Porcupine Seabight Research Articles

Reproductive biology of the deep-sea asteroid Henricia abyssicola from the NE Atlantic Ocean

The reproductive biology of the North Atlantic asteroid Henricia abyssicola is determined in time series samples taken from 1250 to 1900 m depth in the Porcupine Seabight (NE Atlantic). The male-female ratio, gonadosomatic index (GI), pyloric caecum index (PCI), fecundity and oocyte size distribution were determined and the results were corroborated by histological examination of the gonads. This species is a gonochoric asteroid and the male-female ratio did not differ significantly from the equality. Oogenesis was asynchronous. The previtellogenic oocytes grow to a size of ~230 µm before undergoing vitellogenesis. Maximum oocyte size was ~970 µm. The ovary volume was mainly occupied by large vitellogenic oocytes (400–500 µm) at any time. There were no seasonal variations in GI, PCI and fecundity. There were, however, variations in the production of vitellogenic oocytes of H. abyssicola superimposed on the continuous reproduction. We suggest that H. abyssicola makes use of the energy source provided by the deposition of phytodetritus by storing it in the developing oocytes and that it might have the benefit of a facultative planktotrophic larva.

Biodiversity and ecological composition of macrobenthos on cold-water coral mounds and adjacent off-mound habitat in the bathyal Porcupine Seabight, NE Atlantic

The cold-water scleractinian corals Lophelia pertusa and Madrepora oculata form mound structures on the continental shelf and slope in the NE Atlantic. This study is the first to compare the taxonomic biodiversity and ecological composition of the macrobenthos between on- and off-mound habitats. Seven box cores from the summits of three mounds and four cores from an adjacent off-mound area in the Belgica Mound Province in the Porcupine Seabight yielded 349 species, including 10 undescribed species. On-mound habitat was three times more speciose, and was richer with higher evenness and significantly greater Shannon's diversity than off-mound. Species composition differed significantly between habitats and the four best discriminating species were Pliobothrus symmetricus (more frequent off-mound), Crisia nov. sp, Aphrocallistes bocagei and Lophelia pertusa (all more frequent on-mound). Filter/suspension feeders were significantly more abundant on-mound, while deposit feeders were significantly more abundant off-mound. Species composition did not significantly differ between mounds, but similarity within replicates decreased from Galway Mound<Thérèse Mound<off-mound. We propose that, despite having greater vertical habitat heterogeneity that supports higher biodiversity, coral mounds have a characteristic “reef fauna” linked to species’ biology that contrasts with the higher horizontal habitat heterogeneity conferred by the action of deposit feeders and a varied seabed sedimentary facies off-mound. Standardisation of equipment and restriction of analyses to higher taxonomic levels would facilitate prospective comparative analyses of cold-water coral biodiversity across larger spatio–temporal scales.

Benthic bioluminescence in the bathyal North East Atlantic: luminescent responses of Vargula norvegica (Ostracoda: Myodocopida) to predation by the deep-water eel (Synaphobranchus kaupii)

Using two autonomous lander systems, one equipped with a high sensitivity intensified silicon intensifier target (ISIT) video camera and a second with a 4.1-megapixel digital stills camera, observations were made of bioluminescent emissions and fauna attracted to artificial food falls at ca. 1,000 m depth in the carbonate mound provinces of the Porcupine Seabight and Rockall Bank, North East Atlantic. On the Galway coral mound (Belgica Mound Province), seven bioluminescent events per hour were observed, whereas on an area of sediment at the base of the Kiel Mount on the Rockall Bank 133 events per hour were observed. This increase in bioluminescent activity was associated with the presence of the deep water eels Synaphobranchus kaupii, and the ostracods, Vargula norvegica. Captured ostracods luminesced readily in response to mechanical stimulation and were also observed emitting a luminous secretion. We hypothesise that V. norvegica, attracted to bait, luminesce as a defence response against the predatory activity of S. kaupii that compete for bait but also feed on the ostracods. Benthic bioluminescence in the carbonate mound provinces was not directly linked to the presence of corals.

