Drivers of distribution

Studies suggest that a mix of chemical, physical and biological factors determine distribution and these are summarised in Fig.3.

Fig.3. Summary of factors affecting the distribution of cold-water coral reefs.

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).

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Radial DiagramCold-water coral reefs have a cosmopolitan distribution and are located from high to low latitudes, within all of the major ocean basins (Fig.1). The most widely recognised reefs are those on the Norwegian and Irish Margins which include the Darwin Mound, (e.g. Fosså, 2002, Mol et al. 2002; Wheeler et al., 2005). Recent studies have identified reefs in the Gulf of Cadiz (Wienberg et al., 2009), Gulf of Mexico (e.g. Cordes et al., 2008) and on the Angola Margin, off the west coast of Africa (Guilloux et al., 2009). Research bias within developed countries tends to skew distribution patterns (Roberts et al., 2009), hence global distribution is likely to be greater than current evidence suggests, especially in areas such as the Indian Ocean (Davies et al., 2010).

Cold-water corals reefs occur on a variety of geological features: seamounts, ridges, canyons, continental slopes and shelf margins (Mol et al., 2002, … ). The size of the reefs also varies in surface area from small patches approx. 10m wide to reefs 2000 km across (Ref). In terms of depth, reefs have so far been found at depths from 50 to 1000 m with some as deep as 4000m e.g. …. Studies suggests that a suite of chemical, physical and biological factors determine distribution and these are summarised in Fig.2.

Cold water reef-forming corals are mostly found between 4-12°C (Rogers, 1999; Roberts et al., 2003) with species-specific distribution controlled by thermal tolerance e.g. range of 6-8°C for Lopehlia pertusa (Fossa, 2002). Thermal tolerance has a subsequent impact on physiological function and calcification rates (Guinotte et al., 2006; Dodds et al., 2007). Hydrodynamics are a key driver of distribution with currents forming as a result of downwellings, water channelled down canyons and over topographic highs (Davies et al., 2009), or through the disruption of internal waves (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). Currents also reduce the risk of being smothering and control larval supply, which determines the local and spatially patterning of reefs (ref#_). Since cold-water corals are sessile, they rely on currents to supply nutrients which are either photo-detrial material or zooplankton carried by near bottom currents (Guinotte et al, 2006). Reefs have been associated with topographic highs in areas of high surface productivity for these reasons(Guinotte et al, 2006). Substrate is needed for larval settlement and development of the coral framework and is one reason for the distribution of reefs on volcanic mounds, atop glacial deposits and growing on the sides of canyons. The reef structure also provides a suitable substrate and primary framework corals e.g. L. pertusa develop first with a settlement of oto-corals e.g. on the corals. Alkalinity strongly determines the formation of reefs through the availability of aragonite, a soluble form of CaCO3 for skeletal formation. 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). The last factor of consideration is depth which controls ultimately determines the factors discussed. Cold-water reefs occur at shallow depths but also deeper, but only at sites facilitating nutrient supply and gamete dispersal (ref).

Radial Diagram

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