improved our knowledge
of MCEs. Multibeam sonar allowed the first detailed mapping
of mesophotic areas, providing accurate depictions of slope
and geomorphology. Small remotely operated vehicles
or ROVs intended for relatively shallow water use (down
to approximately 300 m depth) have also become widely
available. Autonomous underwater vehicles (AUVs) provide
new environmental information, often including otherwise
hard-to-obtain time-series data.
Figure 2.13. Small submersibles make it possible for researchers
to study mesophotic coral ecosystems in situ for longer time
periods than technical diving (maximum of 20 minutes) permits.
The author (Patrick Colin) pictured with Adrien “Dutch Schrier
off western Curacao (photo Barry Brown).
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
19
While MCEs are viewed as extensions of shallow-water coral
reef ecosystems, there are some notable dierences between
them (Table 2.1). It is important to note that the transition from
shallow-water reefs to MCEs does not occur at a specic 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. ere is no specic
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 oshore, such
Depth range
Dominant habitat-
building taxa
Light levels
ermal 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, stratied 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
seaoor 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
dierences 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 aected by breaking waves.
• Seaoor generally unaected 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.
e hydrodynamic environment of surface coral reefs is
quite dierent from that of MCEs. Breaking waves over
surface reefs induce ow 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 stratied (e.g.
Andrews et al. 1984). ese 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.
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
20
MCEs are found worldwide in tropical and subtropical
waters. e existence of corals at mesophotic depths has
been known since at least 1889, when Darwin reported
the discovery of corals at depths of 128m (Darwin 1889).
However, it was not until the 1960s and 1970s that direct
observation of MCEs began in earnest (Starck and Starck
1972, Wells 1973). As deep-diving technologies have
advanced and been adopted by scientists, so has our ability to
access and study MCEs. is chapter takes an in-depth look
at some of the MCEs that have been studied to date (Figure
3.1) and demonstrates that while there are commonalities
among MCEs, there are also dierences; just as the shallow
coral reefs of the Great Barrier Reef are similar but dierent
from those found in the Florida Keys.
e MCEs discussed have a wide variety of geomorphologies.
ey include MCEs found on the edges of continental shelves
and far from land, such as the Great Barrier Reef and Pulley
Ridge in the Gulf of Mexico o the southwest Florida shelf;
submerged fringing reefs and banks, such as in the United States
Virgin Islands, the Main Palau Island group, Okinwa and the
Gulf of Carpentaria in Northern Australia; canyon walls, such
as Eilat in the Red Sea; and insular island shelves and submerged
karst topography found in the Hawaiian Islands. Each MCE
described below provides a snapshot of what is known about
it, the dominant species present, any known limiting factors
(e.g. sedimentation, temperature and terrigenous input) and
any known impacts (e.g. hurricanes and El Niño Southern
Oscillation), as well as whether there is a management regime
in place. ese case studies show the inuences on MCEs and
that there is still a lot to learn about them.
Mesophotic coral ecosystems
examined
3.1. Introduction
Chapter 3.
Figure 3.1. Location of MCE case studies.
Mesophotic coral ecosystem case studies
Palau Island Group
Pulley Ridge, Gulf of Mexico
La Parguera, Puerto Rico
U.S. Virgin Islands
Eilat, Red Sea
Great Barrier Reef
Gulf of Carpentaria
Hawaiian Archipelago
Ryukyu Archipelago, Japan
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
21
e Great Barrier Reef (GBR) Marine Park contains over 2,900
individual shallow reefs and covers an area of 344,000 km
2
, of
which approximately 7 per cent (20,679 km
2
) is occupied by
shallow-water coral reefs, mapped using aerial photography
and satellite imagery (GBR Marine Park Authority, http://
www.gbrmpa.gov.au/resources-and-publications/spatial-
data-information-services).
MCEs are common within the GBR Marine Park and occur
on the deeper anks of shallow reefs and on submerged banks,
both along the shelf edge and inside the GBR lagoon (Bridge
et al. 2012a, Harris et al. 2013). e morphology of the GBR
shelf changes signicantly with latitude, being narrower and
steeper in the north than in the south. ese changes aect
reef morphology, inuencing both the amount and nature of
habitats available for MCE development. e northernmost
800 km of the GBR is characterized by a relatively narrow
continental shelf with a shallow lagoon (approximately 30 m),
and long, narrow ribbon reefs separated by narrow passages
occurring along the shelf edge (Figure 1). e seaward slope
of the reefs drop steeply into very deep water, leaving limited
room for the development of submerged reefs along the shelf
edge. However, MCEs inhabited by diverse scleractinian and
octocoral assemblages are known to occur along narrow
submerged reefs seaward of the Ribbon Reefs at depths of
approximately 50 to 70m (Hopley et al. 2007, Beaman et al.
