Not to be cited without prior reference to the authors

ICES CM 2007/B:10

Mechanisms of bio-physical coupling at submarine bank ecosystems

Christian Mohn and Martin White

Dept. Earth and Sciences, National University of , Galway, Ireland

Abstract

Submarine banks, like many other isolated or quasi-isolated topographic features have long been recognized as important hot spots of bio-physical interactions. At a smaller spatial and temporal scale, physical processes can have a significant effect for chlorophyll and benthic dynamics. A sufficiently long residence time of primary production is important for any sessile benthic community resident over the topography to (i) transfer surface productivity to higher trophic levels or to (ii) transfer organic material directly to benthic communities before it is lost to the system. An important benthic ecosystem at the Rockall and Porcupine Bank slopes at the European is that of cold water corals. These reef-forming corals are generally found in regions of strong benthic currents and enhanced surface productivity. We analysed several years of remote sensing data (SST, Chlorophyll-a) to identify robust bio-physical distribution patterns. In a second step, data from recent surveys (ADCP and current meter data) and results from model simulations were used to investigate the relative importance of physical processes on various spatial and temporal scales (rectified flow, , internal waves) as a possible feeding mechanism for benthic communities at these locations.

1. Introduction

The oceanic regions west of Ireland have always been considered as an important area for the accumulation and propagation of warm and saline North Atlantic waters to the Arctic Ocean where they are transformed to cold, fresh deep water and contribute to the North Atlantic thermohaline overturning circulation. They are also a transition zone between the subpolar and subtropical gyre systems. A wide spectrum of localised dynamics associated with the complex and abrupt topography of the region is superimposed to the far field forcing, including strong barotropic and internal tidal activity, up- and events and activity. These processes act together to support a variety of biological phenomena and patterns, such as enhanced primary and secondary production as well as aggregation and retention of biological material and higher tropic organisms. In this study, a special focus is given to processes and patterns associated with the large submarine banks including the Porcupine Bank / Irish shelf transition zone and the Rockall Bank (see Fig. 1). We introduce and summarize concepts and mechanisms of physical forcing as an important contributor for biological variability to illustrate the importance of a better understanding of the different aspects of bio-physical coupling in the region.

60oN

RB 0.2 57oN

0.5 1.0 RT

2.0 54oN 3.0 0.2 IS 4.0

1.0 0.5 o 51 N PB 2.0 0.1 PS CS

GS 48oN

45oN 20oW 18oW 16oW 14oW 12oW 10oW 8oW 6oW 4oW

Figure 1: Map showing the study area. Depth contours are in km and topographic features are labelled CS (Celtic Shelf), GS (Goban Spur), IS (Irish Shelf), PB (Porcupine Bank), PS (Porcupine Seabight), RB (Rockall Bank) and RT (Rockall Trough).

2. Material and methods

Based on a combination of literature review and data analysis we describe the spectrum of physical processes associated with the submarine banks of the Rockall Trough and their potential to affect biological material transport as well as particle aggregation, trapping and retention. We analysed 8 years (1998-2005) monthly composites of remotely sensed SST and Chlorophyll-a (SeaWiFS) data to identify spatial bio-physical distribution patterns and their temporal variability. These data were combined with results from model simulations using the 3-dimensional ocean circulation model SPEM (s-coordinate primitive equation model). To describe the different aspects of benthic dynamics we reviewed results from early and recent studies and summarized possible implications for benthic ecosystems.

