The fact that Lophelia pertusa was most found significantly more on sand/mud (class 1), as well as on reef (class 4) than on gravel and bioclastic may be linked to a number of environmental parameters. Brooke et al. (2009) state that high sedimentation and suspended material in the benthic regions can have huge effects on L. pertusa mortality, leading to smothering of the polyps, while Lim et al. (2017) noted that there is a high intensity of bottom currents coming from the south to north orientation, along with orientated sediment waves coming from the east to west direction. These high energy bottom currents and sediment waves have in turn led to scouring on the outer edge of the Piddington Mound. Hence, due to these currents and high sedimentation, L. pertusa may only be present in the centre of the mound in sand and reef sediment classes to reduce mortalities from smothering or high speed currents tipping these corals over. The currents are weaker in the centre of the mound due to protection from corals, thus providing food particles whilst clearing the cold water corals from high sedimentation which may lead to smothering. Lophelia pertusa is commonly found where there is a current speed of 0.28 m s− 1 and 0.70 m s− 1 (Robert et al., 2005) and this was noted on the Galway Carbonate mound in Ireland. Anything greater than this speed may tip L. pertusa over. Robert et al., 2005 again state that these cold water corals have an upper tolerance limit of around 1.0 m s− 1L. pertusa is most commonly found on hard substrates and can attach to something as small as a pebble. In the north-east Atlantic, most L. pertusa are associated with the Pleistocene period, often referred to as the Ice Age (Roberts et al., 2006; Davies et al., 2008). Along the Piddington Mound, there were many glacial dropstones and pebbles present which may be associated with the late Pleistocene period. Although L. pertusa was mainly found along sand/muddy areas, it needs a hard substrate to attach itself to (Rogers, 1999). This may even be as small as a pebble or a benthic tube worm that is subsequently buried beneath the sediment. Lophelia pertusa may also grow on reef such as the reef formed in the Piddington Mound.
The second most dominant species of stony coral found on the Piddington Mound was Madrepora oculata with a high sediment tolerance ((Brooke et al., 2013). Cairns (1982) pointed out that it is found at an average depth of 2,020 m and rely on a zooplankton diet. However, when compared to L. pertusa which has mono-unsaturated fatty acids along with d15N (Roberts et al., 2006), it has much smaller polyps (Waller & Tyler, 2005), which allows L. pertusa to catch larger zooplankton. Therefore, it may be possible that these two scleractinian species have adapted different feeding habits to prevent outcompeting, since in the present study, similar to L. pertusa, Madrepora was found to be most dominant on sand and/or mud. Furthermore, since Madrepora is a fragile stony coral however, and it most likely found co-occurring with other stony corals such as Lophelia pertusa, as was confirmed in the present study. However, M. oculata was also found in significantly large quantities on the north-west side of the mound where gravel and coral rubble were present.
In the present study, a quantitative relationship between the abundance of cold water corals and other benthic megafauna and seabed terrain parameters was tested. High coral abundances tend to be observed where there is rough seabed terrain nearing the centre of the Piddington Mound. Gage & Tyler 1991 discuss how aspect can provide information on seabed exposure and is important is benthic habitats. In this present study, aspect was weakly correlated with coral location or percentage of coral but shows signs of correlation near the northern section of the mound for certain specimens. This correlates with Lim et al., 2017 study suggesting a strong south-north bottom current, where species may be thriving on the northern side as the currents may be too strong on the southern side. Both slope and bathymetry showed strong correlation with specimens confirming that other terrain parameters highly influence in coral orientation and situation. Slope, being a proxy for high velocity currents at carbonate mounds showed coral coverage associated with high slopes (Yellow Zoanthidea; Alyconea sp.; Antipatharia sp.) where food particles may be present This has been shown in previous studies Frederiksen et al. 1992, Dorschel et al. 2005 coinciding with the Galway Mound, west of Ireland. Furthermore, the data illustrated in. Fig. 1 to 4 shows narrow terrain ranges for certain species. Phelliactis sp. is a suspension feeder which normally situates itself facing the upwelling current (Tyler, 2003). It is generally found in muddy sediment and was situated around the centre of the Piddington Mound, where it could face upwards currents as suggested by Tyler (2003). Species that showed a narrow aspect range were Brachyuran crabs which are burrowing crabs and are often found on muddy, soft substrates (Bellwood, 2002) and have been noted on other cold water carbonate mounds on the Rockall Trough (Duineveld, et al., 2007; Oevelen et al., 2009); Yellow encrusting sponge, Antipatharia sp., which have been observed on mud-draped boulders and gravel in previous studies (Henry & Roberts, 2014) Pink and Yellow Gracilechinus urchin was most often found on mud and gravel which agrees with Bay-Nouailhat (2008) study on its preferred habitat type.