Small mounded contourite drifts associated with deep-water coral banks, Porcupine Seabight, NE Atlantic Ocean

Abstract Numerous studies on sediment drifts have demonstrated a close interaction between sea-bed morphology, palaeoceanography, sediment supply and climate. Contourites have been reported in areas along continental margins directly influenced by the effect of intensive deep-water currents from the global conveyor belt. In this paper, we report the occurrence of a small-scale confined contourite drift from Porcupine Seabight, SW of Ireland, and its association with a province of coral banks. The Porcupine Basin is a relatively shallow, semi-enclosed basin characterized by the presence of cold-water coral bank provinces. These coral banks are often associated to a strong northward-flowing bottom current, created and steered by a complex interaction of the water mass characteristics, tidal influences and sea-bed morphology. Very high-resolution seismic stratigraphy allowed the identification of a small mounded drift, located between a depression created by (1) an irregular palaeotopography caused by a vigorous Late Pliocene erosion event and (2) a north-south alignment of coral banks. Core MD99-2327, taken on the flank of this drift mound, shows the variability of the bottom currents. Sortable silt data show several periods of bottom-current enhancement, which may be linked with warmer periods and an inferred influx of Mediterranean Outflow Water. The glacial part of the core has been interpreted as a muddy contourite with a high content of ice-rafted debris. The lower part of the core is a deep-water massive contourite sand resembling the present-day sea-floor sediments.

Age constraints on the origin and growth history of a deep-water coral mound in the northeast Atlantic drilled during Integrated Ocean Drilling Program Expedition 307

Sr isotope stratigraphy provides a new age model for the first complete section drilled through a deep-water coral mound. The 155-m-long section from Challenger Mound in the Porcupine Sea-bight, southwest of Ireland, is on Miocene siliciclastics and consists entirely of sediments bearing well-preserved cold-water coral Lophelia pertusa. The 87Sr/86Sr values of 28 coral specimens from the mound show an upward-increasing trend, correspond to ages from 2.6 to 0.5 Ma, and identify a significant hiatus from ca. 1.7 to 1.0 Ma at 23.6 m below seafloor. The age of the basal mound sediments coincides with the intensification of Northern Hemisphere glaciations that set up the modern stratification of the northeast Atlantic and enabled coral growth. Mound growth persisted throughout glacial-interglacial fluctuations, reached a maximum rate (24 cm/k.y.) ca. 2.0 Ma, and ceased at 1.7 Ma. Unlike other buried mounds in Porcupine Seabight, Challenger Mound was only partly covered during its growth interruption, and growth restarted ca. 1.0 Ma.

Seasonal development of a deep pelagic bioluminescent layer in the temperate NE Atlantic Ocean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 341:37-44 (2007) - doi:10.3354/meps341037 Seasonal development of a deep pelagic bioluminescent layer in the temperate NE Atlantic Ocean E. J. V. Gillibrand1,3,*, A. J. Jamieson1, P. M. Bagley1, A. F. Zuur2, I. G. Priede1,** 1Oceanlab, University of Aberdeen, Main Street, Newburgh, Aberdeen AB41 6AA, UK 2Highland Statistics Ltd, 6 Laverock Road, Newburgh, Aberdeen AB41 6FN, UK 3Present address: Robert Gordon University, Schoolhill, Aberdeen AB10 1FR, UK *Née Battle**Corresponding author. Email: i.g.priede@abdn.ac.uk ABSTRACT: Vertical distribution of bioluminescent organisms throughout the water column in the Porcupine Seabight and Porcupine Abyssal plain area of the NE Atlantic Ocean was measured using a free-falling autonomous vehicle equipped with a high-sensitivity video system viewing stimulated flashes of light from impacts on a mesh screen. Data were recorded from 500 m depth down to the sea floor at depths from 740 to 4808 m during spring and autumn of 2001 and 2002. Bioluminescent organisms decreased in abundance from a maximum of 80 m–3 at 600 m depth to a mean of 0.5 to 1.2 m–3 at depths greater than 3000 m. During autumn, a seasonally enhanced peak of abundance (mean 27.19 m–3, compared with 7.52 m–3 in spring) centred at 1420 m within the depth range of occurrence of Mediterranean intermediate water. It is hypothesised that this bioluminescent layer reflects seasonal increase in deep pelagic biomass fed by downward transport of organic matter following the spring peak of primary production in the surface layers. The bioluminescent layer is a large-scale regional phenomenon extending over 100s of kilometres. KEY WORDS: Bioluminescence · Deep pelagic · NE Atlantic Ocean · Mediterranean intermediate water · Pelagic–benthic flux Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 341. Online publication date: July 04, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Progressive reduction in NE Atlantic intermediate water ventilation prior to Heinrich events: Response to NW European ice sheet instabilities?