3. Physical controls

The most notable circulation feature at the continental margin is the shelf edge current (SEC), defined by a distinctive high salinity core centered at depths between 200 - 500 m (e.g. Hill and Mitchelson-Jacob, 1993). The inter-annual signal is the dominant mode of the SEC variability. In the Rockall Channel it is described as a narrow (20-50 km), but steady poleward with typical velocities varying between 10 cm/s in summer and up to 30 cm/s in winter (e.g. Booth and Ellett, 1983). The SEC is less energetic at the more southerly regions between the northern Bay of Biscay and Goban Spur (Celtic and Armorican slopes) with some evidence of occasional reversal of flow: Pingree and Le Cann (1990) identified a persistent northwestward slope residual current of 6 cm/s accompanied by a weak southeastward counter-current of 2 cm/s on the outer Celtic shelf. At Goban Spur, maximum poleward flow also occurs in winter (December – January), whereas a remarkable modulation of the upper slope current amplitude and direction is manifested in spring (March-April) and autumn (September-October). In these periods the northward along-slope flow turns into an equatorward flow with occasional inertial overshooting into the deep oceanic regions west and north of Goban Spur (Pingree et al., 1999). This seasonal variability pattern is known as SOMA (September-October, March-April) and occurs as a response to seasonal changes of the local mean wind stress field. However, this response mainly affects the upper SEC layers, whereas near-bottom flow appears to be more topographically controlled with a generally northwestward and along-slope direction (Pingree et al., 1999). The Porcupine Bank and Porcupine Seabight region west of Ireland is generally considered as a critical area for the continuity of the SEC. A continuous poleward flow along the Porcupine Seabight margins is less readily observed. However, there is observational evidence for a weak mean along-slope flow in the near-bottom layers of the eastern and northern Porcupine Seabight in the order of 5 cm/s. West of Porcupine Bank, the SEC is apparent at all available depth levels with an average velocity of 10 cm/s. At the northern Porcupine Bank poleward flow again increase to magnitudes which are frequently observed along the Scottish slope (e.g. White and Bowyer, 1997; Mohn, 2000).

Porcupine and Rockall Bank are large submarine topographic features at the entrance of the Rockall Trough. Whereas Rockall Bank is largely isolated from its surroundings, Porcupine Bank is attached to the shelf break, separated from the Irish continental margin by a shallow (300 m deep) channel (see Fig. 1). Systematic observations over longer periods are sparse, but scientific attention to these areas largely improved over the last decade with the discovery of giant carbonate mound provinces and reef-forming cold water corals with species-rich benthic communities (Roberts et al., 2006). A frequently observed feature at both banks is a region of cool temperatures above the summit regions compared to warmer waters of the surrounding oceanic areas (e.g. Mohn and White, 2007). The cold cores are associated with a closed, clockwise, bottom-intensified recirculation cell along the bank slopes (see Fig. 2). The time-mean solution of this phenomenon is commonly referred to as a Taylor cap and is generated by impinging far field currents and/or the resonant generation of large amplitude topographically-trapped waves through diurnal tidal currents. It has been described for tidally dominated regimes (e.g. Beckmann and Haidvogel, 1997) as well as locations with prevailing low-frequency forcing (e.g. Chapman and Haidvogel, 1992). Enhanced turbulent vertical mixing (e.g. Kunze and Toole, 1997), internal tidal activity and uplifting of cold, nutrient rich water (e.g. Genin and Boehlert, 1985) also contribute to the local flow dynamics and may generate favourable conditions for enhanced biological activity and biodiversity (e.g. Genin, 2004).

At Porcupine Bank, the clockwise recirculation pattern is most likely generated by a combination of diurnal tidal rectification and the influence of the SEC. However, it is strongly modulated by seasonal changes of the far field forcing. In summer, tidal rectification is the major contributor to the observed flow pattern over the bank, whereas in winter, with the intensification of the SEC, tidal influence is decreasing and a more asymmetric recirculation with enhanced flow along the western slopes can be expected. In addition, the Taylor cap flow over Porcupine Bank is weak in periods of strong stratification and strengthens towards winter when the seasonal is replaced by vertically mixed surface layer (Mohn et al., 2002). White et al. (1998) found that a Taylor cap and associated doming of cold, nutrient rich water over Porcupine Bank persisted from April to July 1995. However, Kloppmann et al. (2001) demonstrated that severe storms can partially destroy the Taylor cap over the bank summit with significant consequences for the biological environment. A steady impinging flow of similar magnitude as the SEC is not present at Rockall Bank, and tidal rectification is assumed to be the dominant process for the formation of a time-mean along-isobath flow (Mohn and White, 2007). A comprehensive image of the mean flow at Rockall Bank based on observations alone is not available for many parts of the bank. However, from a series of historical (Dooley, 1984) and recent current meter data (White, pers. comm.) there is strong evidence for a symmetric, clockwise residual re-circulation centered at the 500 m isobath with maximum speeds of up to 20 cm/s. In addition, strong amplification of the major diurnal tidal constituent K1 seems to be a persistent phenomenon at or near the topographic boundaries of Rockall Bank. Historic observations by Huthnance (1974) revealed a up to 20-fold amplification of the K1 compared to typical amplitudes in the oceanic far field outside the bank, recently supported by measurements of White (pers. comm.). It is still unclear, however, whether this recirculation pattern is limited to the near-bottom layers or is extending towards the sea surface. White et al. (2002) found large meso-scale variability in the upper 1000 m at some locations along the outer eastern flank, but a persistent southwestward flow in deeper layers. Mohn and White (2007) presented model results showing a bottom-intensified clockwise recirculation inside the 1000 m isobath extending throughout the whole . The model results were supported by SST remote sensing data indicating low surface temperatures over the summit to be comparatively robust against seasonal variations (Mohn and White, 2007).