2008, Bridge et al. 2012b).
South of Cairns, the shelf widens and shallow reefs are set
back from the shelf edge. e more gently sloping seaoor has
resulted in a series of submerged reefs and terraces occurring
along the shelf edge at depths of 50 to 130 m (Figure 2).
Ecological communities inhabiting these MCEs have been
examined at Noggin Pass, Viper Reef and Hydrographers
Passage (Bridge et al. 2011a, b). In general, phototrophic
taxa including hard and so corals, phototrophic sponges
and macroalgae are the dominant habitat-forming benthos at
depths shallower than 65m (Figure 3). In some regions, inter-
reef terraces are occupied by dense elds of the macroalgae
Halimeda (Bridge et al. 2011b). Below 65m, hard substratum
is increasingly dominated by heterotrophic lter-feeders,
particularly octocorals, with very large benthic foraminifera
(particularly Cycloclypeus carpenter) occurring on so
sediments (Bridge et al. 2011a).
Given that submerged shelf-edge reefs appear to be consistent
features of the GBR shelf edge over hundreds of kilometres, it is
likely that MCEs also occur more or less continuously along
the GBR shelf edge to at least the southernmost extent of the
Swain Reefs at 23°S (Figure 1).
3.2. The Great Barrier Reef, Australia
Thomas C.L. Bridge, Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University & Australian Institute of Marine
Science, Australia
Figure 1. Great Barrier Reef.
Figure 2. Bathymetry of the GBR outer-shelf at Hydrographers Passage, showing submerged shelf-edge reefs (from Bridge et al. 2011a).
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
22
e deeper lagoon in the central GBR allows greater MCE
development on the mid-shelf. e lower slopes of some reefs
extend to depths of at least 50m (Chalker and Dunlap 1983), and
are occupied by scleractinian or hard corals. Submerged banks
and shoals are also abundant throughout the GBR (Pitcher et
al. 2007) covering an area of about 25,600 km
2
(Harris et al.
2013). ree types of banks having a vertical relief exceeding
15m were recognized: Type 1 (n = 1,145), with a mean depth
of 27 m, have some portion of their surface covered by shallow
coral reefs (and are thus co-located with shallow reefs); Type 2
(n = 251), with a mean depth of 27 m, are located landward of
the shelf-edge barrier reef on the middle- to outer-shelf, with
no shallow reefs superimposed; and Type 3 (n = 150), with a
mean depth of 59 m, are located on the outer shelf, commonly
seaward of the outer-shelf barrier reef (Harris et al. 2013).
e shelf position of the dierent bank types is an important
determinant of their ecological composition (Harris et al.
2013). Shallower shoals are dominated by hard corals, while
deeper shoals are oen colonized by gorgonians or calcareous
algal species such as Halimeda (Hopley et al. 2007, Pitcher et al.
2007, Roberts et al. 2015).
Interest in the biodiversity associated with MCEs in the GBR
Marine Park has increased in recent years, although the majority
of this research has focused on hard corals (Bridge and Guinotte
2012, Muir et al. 2015). Broad-scale patterns in community
composition have been investigated primarily using an
autonomous underwater vehicle (Williams et al. 2010). Several
expeditions from 2011 to 2013 conducted extensive sampling of
hard corals on lower reef slopes in the north and central GBR,
with most sampling occurring in the upper mesophotic (30–
40 m), although some specimens were collected from deeper
than 100 m (Englebert et al. 2014). MCEs clearly support a
considerable diversity of hard corals, including common shallow-
water species such as Acropora (Muir et al. 2015).
Considerable interest surrounds the question of whether
MCEs are capable of providing refuges for shallow-water coral
reef biodiversity. Quantitative, long-term data are currently
unavailable for MCEs on the GBR, and understanding their
potential vulnerability to disturbances is dicult. MCEs
are well represented in no-take areas, aided by the robust
and precautionary management approach taken in the 2003
rezoning process (Bridge et al. 2015), but severe tropical
cyclones are currently the leading cause of coral decline on
the GBR. Very severe storms, such as Tropical Cyclone Yasi in
2011, caused damage to depths of at least 70m at Myrmidon
Reef (Bongaerts et al. 2013a), although in general MCEs are less
impacted by storms than shallower reefs (Roberts et al. 2015).
ere have been no observations of warm-water bleaching of
MCEs in the GBR to date, although observations are limited.
