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Marine Science
ICES Journal of Marine Science (2013), 70(2), 284 –293. doi:10.1093/icesjms/fss165
Submerged banks in the Great Barrier Reef, Australia, greatly
increase available coral reef habitat
Peter T. Harris 1 *, Thomas C.L. Bridge 2, Robin J. Beaman3, Jody M. Webster 4, Scott L. Nichol 1,
and Brendan P. Brooke 1
1
Geoscience Australia, GPO Box 378, Canberra ACT 2601, Australia
ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville QLD 4811
3
School of Earth and Environmental Science, James Cook University, Cairns, QLD 4870, Australia
4
Geocoastal Research Group, School of Earth Sciences, University of Sydney, Sydney NSW 2006, Australia
2
*Corresponding Author: tel: +61 2 6249 9611; fax: +61 2 6249 9920; e-mail: [email protected]
Harris, P. T., Bridge, T. C. L., Beaman, R. J., Webster, J. M., Nichol, S. L., and Brooke, B. P. 2013. Submerged banks in the Great Barrier Reef,
Australia, greatly increase available coral reef habitat. – ICES Journal of Marine Science, 70: 284 – 293.
Received 12 June 2012; accepted 14 September 2012; advance access publication 29 November 2012.
Anthropogenic global ocean warming is predicted to cause bleaching of many near-sea-surface (NSS) coral reefs, placing increased
importance on deeper reef habitats to maintain coral reef biodiversity and ecosystem function. However, the location and spatial
extent of many deep reef habitats is poorly known. The question arises: how common are deep reef habitats in comparison with
NSS reefs? We used a dataset from the Great Barrier Reef (GBR) to show that only about 39% of available seabed on submerged
banks is capped by NSS coral reefs (16 110 km2); the other 61% of bank area (25 600 km2) is submerged at a mean depth of
around 27 m and represents potential deep reef habitat that is spatially distributed along the GBR continental shelf in the same latitudinal distribution as NSS reefs. Out of 25 600 km2 of submerged bank area, predictive habitat modelling indicates that more than
half (around 14 000 km2) is suitable habitat for coral communities.
Keywords: benthic habitats, biodiversity, Great Barrier Reef, mesophotic, refugia, submerged banks.
Introduction
There has been a well-documented decline in coral reef ecosystems
due to natural and anthropogenic causes (Hoegh-Guldberg, 1999;
Hughes et al., 2003; Hoegh-Guldberg et al., 2007). Globally, about
19% of coral reefs have already been lost, with a further 35%
expected to be lost in the next 40 years (Wilkinson, 2008). Even
those reefs not currently under threat are predicted to be affected
by climate change impacts, particularly bleaching events due to elevated sea temperature. However, these estimates are largely based on
the known extent of reef habitat, which generally only includes
near-sea-surface (NSS) coral reefs. Submerged reefs that occur
below a depth of around 20 m cannot easily be detected using satellites or aerial photography, even in clear waters. Consequently, their
spatial distribution and even their existence are unknown in most
reef provinces. For this reason, submerged reefs have been largely
neglected in estimates of the available area of coral habitat, despite
recent evidence that these areas may be significant (Locker et al.,
2010). Understanding the extent of submerged reefs is therefore important because they can support large and diverse coral
# Crown
communities (Bridge et al., 2012) and hence may provide vital
refugia for corals and associated species from a range of environmental disturbances (Glynn, 1996; Riegl and Piller, 2003; Bongaertset al.,
2010).