3.2 Implications for marine biology

In this section we present and discuss different aspects of possible bio-physical interactions at Porcupine and Rockall Bank. The European continental margin is characterised by its widely distributed deep water ecosystems and its role as a major spawning and nursery are for commercially important fish species. With the discovery of highly abundant carbonate mounds and reef-forming cold water coral communities along the European continental margin (Freiwald, 2002) a better understanding of the main factors controlling food supply to bentho-pelagic species communities was the main focus of subsequent research activities. The number of living deep water corals is particularly high around the and submarine banks of the Rockall Trough (e.g. Roberts et al., 2003; Kenyon et al., 2003). These corals are reef-forming and are among the most diverse ecosystems in the ocean. Enhanced levels of primary and secondary production in these areas also provide a rich food source for local fish populations, such as Atlantic mackerel and the deep-water species Orange Roughy. Many studies emphasize the important role of physical processes and their variability to generate and maintain energy and mass flux to higher trophic levels at abrupt topographies (e.g. Genin, 2004).

3.2.1 Bio-physical distribution patterns

An analysis of characteristic bio-physical patterns and their variability showed that temporally robust patterns of SST and Chlorophyll-a over are closely related to the location of major frontal zones and transition areas (Fig. 3a). Chlorophyll enhancement is strongest on the Irish shelf and is associated with the Irish Shelf Front. Other chlorophyll extrema can be found above Porcupine and Rockall Bank where a combination of Taylor cap and non-linear tidal dynamics contributes to a substantial enrichment and retention of cold, nutrient rich waters and phytoplankton over the shallow summit regions. This is supported by the observations of White et al. (1998) who found enhanced nutrient levels over the Porcupine Bank summit persisting over a 3 month period from April to July 1995. The seasonal onset of the spring bloom is the dominant mode of variability (Fig. 3b). It is highest in the central Rockall Trough and lowest on the Irish shelf. Over the banks seasonal variability is moderate compared to the central Rockall Trough indicating generally elevated or retained Chlorophyll levels. The standard deviation of monthly anomalies as a measure of the inter-annual variability clearly emphasizes the bank regions as areas of generally low variability and robust spatial distribution patterns (Fig. 3c).

mean (1998-2005) seasonal variability inter-annual variability

(a) (b) (c)

Figure 3: SeaWiFS Chlorophyll-a distributions: (a) climatological average, (b) standard deviation of the climatological mean, (c) standard deviation of monthly anomalies.

Mohn and White (2007) analysed the annual climatological average of SST and chl-a in more detail along transects through the central summit areas of Porcupine and Rockall Bank (Fig. 4 a, b). The annual mean SST at both transects are dominated by an uplifting of isotherms resulting in a cold summit signal. SST values over Porcupine Bank are 0.3ºC and 0.1ºC cooler than surrounding oceanic and shelf/slope waters, respectively. At Rockall Bank the cold dome is even more pronounced with and SST decrease of 0.3ºC and 0.7ºC relative to Hatton- and Rockall Trough levels, respectively. Climatological chl-a levels over Porcupine Bank are slightly enhanced, but generally lower than shelf/slope concentrations (Fig. 10 b). At Rockall Bank, however, the annual mean concentration exceeds Porcupine Bank values by 30%. The spring bloom is the major contributor to the elevated levels over the Porcupine Bank summit, whereas even in periods of light limitation a substantial part of biological material is retained over Rockall Bank to add to the observed high chl-a levels.

chl-a SST 0.9 0.9 12.5 11.2 0.8 0.8

12.4 11 0.7 0.7

0.6 0.6 12.3 10.8 0.5 0.5 12.2 10.6 0.4 0.4 -16 -14 -12 -16 -14 -12

Figure 4: Climatological averages (1998-2005) of SST and chl-a along transects at the central summit regions of Porcupine (black) and Rockall Bank (blue).