Sediment accumulation, due to the lack of wave energy in
deeper waters, appears to be a signicant factor limiting the
growth of corals in mesophotic depths. Controlling sediment
loads is therefore likely to be important for MCEs, particularly
on submerged banks closer to shore. Lack of knowledge of the
spatial location and extent of submerged banks may increase
their incidental exposure to threats such as dumping of dredge
spoil and ship anchoring (Kininmonth et al. 2014).
Figure 3. Examples of MCEs on the Great Barrier Reef: (a) hard-coral dominated community at Mantis reef (photo Ed Roberts),
(b) soft-coral dominated assemblage at Hydrographers Passage, (c and d) heterotrophic octocoral-dominated assemblages at
Hydrographers Passage (photos Australian Centre for Field Robotics at the Unviersity of Sydney, figure from Bridge et al. 2012a).
(a)
(c)
(b)
(d)
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
23
Pulley Ridge, located in the Gulf of Mexico, lies about 250
km west of the Florida coast and extends from north to
south along the southwestern Florida platform at depths
of 60–90m for nearly 300 km (Figure 1; Hine et al. 2008).
Only the southernmost 34 km of Pulley Ridge, referred to as
southern Pulley Ridge, supports MCEs (Figure 2; Cross et al.
2005, Reed et al. 2014, 2015). Southern Pulley Ridge is about
160 km
2
in size (Cross et al. 2005, Culter et al. 2006) with
10m relief and represents a drowned barrier island from the
last glacial period. Pulley Ridge is the deepest known light-
dependent coral reef ecosystem o the continental United
States (Halley et al. 2003).
Seismic maps indicate that drowned shoreline ridge complexes
and pinnacles extend west of Pulley Ridge to depths of 100–150
m, suggesting the potential for other MCE habitat in the region
(Ballard and Uchupi 1970, Holmes 1981, Phillips et al. 1990).
In 2015, an additional 321 km
2
of MCE habitat adjacent to
southern Pulley Ridge was documented (Reed et al. 2015). An
analysis of the total area of mesophotic depth habitat at depths
of 30–150m indicates that the northern Gulf of Mexico region
(Figure 1; 178,867 km
2
) has an order of magnitude area greater
for potential MCEs than either the U.S. Caribbean or the Main
Hawaiian Islands (Locker et al. 2010).
Coral growth is supported by the Loop Current, the
prevailing western boundary current in the Gulf of Mexico,
which provides warm, clear, nutrient-poor waters to Pulley
Ridge (Jarrett et al. 2005). is current separates the clear,
oligotrophic, outer-shelf waters from cooler, higher nutrient,
interior-shelf waters (Hine et al. 2008). Seaoor light
measured at southern Pulley Ridge (65–70 m) is only 1–2
per cent (5–30 µE m
-2
s
-1
) of available surface light, which is 5
per cent of the light typically available to shallow-water reefs
(Jarrett et al. 2005).
Recent surveys of Pulley Ridge in 2012–2014 (Reed et al. 2014,
2015) show that the reef habitat supports a biologically diverse
and dense community that is dominated by macroalgae (53.8
per cent cover), including plates of crustose coralline algae,
Peyssonnelia spp., and the green alga Anadyomene menziesii;
1.6 per cent cover of sponges (102 taxa); and 1.3 per cent cover
of hard coral. A total of 216 benthic macrobiota taxa have been
identied from Pulley Ridge, including 14 Scleractinia, 15
Octocorallia (gorgonacea), and four Antipatharia (black corals;
Figure 3). e scleractinian hard corals are dominated by the
plate corals Agaricia sp., A. fragilis, A. lamarcki/grahamae,
Helioseris cucullata and plate-forms of Montastraea cavernosa.
Previous surveys indicate that there has been a signicant
loss of coral cover on Pulley Ridge over the past 10 years.