Scientific drilling has demonstrated that most NSS reefs have
(relict) limestone foundations, forming a “pedestal” or geomorphic bank upon which modern reef growth occurs (Marshall and
Davies, 1984; Hopley et al., 2007). However, not all banks are colonized by NSS coral reefs, and in many cases reef colonies that were
established in the late-Pleistocene to early-Holocene were unable
to keep pace with post-glacial sea level rise and were subsequently
“drowned” (Vecsei, 2003; Abbey and Webster, 2011). Even though
their slow vertical growth rate has inhibited them from reaching
the sea surface, these submerged reefs do provide habitat for a
range of coral reef species, including reef-building corals (Bridge
et al., 2011a, b). Now commonly referred to as mesophotic coral
ecosystems (MCEs), they are typically found at depths ranging
from 30 to 150 m in tropical and subtropical regions, depending
on water quality and light penetration (Hinderstein et al., 2010,
copyright 2012. For Permissions, please email: [email protected]
Coral reef habitat availability in the Great Barrier Reef, Australia
Kahng et al., 2010). Their potential importance as coral refugia has
generated significant interest from both scientists and managers in
recent years because observations indicate that deep (.30 m)
coral habitats can support diverse coral communities (Puglise
et al., 2009; Hinderstein et al., 2010; Bridge and Guinotte, 2012).
MCEs have been well documented in the western Atlantic
(Armstrong et al., 2006; Garcia-Sais, 2010; Smith et al., 2010;
Armstrong and Singh, 2012) and have been reported from the
Pacific and Indian Oceans (Kahng and Kelley, 2007; Banks et al.,
2008; Bare et al., 2010; Bongaerts et al., 2011; Bridge et al.,
2011a, b). In Australia, submerged reefs and associated coral communities have been described in the Gulf of Carpentaria (Harris
et al., 2004, 2008), adjacent to Lord Howe Island (Woodroffe
et al., 2010), in the Timor Sea off Australia’s North West Shelf
(Heyward et al., 1997), adjacent to the Ningaloo Reef in Western
Australia (Nichol and Brooke, 2011), in the western Coral Sea
(Bongaerts et al., 2011), and across the Great Barrier Reef (GBR)
continental shelf (Harris and Davies, 1989; Beaman et al., 2008;
Bridge et al., 2011a, b).
285
NSS coral reefs are exposed to a range of natural and anthropogenic threats, particularly the increasing incidence and severity of
coral bleaching due to anthropogenic warming of sea temperatures
and high light irradiance (Glynn, 1996; Hoegh-Guldberg, 1999;
Hughes et al., 2003). In comparison, submerged reefs may be buffered from many of these bleaching events due to the greater depth
of the overlying water column and reduced light irradiance
(Glynn, 1996; Riegl and Piller, 2003). Importantly, NSS coral
reef recovery may be assisted by seed stock supplied from nearby
submerged reefs (Harris et al., 2008; Bongaerts et al., 2010, van
Oppen et al., 2011). Global ocean warming, combined with a
range of other threats (Pandlofi et al., 2003), could potentially
result in submerged reefs functioning as refugia for corals and
associated species. Therefore, their conservation may be crucial
for the persistence of corals and associated species under future
climate change impacts and other local stressors.
The Great Barrier Reef (GBR) extends over 15 degrees of latitude and comprises a wide variety of reef systems and different
morphotypes (Hopley et al., 2007). The GBR therefore provides
an ideal case study to illustrate that the lack of information on
Figure 1. Map of the Great Barrier Reef, Australia, showing location of submerged geomorphic banks. (a) Histogram of bank occurrence
illustrating the spatial distribution of Type 1, 2 and 3 banks along the length of the GBR; and (b) histogram of NSS reef occurrence. Type 1, 2
and 3 banks are defined in the text (see also Figure 3). The location of Figure 5 is indicated.
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P. T. Harris et al.
Table 1. Statistics on geomorphic banks (mesophotic coral habitat) and near-sea-surface (NSS) coral reefs in the GBR.
All banks incl. NSS reefs
All NSS reefs
Type 1 banks
Type 2 banks
Type 3 banks
All banks with NSS reef area subtracted
Number
1 581
3 457
1 145
251
150
1 546
Mean
depth (m)
27.29
14.9
26.79
27.23
58.57
29.94
Mean
depth SD
15.69
15.4
11.26
6.29
16.63
14.66
Mean
height (m)
41.67
N/A
43.62
26.29
36.30
40.09
Mean
area km2
26.38
5.98
20.92
1.98
8.50
16.56
Total
area km2
41 709
20 679
23 827
497
1 275
25 599
Mean P/A
ratio
3.10
8.75
2.29
7.06
2.84
3.10
Banks with reef area subtracted deletes 35 banks that are 100% covered by NSS coral reefs. Types 1, 2 and 3 banks are as defined in the text. The fraction of
bank area covered by NSS reef averages 40.2 + 27.6% over the 1180 banks that support NSS reefs. NSS reef depths estimated using reef polygons available
from the GBRMPA, overlain on Beaman’s (2010) bathymetric model. Sources of error in the surface area estimates are associated with the pixel size of the
bathymetric grid (0.01 km2) plus any (unquantified) human errors associated with digitizing the NSS reefs and banks.