3.2.2 Passive particle

Previous studies provided some evidence for the Porcupine Bank as a significant retention area for biological material. This was confirmed by modelling studies of the early life history of mackerel of Bartsch and Coombs (2001) showing reduced dispersal and subsequently reduced spread of larvae length classes. However, Kloppmann et al. (2001) also showed that strong wind events are capable of weakening or disrupting the retentive circulation over the bank with larvae being advected out of the bank region into a different physical regime. Mohn and White (2007) quantified the retention time of passive biological particles for an idealized (no wind) summer regime based on model simulations. The retention time at Porcupine Bank was estimated to be 75 days (95 days) at the surface (bottom) and in excess of 150 days at Rockall Bank (Mohn and White, 2007). The larger and more persistent concentration of biological material over Rockall Bank would appear due to its isolated nature as well as its size. In contrast, Porcupine Bank is exposed to the poleward flowing SEC which may strip particles from the central summit region resulting in a weaker retention and enhanced particle dispersal (Fig. 5).

Surface Bottom 90 days 180 days 90 days 180 days

Figure 5: Predicted passive particle distribution at 90 and 180 days after particle release in the surface (left panel) and bottom (right panel) layers at Porcupine and Rockall Bank based on the model simulations by Mohn and White (2007).

3.2.3 Benthic dynamics

At near bottom layers low frequency forcing becomes the dominant contributor to particle dynamics including amplification of diurnal tides and internal tidal activity.

Porcupine Bank Rockall Bank

200 200

100 100 ) ) m m

(m) 0 (m) 0 ( ( vertical p p

d d d d −100 −100

−200 −200

0 20 40 60 0 20 40 60 days adaysfte afterr t rfloatac releaseer release days adaysfte afterr t rfloatac releaseer release

+ + horizontal + +

Figure 6: Relative along-track particle depths (m, upper panel) and particle paths (plan view, lower panel) for individual Lagrangian bottom floats at Porcupine and Rockall Bank from model simulations.

Huthnance (1974) and recent observations by White et al. (2007) report strong amplification of diurnal tidal currents at Rockall Bank and Porcupine Bank. However, the magnitude of flow amplification at Porcupine Bank is significantly lower. Mohn and White (2007) showed that near- tidal oscillations may help to establish a periodic pulse of retained material, but the overall effect of tidal forcing for passive particle drift strongly varies between the two banks (Fig. 6). At Porcupine Bank tidally induced particle excursions are small in the order of 20-50 m and particle drift is mainly controlled by the quasi-steady far field flow. In contrast, particle displacement at Rockall Bank is dominated by tides and can reach 100-150m within one tidal cycle. From these results it appears that passive tracer retention in tidally dominated systems, such as Rockall Bank, is more effective compared to regions where quasi-steady background currents are the major source of physical forcing. However, strong tidal activity and tidal amplification might be able to move particles from tidally active to less active bank areas with a strong potential for particle redistribution within one tidal cycle.

Evidence for internal tidal wave and trapped diurnal wave dynamics for continental slope sedimentation and the generation of bottom and intermediate nepheloid layers in the study region has been given by early studies (e.g. Dickson and McCave, 1986). Localised benthic physical processes have been identified as playing an influential role in the development of carbonate mound ecosystems by recent studies (e.g. White, 2007).

(a) (c) (b)

K1 O1

M2

Bel Bel

Figure 7: (a) Location of major carbonate mound provinces along the European continental margin. (b) Tidal ellipsoids of major tidal constituents from current meter measurements at the the Galway mounds (Belgica province). (c) Location of carbonate mounds at the northern Belgica province and orientation of the K1 ellipsoid. Figures were taken from White et al. (2007).

The influence of internal waves, in combination with other tidal period baroclinic waves and diurnal tidal amplification, may enhance the bottom currents and therefore add to both the residual and maximum flow strength. The requirement for the development and distribution patterns of live deep-water corals is a combination of a suitable hard substrate on which to settle, a dynamical environment to keep such substrates clear of sediment, and sufficient food availability. Observations show that physical conditions strongly vary across different mound provinces. Stronger currents are found at Belgica (Southeastern Porcupine Seabight) and Pelagia (Northwest Porcupine Bank) mounds, whereas the weakest currents are recorded at the Hovland and Magellan mound provinces at the northern Porcupine Seabight margin (White, 2007). These differences may be attributed to the presence of internal waves (Pelagia) or bottom intensified diurnal tides (Belgica, see Fig. 7). White et al. (2007) have suggested a correlation between the direction of the tidal currents and the orientation of the mound clusters at two of the Belgica mound provinces where enhanced diurnal tidal currents have been measured (Fig. 7). Their results also indicate that at some mound locations, bottom intensification of diurnal tidal currents may have played a major role for the determination and development of the shape of mound clusters. Enhanced bottom currents appear to be the dominant physical control at other mound locations, but may be created by different mechanism, e.g. amplification of internal waves.