In 2003, the mean coral cover at southern Pulley Ridge was
11.9 per cent, with a maximum of 23.2 per cent in the central
region of the ridge; and platy corals were up to 50 cm in
diameter with coral cover as high as 60 per cent (Halley et
al. 2003, Jarrett et al. 2005, Hine et al. 2008). By 2013, the
3.3. Pulley Ridge, Gulf of Mexico, USA
John Reed, Harbor Branch Oceanographic Institute — Florida Atlantic University, USA
Texas Louisiana
Mississippi
Alabama
Florida
Georgia
Habitat Areas of Particular Concern (HAPC)
National Marine Sanctuaries (NMS)
Marine Protected Areas (MPA)
Other
Stetson Bank
McGrail Bank
Florida Middle
Grounds
Twin Ridge
Madison Swanson
The Edges
Steamboat Lumps
Sticky
Grounds
Pulley Ridge
Tortugas North
Tortugas South
Florida
Keys
The Pinnacles
Yellowtail and
Roughtongue Reef
Flower Gardens
0 100 200 300 400 kilometres
1000 metres isobath
500 metres isobath
150 metres isobath
Bathymetry
30 metres isobath
Gulf of Mexico
85°W95°
W
90° W
30° N25°N
Figure 1. Map of U.S. Gulf of Mexico showing extent of mesophotic depth habitat (darker 30 m to 150 m depth contours) and major
mesophotic reefs (boxes). Mesophotic depth marine reserves include marine protected areas (MPA), Habitat Areas of Particular Concern
(HAPC), and National Marine Sanctuaries (NMS).
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
24
73
86
77
Depth in meters
60
100+
average hard coral cover was 0.85 per cent, with a maximum
of 5.6 per cent, which is a 92.8 per cent loss of coral cover
in a decade (Reed et al. 2014). In 2014, additional surveys to
the west of southern Pulley Ridge, in an area known as the
Pulley Ridge Central Basin, discovered a new coral area with
the densest cover of mesophotic Agaricia corals known in
the Gulf of Mexico (2.6–4.98 per cent cover with an average
coral density of 5.6–16.8 colonies per m
2
; Figure 2). is new
area is unprotected and outside of the Pulley Ridge marine
protected area (Reed et al. 2015). On a positive note, a large
number of these corals are relatively new recruits: 47.7 per
cent are less than 5 cm in diameter, and 35.4 per cent are 5–9
cm. So it appears that the coral is growing back from the die-
o that occurred aer 2003.
A total of 78 sh taxa were identied in Pulley Ridge in 2012
and 2013 (Reed et al. 2014). e most common species included
chalk bass, bicolour damselsh and cherubsh. Fieen species
of commercially- and recreationally-important grouper and
snapper species were found (681 individuals in total), with
the dominant species being vermilion snapper, black grouper,
graysby, mutton snapper, red grouper and scamp. On southern
Pulley Ridge, red groupers have excavated over 155,000 burrow
pits from 5m to over 15m in diameter and 1–2m in depth.
Most active burrows have one adult red grouper with a total
length of 50 cm or greater. e burrows provide habitat and act
as oases for many small reef sh, but unfortunately most of the
burrows seen in 2013 and 2014 had from several to 60 invasive
lionsh per burrow (Reed et al. 2014; see Chapter 6).
Figure 2. Multibeam map of the Pulley Ridge MCE in the U.S. Gulf of Mexico, the deepest known photosynthetic reef in U.S.
continental waters. Pulley Ridge South (60–70 m depth) is a submerged intact barrier island. Pulley Ridge Basin and West Pulley
Ridge are deeper geological features (80–90 m depth), which also provide MCE habitat. Yellow box= Pulley Ridge Habitat Area of
Particular Concern, 346 km
2
(Multibeam Bathymetry Survey data, University of South Florida).
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
25
In the 2003 the corals generally appeared to be healthy, with
little evidence of coral bleaching or disease (Jarrett et al. 2005,
Hine et al. 2008). In 2014, a total of 7,329 individual plate
corals (Agaricia spp. and Helioseris cucullata) were counted
from the transect photos, of which 247 were noted to be
bleached, partially bleached, totally bleached, partly dead,
recently dead or diseased; resulting in 4 per cent morbidity
of the total population measured (Reed et al. 2015). Bleaching
(partial to total) ranged from 0 to 11.5 per cent per km
2
block.
e Gulf of Mexico Fishery Management Council (GMFMC)
expressed concern over ongoing damage by shing
operations to Pulley Ridge habitat, and in 2005 designated
Pulley Ridge a Habitat Area of Particular Concern (HAPC;
Figure 3. Habitat and biota of Pulley Ridge MCE in the U.S. Gulf of Mexico. (a) Helioseris cucullata, depth 74 m. (b) Agaricia grahamae,
depth 82.5 m. (c) Swiftia exserta (octocoral with lionfish Pterois volitans), depth 79 m. (d) Peyssonnelia sp. (crustose coralline algae)
and Halimeda copiosa (green algae), depth 80 m. (e) Geodia neptuni (sponge) and Anadyomene menziesii (green algae), depth 73 m. (f)
Epinephelus morio (60 cm red grouper) guarding its burrow, depth 80 m, laser scale is 10 cm (photos Reed et al. 2015).