Lophelia pertusa, being the most dominant cold water coral species found on the Piddington Mound has an average depth of 200-1,000 m on carbonate mound structures (Rogers, 1999; Roberts et al., 2006). The present study agrees with this, since the depth at which Lophelia pertusa was found on the Piddington Mound in 2011 (Lim et al., 2017) was at an average of 980 m, confirming data from previous years (Squires, 1959). Rogers (1999) also states Lophelia pertusa is found at an average temperature between 4- 12 °C. Furthermore, Thiem et al., (2006) claim that cold water corals being sessile feeders, are found where there is high rates of organic particles present. This confirms Lim et al., 2017 study on the biogeology of the spatial distribution of the Piddington Mound that states enhanced bottom currents concentrate organic particles to these carbonate mounds. Dorschel et al., 2005 & Fosså et al., 2002 both state how L. pertusa reef structures are commonly found where regions of elevated benthic flow such as on carbonate reef mounds are present.
The factors controlling the distribution of cold water coral mounds in the North Atlantic are not well understood. The Darwin Mounds, located on north west coast of Scotland (De Santo, 2013) is a cold water coral province that is located at a depth of 1,000 m (Wheeler et al., 2008). However, it is important to note that these are sand mounds and not carbonate. It is dominated by Lophelia pertusa and Madrepora oculata (Wheeler et al., 2007) and is 70 m in diameter. It is suggested that the Darwin Mounds, formed as a result of sand volcanoes caused by water escape. This provided settlement for the first species of cold water corals (Masson et al., 2003).
The Darwin mounds, however, show similar scouring patterns and sediment accumulation to that of the Piddington Mound (Wheeler et al., 2007). Furthermore, CTD data from the Darwin Mounds also suggested an average temperature ranging between 7.8-8.5 °C, similar of that to the Piddington Mound.
The Galway Mound, is a giant double-peaked carbonate mound at 100 m in height, over 1.6 km long and 800 m in diameter. The mound is topped by dense reefs of L. pertusa and M. oculata, and thus it is termed a fully “active” mound (Huvenne et al., 2005). Compared to the Moira Mounds which are clusters of hundreds of small, mostly subcircular-shaped mounds, about 5–10 m high and 15–40 m across (Wheeler et al., 2005b). They occur in areas of active sediment transport on rippled sand sheets, with glacial dropstones and corals scattered between mounds (Foubert et al., 2005; Wheeler et al., 2005b). These mounds may represent the “incipient” early developmental stage of large carbonate mounds (Wheeler et al., 2005b).
In this present study, spatial organisation was seen on the Piddington Mound for specific species. Coral facies distributions, current induced seabed features and sediment size distributions over the Piddington Mound highlights a correlation between the abundance of living CWC with areas of enhanced bottom currents (Wheeler et al., 2011; Lim et al., 2017; Lim et al., 2018).
Aspect showed weakly correlated results whilst rugosity, slope and bathymetry showed a spatial distribution between certain species in abundance and terrain parameters. Similar species had narrow seabed terrain ranges on the Piddington Mound. However, it is suggested that future studies should be carried out on the surrounding megafauna and their distribution on the Piddington Mound as little is known about its surrounding fauna. With this kept in mind, it is suggested that this approach could provide the basis for predicting the distribution of cold-water corals and its surrounding megafauna. Further studies must also be carried out in determining accurate environmental tolerances and factors for species within the Piddington Mound.