4. Summary and conclusions

The combination of high primary and secondary productivity with an effective retention mechanism may well provide optimal feeding conditions for benthic species communities at the banks, such as cold water corals. White et al. (2005) illustrated the bentho-pelagic coupling under such conditions using a conceptual model (see Fig. 8).

Figure 8: Conceptual model of processes acting at submarine banks of the Rockall Trough: (A) formation of cold nutrient-rich water and enhanced productivity over the bank, (B) sinking and transport of organic matter downslope to benthic species communities (White et al., 2005).

This process can be considered in two stages. First, nutrient rich water masses are formed over the banks during winter convection. These waters are resident below the seasonal thermocline later in time, but still shallow enough to be mixed up into the surface layers by or wind mixing events. It then can support enhanced surface productivity over the banks (Fig. 8, stage A). In a second stage, the dynamics around the bank support the transfer of material in two ways: The residual circulation retains this material in the upper and mid water depths, whereas Ekman drainage flow may deliver the material downslope into the benthic boundary layer (Fig. 8, stage B). As another process, dense water cascading may also significantly contribute to exchange between shallow and deep ocean regions over sloping bottom (Shapiro et al., 2003). Rockall Bank was recently identified as a possible location where dense water cascades regularly occur (Ivanov et al., 2004) and could provide an important mechanism for nutrient fluxes. However, both Ekman drainage and dens water cascades act on time scales that are difficult to observe in nature: Whilst Ekman drainage is very slow, dense water cascades are more rapid and of intermittent character. Therefore, the interaction of these processes, particularly the interaction between mid-water retention processes and near-bottom downslope dynamics, would appear complicated and, as yet, not fully quantified (White et al., 2005). A combination of non-linear processes (internal waves, trapped waves and tidal amplification) can generate locally amplified seabed or near-seabed currents that significantly contribute to sediment dynamics, forming patterns in sediment distribution (deposition, erosion, sediment plumes) as well as layers of resuspended organic material and inorganic nutrients. These benthic nepheloid layers are very common at continental slopes and are frequently observed along the Porcupine slopes (e.g. Thorpe and White, 1988). Thorpe and White (1988) suggested that nepheloid layers near the slope are transient events, but scattering layers away from the slope are more persistent and maintained as a result of sediment erosion by large bottom currents associated with internal waves.

5. Acknowledgements

This study is a contribution to the project ‘A simulation and forecasting system for ecosystem dynamics in Irish waters’ and supported under the Marine RTDI Measure (NDP 2000-2006) of the Marine Institute of Ireland. The AVHRR Pathfinder SST data were obtained through the online PO.DAAC Ocean ESIP Tool (POET) at the Physical Distributed Active Archive Center (PO.DAAC), NASA Jet Propulsion Laboratory, Pasadena, CA, USA. The chlorophyll data used in this study were acquired using the GESDISC Interactive Online Visualization ANd aNalysis Infrastructure (Giovanni) as part of the NASA’s Goddard Earth Sciences (GES) Data and Information Services Center (DISC).