criteria for HAPCs include ecosystem services provided
by the habitat, sensitivity to human impact, development
stressors and rarity of habitat type). is 346 km
2
marine
protected area is also considered essential habitat for coral
and sh. Fishing restrictions within the Pulley Ridge HAPC
include prohibition of bottom-tending gear, such as bottom
trawls, bottom longline, buoy gear, pot or trap and bottom
anchoring by shing vessels (GMFMC 2005). In 2014, a
proposal was submitted to the GMFMC to extend the Pulley
Ridge HAPC boundaries to include the newly discovered
MCE habitat (321 km
2
) in the Pulley Ridge Central Basin
and West Pulley Ridge.
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
26
e insular shelf of the United States Virgin Islands (USVI)
supports diverse MCEs that form on steep walls around the
island of St. Croix and on the extensive banks and steep walls
in the northern USVI around the islands of St. John and St.
omas (Figure 1). Seventy-ve per cent of the total shelf
area above 65m depth (1918 km
2
) is potentially MCE habitat
(25–65 m), suggesting that MCEs could be more extensive
than shallow reefs. is is certainly true around St. John and
St. omas on the southeast Puerto Rican Shelf, where the
identied hard bottom habitat below 30m depth constitutes
60 per cent of the total hard bottom habitat (137 km
2
).
*
e northern USVI presents one of the most spectacular
known examples of bank reef MCEs in the Caribbean (Figure
2). Within the well-characterized MCE depths (30–45m) of
the southeastern Puerto Rican Shelf, there is strong habitat
heterogeneity, with shelf-edge reefs forming on a drowned
barrier reef complex and more inshore banks forming at similar
depths (Smith et al. 2010). e most extensive area of reef
development is on the southern entrance to the Virgin Passage,
separating USVI from Puerto Rico. is area may represent
one of the best developed MCEs within the U.S. Caribbean.
e shelf-edge reefs of the Virgin Passage tend to be low in
coral cover (< 10 per cent), most likely as the result of natural
disturbances from storms (Smith pers. obs.), whereas the
* is calculation does not include any of the uncharacterized hard bottom
MCE habitats on the deep and wide northern bank.
secondary and tertiary bank reefs have higher coral cover
(25–50 per cent) — representing the highest in USVI and very
high for the Caribbean (Smith et al. 2010). Importantly, the
dominant coral genus that forms over 85 per cent of coral cover is
Orbicella, which has recently been listed as threatened under the
U.S. Endangered Species Act (NOAA 2014). is genus is very
abundant in the upper mesophotic zone, with a conservative
estimate of 50 million Orbicella colonies on the 23 km
2
of hard
bottom habitats in the Hind Bank Marine Conservation District
(Smith 2013). Other bank reef systems at similar depths in the
Western Atlantic may be similarly dominated by Orbicella spp.,
while only 6 per cent of the MCEs of the south shelf are in the no-
take or restricted-take shery areas (Kadison pers. com.).
MCE development around St. Croix is limited by a mostly
narrow shelf that drops steeply into deeper water, which may
typify many small island MCEs of the Caribbean. Only 13 per
cent (48 km
2
) of the St. Croix shelf is at mesophotic depths (25–
65 m), which is a much smaller area than that of the northern
USVI shelf (1385 km
2
). Most MCE development is on steep
walls and slopes, the exception being some deeper linear reefs
at the eastern extent of the Lang Bank (García-Sais et al. 2014,
Smith et al. 2014). Since the 1970s, a few of the walls have been
very well-studied, such as Salt River Canyon and Cane Bay walls
on the northwest. ese wall systems form dramatic precipices
that extend from shallow depths to below 100 m.
Mesophotic coral cover was historically above 25 per cent for Salt
River Canyon (Aronson et al. 1994) and Cane Bay (Sadd 1984), but
there has been degradation in recent years due to the combination
of several large hurricanes and a thermal stress and bleaching
event in 2005. MCE coral cover at these sites is now below 10
per cent (Smith et al. 2014). e coral communities are a typical
mix of plating forms; predominantly lettuce corals (Agaricia spp.)
and star corals (Orbicella spp.), which form on vertical buttresses
surrounded by channels where sediment is transported o-shelf.
e Salt River Canyon and areas at the eastern end of the Lang
Bank are in Fisheries Protected Areas, covering about 25 per
cent of the potential MCE shelf depths. Despite the moderate
coverage of Marine Protected Areas, shing intensity on the
narrow shelf is quite high and many commercially-important
sh, such as large-bodied snappers and groupers, are absent or