We present high‐resolution benthic δ13C records from intermediate water depth core site MD01‐2461 (1153 m water depth), from the Porcupine Seabight, NE Atlantic, spanning 43 to 8 kyr B.P. At an average proxy time step of 160 ± 56 years this core provides information on the linkage between records from the Portuguese Margin and high‐latitude North Atlantic basin, allowing additional insights into North Atlantic thermohaline circulation (THC) variability during millennial‐scale climatic events of the last glacial. Together, these records document both discrete and progressive reductions in Glacial North Atlantic Intermediate Water (GNAIW) formation preceding Heinrich (H) events 1, 2, and 4, recorded through the apparent interchange of glacial northern and southern‐sourced intermediate water signatures along the European Margin. Close coupling of NW European ice sheet (NWEIS) instability and GNAIW formation is observed through transient advances of SCW along the European margin concurrent with pulses of ice‐rafted debris and meltwater release into the NE Atlantic between 27 and 16 kyr B.P., when the NWEIS was at maximum extent and proximal to Last Glacial Maximum convection zones in the open North Atlantic. It is such NWEIS instability and meltwater forcing that may have triggered reduced North Atlantic THC prior to collapse of the Laurentide ice sheet at H1 and H2. Precursory reduction in GNAIW formation prior to H4 may also be inferred. However, limited NWEIS ice volume prior to H4 and convection occurring in the Norwegian‐Greenland Sea require that if a meltwater trigger is invoked, as appears to be the case at H1 and H2, the source of meltwater prior to H4 is elsewhere, likely higher‐latitude ice sheets. Clarification of the sequencing and likely mechanisms of precursory decrease of the North Atlantic THC support theories of H event initiation relating to ice shelf growth during cold periods associated with reduced North Atlantic THC and subsequent ablation through subsurface warming and sea level rise associated with further reductions in meridional overturning.

Microhabitat type determines the composition of nematode communities associated with sediment-clogged cold-water coral framework in the Porcupine Seabight (NE Atlantic)

The nematofauna associated with a cold-water coral degradation zone in the Porcupine Seabight (NE Atlantic) was investigated. This is the first comprehensive study of nematodes associated with cold-water corals. This research mainly aimed to investigate the influence of microhabitat type on nematode community structure. Three distinct microhabitats for nematodes were distinguished: dead coral fragments, glass sponge skeletons and the underlying sediment. The nematode assemblages associated with these three microhabitats were significantly different from each other. Coral and sponge substrata lie relatively unprotected on the seafloor and are consequently more subjected to strong currents than the underlying sediment. As a result, both large biogenic substrata were characterized by higher abundances of taxa that are less vulnerable and more adapted to physical disturbance, whereas the underlying sediment yielded more slender, sediment-dwelling taxa. Typically epifaunal taxa, such as Epsilonematidae and Draconematidae, were especially abundant on dead coral fragments, where they are thought to feed on the microbial biofilm which covers the coral surface. Several epifaunal genera showed significant preferences for this microhabitat, and Epsilonema (Epsilonematidae) was dominant here. Sponge skeletons are thought to act as efficient sediment traps, resulting in a lower abundance of epifaunal taxa compared to coral fragments. The underlying sediment was dominated by taxa typical for slope sediments. The considerable degree of overlap between the communities of each microhabitat is attributed to sediment infill between the coral branches and sponge spicules. It is assumed that the nematofauna associated with large biogenic substrata is composed of a typical sediment-dwelling background community, supplemented with taxa adapted to an epifaunal life strategy. The extent to which these taxa contribute to the community depends on the type of the substratum. Selective deposit feeders were dominant on sponge skeletons and in the underlying sediment, whereas coral fragments were dominated by epistratum feeders. The presence of a microbial biofilm on the coral fragments is proposed as an explanation for the significant preference of epistratum feeders for this microhabitat. Densities in the underlying sediment were low in comparison with other studies, but biodiversity was higher here than on the coral and sponge fragments, a difference which is attributed to lower disturbance. Nevertheless, the large biogenic substrata provide a microhabitat for rare, epifaunal taxa, and fragments of both substrata within the sediment increase habitat complexity and hence biodiversity.

