Studies suggest that a mix of chemical, physical and biological factors determine distribution and these are summarised in Fig.3.
Cold water, scleractinian corals are mostly found between 4 and 13°C (Freiwald et al., 2004), with species-specific distribution controlled by thermal tolerance e.g. range of 6-8°C for Lopehlia pertusa (Fosså, 2002). This is because thermal tolerance has an impact on physiological function and calcification rates (Guinotte et al., 2006; Dodds et al., 2007). Cold-water corals are located in water of 32-39 PSU and it is thought that a combination of stable temperatures and salinity in the deep-sea are major drivers for determining distribution of reefs (Freiwald et al., 2004).
Hydrodynamics are a key driver of distribution with currents created by downwellings, channelling through canyons or over topographic highs (White et al., 2005; Mienis et al., 2007; Davies et al., 2009;). Currents are also formed by the disruption of internal waves between water layers (Frederiksen et al., 1992). All these processes increase food supply through the re-suspension or concentration of organic materials (Mortensen et al., 2001; Thiem et al., 2006). Since corals are sessile, they rely on currents to supply photo-detrial material and zooplankton and this explains the location of many reefs on topographic highs, below areas of high productivity in surface waters (White et al., 2005) . Currents also reduce the risk of smothering and control gamete and larval dispersal (Waller, 2005).
Substrate is needed for larval settlement and development of the coral framework and is one reason for the distribution of reefs atop features topograhic highs (Freiwald, 2002). This location also takes advantage of the suitable hydrodynamic conditions (e.g Wheeler, 2005). The reef framework formed by primary framework corals e.g. L. pertusa, provides a suitable substrate for the settlement of other corals such as octocoralline soft corals and sea fans (Freiwald et al., 2004).
Alkalinity strongly determines the formation of reefs through the availability of aragonite, a soluble form of CaCO3 for skeletal formation (Guinotte et al.,2006). The aragonite saturation horizon (ASH) describes the distribution of aragonite by depth and studies show that large areas of cold-water corals occur in the North Atlantic because this horizon is deeper (Guinotte et al.,2006).