deep reef habitats has likely caused a substantial underestimation
of the available coral reef habitat in this region. Such a case
study, by extension, can be used to highlight an underestimation
of the global spatial extent of coral reef habitat. Given that the
NSS coral reefs in the GBR are among the best-quantified in the
entire Indo-Pacific, we suggest that a significant proportion of
global coral reef habitat is currently undocumented, limiting its
environmental management as well as our understanding of
coral reef biodiversity and also important ecosystem processes,
such as connectivity and the effects of climate change on coral
reef ecosystems.
Here, we have used a recently developed high-resolution digital
elevation model (DEM) for the Great Barrier Reef and Coral Sea
(Beaman, 2010) to re-evaluate the geomorphology of the GBR
shelf, and address the question of how much surface area of geomorphic banks (potential submerged reef habitat) exists compared
to the known area of NSS coral reefs in the GBR. The potential for
submerged bank areas to be colonized by corals is assessed based
on a test area from the central GBR outer-shelf, where highresolution (5 m grid) bathymetric data together with observations
of mesophotic coral communities are available.
Methods
Mapping geomorphic banks
The geomorphic mapping in this study is based on a GIS analysis of a
100 m bathymetry grid produced by Beaman (2010). The bathymetric data were contoured at 5 m intervals and used to interpret the location of geomorphic bank features, defined as having at least one
steep (i.e. greater than 2 degrees) slope rising more than 15 m
above the level of surrounding seafloor. A “geomorphic bank” is
an underwater feature defined by the International Hydrographic
Organisation (IHO, 2008) as “isolated (or group of) elevation(s) of
the sea floor, over which the depth of water is relatively shallow,
but sufficient for safe surface navigation”. Due to their complex,
asymmetric morphology (exhibiting both elongate and oval shapes
in plan view and both steep and gentle slopes), all banks were digitized by hand (aided using 3D imagery). Bank polygons were
created in ArcGIS with the base of slope taken as the outer edge of
the bank. Mean bank elevation estimates thus include the bank
slopes as well as planar bank-tops. Only banks occurring on the continental shelf of the Great Barrier Reef between the 20 and 200 m isobaths, and between the latitudes of 10 to 258S were included.
GBR_FEATURES.shp, an ArcGIS layer for NSS coral reefs published
by the Great Barrier Reef Marine Park Authority (GBRMPA), was
used. Land (island) areas were removed, as required, to generate
total submerged bank areas. Statistical analyses of depths and
surface areas of banks were carried out using ArcGIS. Great Barrier
Reef Marine Park zones were downloaded from the GBRMPA web
site (http://www.gbrmpa.gov.au/) and used to derive bank areas
within the different zones.
Predictive habitat models
In order to estimate the likely occurrence of coral habitat on submerged banks in the GBR, we used Maxent 3.2.19 (Phillips et al.,
2004, 2006; Elith et al., 2011) to generate predictive habitat models
for the Hydrographers Passage region. Maxent uses maximum
entropy techniques to create models of the relative probability of
species/community distribution across a study area. It has the advantage of requiring presence-only data, which is beneficial for
modelling inaccessible ecosystems, such as deep reefs where occurrence data are sparse, and the lack of reliable absence data renders
them unsuitable for traditional modelling methods. Maxent has
been used in both terrestrial and marine ecosystems and has
been shown to perform favourably relative to other presence-only
modelling techniques (Pearson et al., 2007; Elith et al., 2011).