Deep sea benthic bioluminescence at artificial food falls, 1,000–4,800 m depth, in the Porcupine Seabight and Abyssal Plain, North East Atlantic Ocean

Natural bioluminescence (that not mechanically stimulated by human intervention) produced by organisms on the seafloor of the northeast Atlantic ocean between 970 and 4,800 m depth was examined using an image intensifying (ISIT) camera mounted on an autonomous lander system. In the absence of bait little or no luminescence was observed but with bait present there was a significant inverse relationship with depth, Log10 (1 + number of events h−1) = 1.7627–0.3235 depth (km) (r2 = 0.8158, P < 0.001) indicating an average of 2.6 events h−1 at 4 km and 28 h−1 at 1 km. But in an area at ca. 1 km depth near carbonate and coral mounds the mean was 133 events h−1, much higher than predicted. In this bioluminescent hot spot 52–483 events h−1 were observed including moving luminescent targets and release of patches of luminescent material into the water around the bait so that on occasions the whole area around the bait was illuminated persisting on a time scale of minutes. At abyssal depths, luminescence was much less than reported at similar depths in the tropical NE Atlantic off Cape Verde. The sources of luminescence could not be determined but in the most active areas were associated with presence of eels Synaphobranchus kaupii which although themselves not luminescent may have stimulated luminescence from prey organisms such as ostracods (Vargula norvegica).

Benthic dynamics at the carbonate mound regions of the Porcupine Sea Bight continental margin

A brief review is given of some dynamical processes that influence the benthic dynamics within the carbonate mound provinces located at the Porcupine Bank/Sea Bight margin, NE Atlantic. The depth range of the mounds in this region (600–1,000 m) marks the upper boundary of the Mediterranean outflow water above which Eastern North Atlantic Water dominates. Both water masses are carried northwards by the eastern boundary slope current. In the benthic boundary layer both the action of internal waves, and other tidal period baroclinic waves, may enhance the bottom currents and add to both the residual and maximum flow strength. Both residual and maximum bottom currents vary at different mound locations, with stronger currents found at Belgica (SE Porcupine Sea Bight) mound and Pelagia (NW Porcupine Bank) mound regions, whilst weakest currents are found at the Hovland and Magellan Mounds at the northern Sea Bight margin. The differences may be attributed to the presence of internal waves (Pelagia) or bottom intensified diurnal waves (Belgica). These different dynamical regimes are likely to have implications for the distribution patterns of live coral at the different locations.

X-ray imagery and physical core logging as a proxy of the content of sediment cores in cold-water coral mound provinces: a case study from Porcupine Seabight, SW of Ireland

Three provinces, characterized by the presence of carbonate mounds interpreted as cold-water coral banks have been reported in Porcupine Seabight, SW of Ireland and were recently subjected to many detailed studies. This contribution discusses the use of X-ray imagery and physical properties in deciphering palaeoceanographic, sedimentological and biological processes. Physical property core logging and X-ray imagery are used to identify and describe sedimentation regimes and so their respective palaeoceanographic and palaeoclimatological settings in two mound provinces, respectively the Belgica mound province and the Magellan mound province. Both provinces show at present time clear differences in the hydrodynamic environment. This study confirms that also during the past the oceanographic and sedimentological environment of both provinces differ clearly. Impacts of glacial–interglacial variations and locally derived ice rafting events (IRE), comparable with the North Atlantic Heinrich events (HE) have been recognized in both provinces. Moreover, the combination of X-ray imagery, magnetic susceptibility, gamma density and P-wave velocity makes it possible to estimate the coral content and coral distribution in unopened cores localized on top of carbonate mounds. A comparison between on-mound and off-mound cores in both provinces allowed revealing some mechanisms of mound evolution and coral growth versus time.

