coral formations than anticipated hitherto (Freiwald et al., 2009). Habitat
modelling has
thus far mostly been applied to a few of the most common species at a global and
regional scale (Rengstorf et al., 2013; Yesson et al., 2012) at a coarse spatial resolution
(Ross and Howell, 2013). However, models are now being applied at finer resolution
levels in order to guide surveys with the visual tools of remotely-operated and manned
submersibles (Georgian et al., 2014). Additional fine-grained and broad-scale habitat
modelling, specifically incorporating the best available taxonomic identifications (Henry
and Roberts, 2014) is still needed to discover additional habitats, and to forecast the
fate of CWC facing both direct (fisheries) and indirect (environmental) impacts (Guinotte
et al., 2006; Clark and Tittensor, 2010).
Cold-water coral reefs, mounds, and gardens support a highly diverse community,
comprising faunal biomass that is orders of magnitude above that of the surrounding
seafloor (Mortensen et al., 1995; Henry and Roberts, 2007; Cordes et al., 2008; Roberts
et al., 2008; Rowden et al., 2010). In addition to this tightly-associated community, cold-
water corals may also serve as important spawning, nursery, breeding and feeding areas
for a multitude of fishes and invertebrates (Koslow et al., 2001; Fossa et al., 2002;
Husebo et al., 2002; Colman et al., 2005; Stone, 2006; Ross and Quattrini, 2009; Baillon
et al., 2012; Henry et al., 2013), and habitat for transient diel vertical migrators (Davies
et al., 2010). The ability to construct massive calcium carbonate frameworks, which
makes both shallow and deep-water coral reefs unique, provides an important
biogeochemical function in both the carbonate system (Doney et al., 2009) and in
calcium balance (Moberg and Folke, 1999). CWC skeletons also provide an information
function (sensu de Groot et al., 2002) through their archiving of paleoclimate signals
(Adkins et al., 1998; Williams et al., 2006). Besides this, CWC ecosystems possess an
inherent aesthetic value (sensu de Groot et al., 2002) demonstrated through countless
films, photographs, and paintings of reefs or reef organisms.
Cold-water corals and the communities they support rely on surface productivity as
their primary source of nutrition; either through the slow, relatively steady deposition of
particulate organic carbon (POC) in the form of marine snow, which may be enhanced
by hydrographic mechanisms (e.g. Davies et al., 2009; Kiriakoulakis et al., 2007), or
through more active transport of carbon provided by vertical migrators (Mienis et al.,
2012). However, L. pertusa has been shown to incorporate everything from dissolved
organic carbon (DOC) to POC to algal biomass to small zooplankton (van Oevelen et al.,
2009). As in shallow-water systems, corals and sponges of the deep reefs recycle these
nutrients and form both the structural and trophic foundation of the ecosystem. In
addition to these ties from shallow to deep water, the transport of nutrients from deep
to shallow water is accomplished both by the diel vertical migrations of plankton and
small fishes (Davies et al., 2010) as well as by periodic down- and upwelling that can
occur near some of the reefs (Mienis et al., 2007; Davies et al., 2009). Although the
mechanisms for deep-to-shallow water transport are well established, the input of
deep-water secondary productivity to shallow ecosystems remains unquantified.
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2.
Features and Trends
All geological structures mentioned share some environmental factors that facilitate
coral settlement and subsequent growth: provision of current-swept hard substrate,
and often topographically-guided hydrodynamic settings. It has been suggested that
corals are preferably confined to narrow seawater density (Sigma-theta) envelopes
(Dullo et al., 2008) in which along-slope larval dispersal propagation may be facilitated.
Survival and growth may be most closely associated with specific hydrodynamic settings
including tidal-driven internal-wave fronts hitting continental slopes and seamounts
(Mienis et al., 2007; Henry et al., 2014), specific up- and downwelling currents affecting
the summits of shallow-water seamounts (Ramirez-Llodra et al., 2010), and tidal-driven
downwelling phenomena on inner shelf settings (Davies et al., 2009; Findlay et al.,
2013). These hydrographic transfer processes tend to concentrate or prolong the
retention time of nutrients and food that sustain the metabolic demands of the
suspension-feeding community.
Another perspective on the occurrence of coral habitat is a combined biogeophysical
and hydrochemical analysis of the ambient seawater, a very recent endeavour in the still
young research history of cold-water coral systems (e.g., Findlay et al., 2014; Flögel et
al., 2014; Henry et al., 2014; Lunden et al., 2013). These forms of data along with species
presence data were incorporated into global habitat suitability study by Davies and
Guinotte (2011) that was conducted on the six major cold-water framework-building
corals (Enallopsammia rostrata, Goniocorella dumosa, Lophelia pertusa, Madrepora
oculata, Oculina varicosa and Solenosmilia variabilis) using the Maximum Entropy
modelling approach (MAXENT). This approach uses species-presence data, global
bathymetry 30-arc second grids (1 km
2
resolution) and incorporates environmental data
from several global databases. Viewed on such a global scale, these corals generally
thrive in waters that: (1) are supersaturated with respect to aragonite, (2) occur
shallower than 1500 m water depth, (3) contain dissolved oxygen concentrations of >4
ml l
-1
, (4) have a salinity range between 34 and 37 ppt, and (5) show a temperature
range between 5 and 10°C. Laboratory experiments have confirmed many of these
ranges, with L. pertusa being the most commonly studied species. Mediterranean L.
pertusa and
M. oculata colonies survived and grew at 12
o
C for three weeks, with M.
oculata showing a greater sensitivity to high temperature (Naumann et al., 2014). Gulf
of Mexico L. pertusa colonies survived and grew at up to 12
o
C, but died when exposed
to 14
o
C for 8 days (Lunden et al., 2014). Studies of L. pertusa from west of Scotland,
United Kingdom of Great Britain and Northern Ireland, demonstrated that this
species
can maintain respiratory independence and even survive periods of reduced oxygen
(Dodds et al., 2007).
However, some remarkable outliers to these trends exist in the Red Sea and the Gulf of
Mexico. The Red Sea represents the warmest and most saline deep-sea basin on Earth,
with temperatures >20°C throughout the water column and salinity in excess of 40 ppt.
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