The Hydrographers Passage (19.708S 150.258E) site was chosen
because its geomorphology and biology have been comparatively
well-documented (Harris and Davies, 1989; Beaman et al., 2008;
Bridge et al., 2011a; Beaman et al., 2012), and because it is one
of very few areas in the GBR where co-located, high-resolution
geophysical and ecological data are available. Areas likely to
support deep-water coral communities were identified using
coral occurrence records derived from optical images taken by autonomous underwater vehicle (AUV) (Williams et al., 2010), and
geophysical data on depth, slope, aspect, rugosity, sidescan acoustic backscatter (a surrogate for substratum roughness and type),
and geomorphic zone (slope, crest, flat or depression) gridded at
5 × 5 m pixel resolution.
In the model, 70% of occurrence records (n ¼ 100) were used
as a training data set, and the remaining 30% used to test model
results. The 70/30 split of occurrence records was done randomly
using an option available in the Maxent program. The performance
of both training and test datasets was evaluated using receiver operating characteristic curves, with the area under the curve (AUC)
being a measure of model performance. AUC is a thresholdindependent measure of model performance ranging from 0–1.
An AUC value of 0.5 represents a model that performs no better
than random, whilst 1 is maximally predictive. AUC values in this
study were very high for both training (0.984) and test (0.977) data
sets, indicating good model performance. Maps of relative habitat
suitability were transformed into Boolean maps using a lowest
287
Coral reef habitat availability in the Great Barrier Reef, Australia
Figure 2. (a) Maps showing the distribution of NSS coral reefs and the three bank types defined in this paper in four selected locations,
representative of the different regions of the Great Barrier Reef. (b) Three-dimensional views of banks and NSS coral reefs in the GBR (same
areas shown in a), illustrating the occurrence of Types 1, 2 and 3 banks and their relative positions in cross-shelf transects. Vertical
exaggeration ¼ ×40. The images illustrate how Type 2 banks are located inboard of the outer-shelf barrier reefs in the northern GBR, whereas
Type 3 banks occur mainly on the outer shelf of the southern GBR.
presence threshold technique (Pearson et al., 2007) to identify areas
with a high probability of containing coral communities. This approach identifies the lowest probability value associated with an occurrence record, and considers all pixels with equal or higher probability
values as being suitable habitat (Pearson et al., 2007). The lowest presence threshold technique therefore provides a conservative estimate of
suitable habitat, identifying the minimum predicted area possible
whilst maintaining zero omission error in the dataset (Pearson et al.,
2007). In this study, the lowest suitability value for any occurrence
record was 0.14. Therefore, this value was used as the lowest presence
threshold.
Results
Geomorphic banks and NSS coral reefs
A total of 1581 geomorphic bank features were mapped in the GBR
(Figure 1), having a mean depth of 27.1 + 15.8 m and a total
surface area of 41 709 km2 in the region mapped (Table 1).
Within the same study area, NSS coral reefs have been mapped
by the GBRMPA using satellite and aerial imagery. Based on our
analysis, NSS reefs have a mean depth of 14.9 m and a negatively
skewed depth distribution (median depth of 12.0 m). NSS reefs
cover an area of 20 679 km2, of which 16 110 km2 (78%) is
located on top of submerged banks mapped in this study.
288
P. T. Harris et al.
Figure 2. (continued)
Therefore, 61% of geomorphic bank habitat (25 599 km2) is not
covered in NSS coral reefs.
Spatial relationship between banks and NSS reefs
Histograms illustrate that banks and NSS coral reefs exhibit a
similar spatial distribution along the length of the GBR. Both
banks and NSS reefs are most abundant in the northern GBR
between 108 and 128S, their occurrence reaches a low between
178 and 188S before increasing again between 208 and 238S. As
noted by Hopley et al., (2007), there is a trend for reef occurrence
and mean depth to be greatest in the northern GBR compared with
the southern GBR; here we document similar trends for geomorphic banks on the GBR shelf (Figure 1). The relevance to
the present study is that MCEs associated with geomorphic
banks are spatially concomitant with NSS reefs and are thus proximal to supply seed stock for their re-colonization.