Quaternary sediment dynamics in the Belgica mound province, Porcupine Seabight: ice-rafting events and contour current processes

The Belgica cold-water coral banks on the eastern slope of the Porcupine Seabight are closely associated with bottom currents. In order to better understand the local temporal and spatial characteristics, as well as the palaeoclimatologic influences, a 26 m long core, taken on a small contourite drift, was studied. This sediment record of approximately 100 ka BP reveals new insights into the regional glacial and sedimentary processes, which are intrinsically linked to several geological, climatological, biological and hydrodynamic variables. The glacial sequences in the core contain six ice-rafting events (IRE). They are comparable with the North Atlantic Heinrich Events, although their characteristics show dominant influences from the proximal British–Irish Ice Sheet (BIIS). These IRE have a low magnetic susceptibility and are deposited during two or three ice-rafting pulses. The record of ice-rafting suggests a millennial-scaled BIIS destabilisation and confirms the start of a final retreat about 25 ka ago. Additionally, the glacial sequence corresponds to a muddy contourite, influenced by bottom-current strength variations during interstadials, possibly triggered by sporadic reintroductions of Mediterranean Outflow Water in a glacial North Atlantic Ocean. The interglacial sequence features an 11-m thick deep-water massive sand unit, probably deposited under a high-energy bottom-current regime.

Consumption of large bathyal food fall, a six month study in the NE Atlantic

We deployed 2 porpoise (Phocoena phocoena) carcasses at bathyal depth (2555 to 2710 m) in the Porcupine Seabight, NE Atlantic for periods of 1 wk and 6 mo respectively. Consumption rates of 0.085 and 0.078 kg h–1 were similar to those observed at abyssal depths in the Atlantic, and 1 order of magnitude slower than at bathyal depth in the Pacific. A distinct succession of scavenging species was observed at both carcasses: the abyssal grenadier Coryphaenoides armatus and the cusk eel Spectrunculus grandis numerically dominated the initial phase of carcass consumption and, once the bulk of the soft tissue had been removed (by Day 15), were succeeded by the squat lobster Munidopsis crassa. The blue hake Antimora rostrata and amphipod numbers were unexpectedly low, and consumption was attributed largely to direct feeding by C. armatus. The interaction of a crustacean prey species (M. crassa) and cephalopod predator (Benthoctopus sp.) was observed for the first time, revealing that large food falls also attract secondary predators that do not utilise the food fall directly. The staying time of a single parasitised C. armatus (18 h) greatly exceeded previous estimates (≤8 h). This study describes the first large food fall to be monitored at high frequency over a 6 mo period, and the first observations of a large food fall at bathyal depth in the NE Atlantic. It enables direct comparison with similarly sized food falls at abyssal depth, much larger megacarrion falls, and similar studies differing in geographic location, in particular those carried out under Pacific whale migration corridors.

Faunal change and bathymetric diversity gradient in deep-sea prosobranchs from Northeastern Atlantic

Despite the plethora of studies, geographic patterns of diversity in deep sea remain subject of speculation. This study considers a large dataset to examine the faunal change and depth-diversity gradient of prosobranch molluscs in the Porcupine Seabight and adjacent Abyssal Plain (NE Atlantic). Rates of species succession (addition and loss) increased rapidly with increasing depth and indicated four possible areas of faunal turnover at about 700, 1600, 2800 and 4100 m. Depth was a significant predictor of diversity, explaining nearly a quarter the variance. There was a pattern of decreasing diversity downslope from ~250 m to ~1500–1600 m, followed by an increase to high values at about 4000 m and then again, a fall to ~4915 m. Processes causing diversity patterns of prosobranchs in the Porcupine Seabight and adjacent Abyssal Plain are likely to differ in magnitude or type, from those operating in other Atlantic areas.