6. References

• Bartsch, J., and S. Coombs, 2001. An individual-based growth and transport model of the early life-history stages of mackerel (Scomber scombrus) in the eastern North Atlantic. Ecological Modelling, 138, 127-141. • Beckmann, A., and D. Haidvogel, 1997. A numerical simulation of flow at Fieberling . Journal of Geophysical Research, 102, 5595-5613. • Booth, D.A. and D.J. Ellett, 1983. The Scottish continental slope current. Research 2 (2/3), 127-146. • Chapman, D.C., and D. Haidvogel, 1992. Formation of Taylor caps over a tall isolated seamounts in a stratified ocean. Geophysical Astrophysical Fluid Dynamics, 64, 31-65. • Booth, D.A. and D.J. Ellett, 1983. The Scottish continental slope current. Continental Shelf Research 2 (2/3), 127-146. • Genin, A. and G. Boehlert, 1985. Dynamics of temperature and chlorophyll structures above a : An oceanic experiment. Journal of Marine Research, 43, 907-924. • Genin, A., 2004. Bio-physical coupling in the formation of zooplankton and fish aggregations over abrupt topographies. Journal of Marine Systems, 50, 3-20. • Hill, A.E. and E.G. Mitchelson-Jacob, 1993. Observations of a poleward-flowing saline core on the continental slope west of Scotland. Deep-Sea Research I, 40 (7), 1521-1527. • Huthnance, J. M., 1974. On the diurnal tidal currents over the Rockall Bank. Deep-Sea Research, 21, 23-35. • Ivanov , V.V., Shapiro, G.I., Huthnance, J.M., Aleynik D.L., Golovin P.N., 2004. Cascades of dense water around the world ocean. Progress in Oceanography, 60, 47–98. • Kenyon, N.H., A. H. Akhmetzhanov, A. J. Wheeler, T.C.M. van Weering, H. de Haas, and M.I. Ivanov, 2003. Giant carbonate mounds in the southern Rockall Trough. , 195, 5-30. • Kloppmann, M., Mohn, C. and J. Bartsch, 2001. The distribution of blue whiting eggs and larvae on Porcupine Bank in relation to and currents. Fisheries Research, 50, 89-109. • Kunze, E., and J.M. Toole, 1997. Tidally driven vorticity, diurnal shear, and turbulence atop Fieberling seamount. Journal of , 27, 2663-2693. • Mohn, C., 2000. On water masses and currents at the European continental margin west of Ireland. PhD thesis, University of Hamburg, 133 pp. • Mohn, C., Bartsch, J. and J. Meincke, 2002. Observations of the mass and flow field at Porcupine Bank. ICES Journal of marine Science, 59, 380-392. • Mohn, C., and M. White, 2007. Remote sensing and modelling of bio-physical distribution patterns at Porcupine and Rockall Bank, Northeast Atlantic. Continental Shelf Research, DOI:10.1016/j.csr.2007.03.006 • Pingree, R.D. and B. Le Cann, 1990. Celtic and Armorican slope and shelf residual currents. Progress in Oceanography, 23, 303-338. • Pingree, R.D.,B. Sinha and C.R. Griffiths, 1999. Seasonality of the European slope current (Goban Spur) and ocean margin exchange. Continental Shelf Research, 19, 929-975. • Roberts J. M., D. Long, J. B. Wilson, P. B.Mortensen, and J. D. Gage, 2003. The cold water coral Lophelia pertusa (Scleractinia) and enifmatic seabed mounds along the northeast Atlantic margin; are they related? Bulletin, 46, 7-21. • Roberts, J. M., Wheeler, A. J., and A. Freiwald, 2006. Reefs of the deep: The biology and geology of cold-water coral ecosystems. Science, 312, 543-547. • Shapiro, G. I., Huthnance, J. M., & Ivanov, V. V. (2003). Dense water cascading off the continental shelf. Journal of Geophysical Research, 108, C12:art no. 3390. • Thorpe, S.A., and M. White, 1988. A deep intermediate nepheloid layer. Deep- Sea Research, 35, 1665-1671. • White, M. and P. Bowyer, 1997. The shelf-edge current north-west of Ireland. Annales Geophysicae, 15, 1076-1083. • White, M., C. Mohn, and M.J. Orren, 1998. Nutrient distributions across the Porcupine Bank. ICES Journal of Marine Science, 55, 1082-1094. • White, M., G. Harker, M. Calverley, F. Geos, and Amergen International Personnel, 2002. Current Measurements along the Rockall Trough Margin, 1999- 2000. Geophysical Research Abstracts, EGS02-A-05765-1. • White, M., C. Mohn, H. de Stigter, and G. Mottram, 2005. Deep-water coral development as a function of hydrodynamics and surface productivity around the submarine banks of the Rockall Trough, NE Atlantic. In Freiwald A., Roberts J.M. (eds), 2005, Cold-water Corals and Ecosystems. Springer-Verlag Berlin Heidelberg, 503-514. • White, M., 2007. Benthic dynamics at the carbonate mound regions of the Porcupine Sea Bight continental margin. International Journal of earth Science, 96, 1-9. • White, M., Roberts, J.M., and van Weering, T., 2007. Do bottom-intensified diurnal tidal currents shape the alignment of carbonate mounds in the NE Atlantic? Geo-Marine Letters, DOI 10.1007/s00367-007-0060-8.