mere 35 banks are completely (100%) covered by NSS coral
reefs. The mean depths of the 1145 banks partially covered by
NSS coral reefs exhibit a positively skewed, unimodal distribution
(Figure 3a). The 401 banks that do not support NSS coral reefs,
however, exhibit a bimodal distribution with modal peaks at
mean depths of approximately 27 m and 56 m (Figure 3a). The
mean and modal depths of banks, combined with co-occurrence
or non-occurrence of associated NSS coral reefs, suggests three different types of bank (Figure 3a). Type 2 banks have a mean water
depth of 27 m, a similar mean to Type 1 banks, but they are an
order of magnitude smaller in surface area and more irregular in
shape (larger perimeter/area (P/A) ratio) than the Type 1 banks
(Figure 4). Type 3 banks have a mean depth of 56 m but are otherwise geomorphologically similar (similar P/A ratio) to Type 1
banks (Figures 3a and 4). Type 2 banks are common in the northern GBR and are rare in the south, whilst Type 3 banks have the
opposite spatial distribution.
Three types of banks
Out of the 1581 banks mapped, 1180 of them (74.6%) have some
portion covered by NSS coral reefs, which are referred to here as
Type 1 banks (Table 1; Figure 2a and b). Almost all Type 1
banks are only partially covered by NSS reefs (n ¼ 1,145); a
Coral cover of submerged banks
Data collected from the test study site of 520 km2 located at
Hydrographer’s Passage (Figures 1 and 5) were used in this
study to investigate coral coverage of submerged banks.
289
Coral reef habitat availability in the Great Barrier Reef, Australia
Observations of depth, substrate characteristics and occurrence of
coral communities were used. In the test site there are nine geomorphic banks covering an area of 71.54 km2 and having a
mean coral coverage estimated at 55.4 + 22.7%.
Discussion
The potential area of MCEs in the Great Barrier Reef is suggested
by the area of submerged geomorphic banks not supporting NSS
coral reefs, which is equal to an area of 25 599 km2. But how much
of the 25 599 km2 of submerged bank area actually supports living
coral communities?
Only a few case studies of MCEs have been published for the
GBR (Bridge et al., 2011a, b; 2012). As noted above, 74.6% of
banks (1180 out of 1581) support some area of NSS coral reefs.
This large proportion of banks colonized by NSS coral reefs is
itself compelling evidence that most banks support at least some
corals and must therefore be considered as potential coral habitat.
Our results from Hydrographer’s Passage indicate the presence
of coral communities on Type 1 and 3 banks, covering about
55% + 23% of bank surface area (Figure 5). Extrapolating this
figure to geomorphic banks of the GBR, we estimate that
around 14 000 + 6000 km2 of bank area supports mesophotic
coral communities. When compared with the surface area of
NSS coral reefs (20 679 km2) it is apparent that the total area of
coral habitat in the GBR is much greater than previously
thought. Furthermore, the 20 679 km2 of NSS reef surface area
includes reefs with sandy lagoons and other habitat types that
are not ideal for coral colonization (Roelfsema et al., 2002).
Overall, it is evident that the surface area of preferred coral
habitat in the GBR is at least 50% larger, and may be as much
as double the size previously believed to exist.
Coral species diversity versus water depth
Figure 3. (a) Histograms of mean bank depths, for 1145 banks that
support from 0 to ,100% NSS coral reefs, plus 401 banks that do
not support any NSS coral reefs. The mean and modal depths of
banks, combined with co-occurrence or non-occurrence of
associated NSS coral reefs, suggests three Types of bank, as shown.
Type 2 banks are ,42 m in mean depth, while Type 3 banks are
.42 m in mean depth (b) Histogram of maximum depths of 675
different coral species occurrences based on IUCN Red List data
reported by Carpenter et al. (2008).
If MCEs located on submerged banks are to provide refugia for
NSS reefs then the question of coral species occurrence versus
depth must be addressed; if communities found in NSS reefs comprise species not found at depth, then deeper MCEs will not
provide a source of seed stock to recolonize them. Published research on depth ranges of coral species is sparse and there are
few published syntheses. Observations of submerged reefs and
banks using both SCUBA and remote imaging methods, such as
AUVs, reveal that zooxanthellate coral communities are
Figure 4. Conceptual diagram showing the three classes of bank identified in this study. Type 1 banks are the most common (n ¼ 1,145) have
the largest mean area (21 km2), support a NSS coral reef of some size, have a mean depth of 27 m, a mean height of 44 m and a perimeter/
area (P/A) ratio of 2.29. Type 2 banks do not have NSS coral reefs, are the smallest (mean area of 2 km2), have a mean depth of 27 m, a mean
height of 26 m and a P/A ratio of 7.06. Type 3 banks do not have NSS coral reefs, are of intermediate size (mean area of 8.5 km2), have a mean
depth of 56 m, a mean height of 36 m and a P/A ratio of 2.84 (see also Bridge et al., 2012). Holocene pinnacle reefs (Heap and Harris, 2008)
occur on all three types of bank but their vertical growth is restricted and they have not reached the sea surface.
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P. T. Harris et al.
Figure 5. Multibeam sonar 3D image showing results of Maxent predictive modelling of zooxanthelate coral community on Type 1 and 3
banks in the Hydrographers Pass area on the Great Barrier Reef (Figure 1; there are no Type 2 banks in the surveyed area). Within the area
mapped using multibeam sonar, nine banks occur covering a total area of 71.54 km2. Coral habitat (shown as black in the lower panel, draped
over the bathymetry) is predicted by the model to cover a total area of 43.10 km2, an average of 55% + 23% of the nine banks.
common features of submerged banks on the GBR outer-shelf to
depths of at least 65 m (Bridge et al., 2011a, b; 2012); Bridge
et al., (2011a, b) report rich communities of phototrophic megabenthos on even the deepest (Type 3) banks.
The available literature suggests that most coral species are
“depth-generalists”, occurring in both shallow and deep water,
with most species occurring from very shallow to at least 40 m
depth (Carpenter et al., 2008). Furthermore, coral diversity
peaks in intermediate depths of 15 to 30 m (Burns, 1985;
Huston, 1985; Cornell and Karlson, 2000) and reef fish diversity
also peaks at around 30 to 35 m (Cappo et al., 2007, Brockovich
et al., 2008). These depth ranges approximately coincide with
the mean depth of submerged bank habitat measured in the
present study (Figure 3a; Table 1). Taken together, these observations are consistent with the conclusion of Harriott and Banks
(2002) that a significant factor controlling coral occurrence is
the availability of hard substrate, in this case the antecedent reef
habitat provided by submerged banks.
Based on IUCN Red List data presented by Carpenter et al.,
(2008), 39% of the maximum depths of occurrence of corals is
at mesophotic depths of 30 m or greater. These maximum depth
values are most likely conservative, because recent studies of mesophotic reefs in the GBR (Bridge et al., 2012, in press) show that
many coral species are found to occur at greater depths than
those reported by Carpenter et al., (2008). The frequency distribution of maximum depths of occurrence of corals has a mean value
of 27.4 + 17.0 m (n ¼ 675; Figure 3b), a value comparable to the
mean depths of Type 1 and 2 banks measured in this study (26.8
and 27.2 m, respectively; Table 1). Therefore, 50% of Type 1 and
Type 2 banks in the GBR are in water depths suitable to at least
50% of coral species.
Regional and global significance
Studies from locations such as the Gulf of Carpentaria in northern
Australia, where NSS platform and patch coral reefs are absent, indicate that the Gulf contains submerged platform and patch reefs
that support MCEs that were only revealed through multibeam
sonar mapping combined with towed video and sampling
(Harris et al., 2004, 2008). In tropical northern Australia west of
Torres Strait, geomorphic banks on the continental shelf are estimated to cover 44 290 km2 (Heap and Harris, 2008), much of
which is potentially submerged coral reef habitat. Diverse coral
communities have been reported on MCEs in other regions (e.g.
Bare et al., 2010), and similar submerged banks and reefs throughout the Indo-Pacific are likely to contain coral communities comparable to those in the GBR. For example, in his global analysis of
the mean depths of carbonate platforms, Vecsei (2003) noted the
modal depth range for all atolls is 20– 30 m. Based on the available
evidence, it seems likely that submerged reefs and associated MCEs
are widespread throughout all of the world’s major coral reef provinces and spatially they extend well beyond the known ranges of
NSS reefs.
Coral reef habitat availability in the Great Barrier Reef, Australia
291
Table 2. Great Barrier Reef Zones, in order of decreasing amount
of protection, listing bank areas, numbers and percentages.
Program (NERP) and is a contribution of the NERP Marine
Biodiversity Hub. RB acknowledges a Queensland Smart Futures
Fellowship for salary support.
ZONE TYPE
Preservation Zone
Marine National Park Zone
Conservation Park Zone
Habitat Protection Zone
General Use Zone
Banks beyond GBR Marine Park
Area (km2)
of banks
included
190
7 301
654
12 983
3 157
1 315
Number
of banks
includeda
35
602
42
889
370
166
Percent
of banks
by area
0.7
28.5
2.6
50.7
12.3
5.1
Preservation and Marine National Park and Conservation Park zones provide
a “no take” level of protection. Bottom trawling is prohibited in all except
the General Use Zone. The study area covers the entire GBR Marine Park,
allowing for an assessment of the protection of bank habitat that exists
within the park zoning plans. The table shows that although only 0.7
percent of the banks are within “Preservation Zones” that offer the highest
level of protection, approximately 87.7% of banks within the GBR marine
park (and 82.6% of banks mapped in this study) are protected from bottom
trawling. aParts of banks may occur in more than one zone.
Implications for management
A major rezoning of the Great Barrier Reef World Heritage Area in
2004 resolved to designate a minimum of 20% of each habitat type
(referred to as “bioregions”) as no-take protected areas (Fernandes
et al., 2005). Although the lack of data on submerged reef habitats
meant they were not considered when determining bioregions, approximately 30% of banks currently occur within designated
no-take areas and 87.7% of banks that occur within the GBR
marine park are protected from bottom trawling (Table 2). This
result is consistent with our conclusion (above) that submerged
banks and NSS coral reefs exhibit the same latitudinal distribution
within the GBR (since NSS reefs were targeted by the GBRMPA for
protection). It also suggests that the GBRMPA’s approach for
design of conservation zones allows for uncertainty and protects
future unknown habitats. However, further research is required
to determine whether the 30% of submerged reefs and banks
that have been protected are those supporting significant areas
of MCE habitat, or if they have the greatest chance of surviving
the impacts of global ocean warming (or other anthropogenic
pressures).
Our results show that the spatial extent of coral reef habitat in
the GBR Marine Park may be underestimated by as much as 100%,
despite the GBR being one of the best-studied coral reef ecosystems on earth. Other reef areas that have received less research
effort may also contain a substantially greater amount of reef
habitat than is currently assumed. Apart from some immunity
from coral bleaching, the water depths of mesophotic coral reefs
may protect them from a range of other threats, such as severe
tropical storms, the frequency and intensity of which is predicted
to increase with climate change (Emanuel, 2005). In the Coral
Triangle, the global epicentre of coral reef biodiversity, many
reefs have suffered from destructive fishing methods such as dynamite and cyanide fishing. Due to both inaccessibility and poor
knowledge of their location, submerged coral reefs (MCEs) seem
likely to have escaped many of these pressures. It is therefore critical to identify and effectively manage submerged reef habitats not
only on the GBR, but around the world.
Funding
This work was produced with the support of funding from the
Australian Government’s National Environmental Research
Acknowledgements
GBR_Features dataset courtesy of the Great Barrier Reef Marine
Park Authority. Thanks to Lachlan Hatch (Geoscience Australia)
for assistance with GIS analysis and statistics. PTH, SLN and
BPB publish with the permission of the Chief Executive Officer,
Geoscience Australia.
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Handling editor: Rochelle Seitz