Organic matter quality and supply to deep-water coral/mound systems of the NW European Continental Margin

Comparison of five deep-water coral (DWC)/mound ecosystems along the European Continental Margin shows that suspended particulate organic matter (sPOM), a potential food source, is lipid rich and of high quality. However, there are differences between the sites. The Darwin and Pelagia Mounds (N. Rockall Trough and N. Porcupine Bank, respectively) have higher proportions of labile particulate lipids (including high proportions of polyunsaturated fatty acids) in the benthic boundary layer than Logachev, Hovland and Belgica Mounds (Rockall Bank, S. Porcupine Bank and Porcupine Seabight, respectively). The high quality sPOM could be transported downslope from the euphotic zone. There is some evidence for inter-annual variability at some sites (e.g. Hovland and Logachev Mounds) as large differences in suspended lipid and particulate organic carbon concentrations were observed over the sampling period. Elevated total organic carbon contents of sediments at mound sites, relative to control sites in some cases (particularly Darwin Mounds), probably reflect local hydrodynamic control and the trapping of sPOM by the DWC. Fresh POM can be relatively rapidly transferred to significant depth (up to 8 cm) through bioturbation that is evident at all sites. There is no clear evidence of present day hydrocarbon seepage at any of the sites.

From sediment to rock: diagenetic processes of hardground formation in deep-water carbonate mounds of the NE Atlantic

Modern cool-water carbonate mounds topped by corals form an extended reef belt along the NW European continental margin at 200–1200 m water depth. An essential element of mound growth are hardgrounds which provide a stable substratum for mound-building invertebrate colonisation and stabilise the inclined mound flanks. Evaluating the degree of lithification and the slope stability against erosion represents an important task within the ESF programme MOUNDFORCE under the umbrella of EUROMARGINS. Sampling of hardgrounds during RV Meteor cruises M61-1 and -3 in 2004 by means of the IFM-GEOMAR TV-grab and the Bremen ROV QUEST focused on carbonate mounds of the Porcupine Seabight and northwestern Rockall Bank off Ireland. Lithified carbonates of mid-Pleistocene age were exhumed during the Holocene and are now exposed on the top and flanks of numerous carbonate mounds showing a patchy to dense colonisation by living corals and associated invertebrates. The sediments, composed of foraminiferal–nannoplankton oozes and admixed mound-derived invertebrate skeletons, range from partly lithified chalks to dense micritic limestones. These wackestones to packstones clearly differ from bacterially induced authigenic carbonate crusts typical of hydrocarbon seep settings by showing current-induced sedimentary structures, a non-luminescing matrix indicating oxic pore fluids, and a marine isotopic signature lacking any depleted carbon regime which is typical of anaerobic methane oxidation. The carbonate lithification is driven by carbonate ion diffusion from supersaturated seawater into the pore fluids in the studied areas. Vigorous bottom currents were the ultimate control not only of carbonate cementation by enhancing the diffusion process and supporting a pumping mechanism, but also of hardground formation and mound shaping by exhuming lithified carbonates and preventing fine-grained sediment accumulation at the downslope mound flanks.

Seafloor classification of the mound and channel provinces of the Porcupine Seabight: an application of the multibeam angular backscatter data

In this study multibeam angular backscatter data acquired in the eastern slope of the Porcupine Seabight are analysed. Processing of the angular backscatter data using the ‘NRGCOR’ software was made for 29 locations comprising different geological provinces like: carbonate mounds, buried mounds, seafloor channels, and inter-channel areas. A detailed methodology is developed to produce a map of angle-invariant (normalized) backscatter data by correcting the local angular backscatter values. The present paper involves detailed processing steps and related technical aspects of the normalization approach. The presented angle-invariant backscatter map possesses 12 dB dynamic range in terms of grey scale. A clear distinction is seen between the mound dominated northern area (Belgica province) and the Gollum channel seafloor at the southern end of the site. Qualitative analyses of the calculated mean backscatter values i.e., grey scale levels, utilizing angle-invariant backscatter data generally indicate backscatter values are highest (lighter grey scale) in the mound areas followed by buried mounds. The backscatter values are lowest in the inter-channel areas (lowest grey scale level). Moderate backscatter values (medium grey level) are observed from the Gollum and Kings channel data, and significant variability within the channel seafloor provinces. The segmentation of the channel seafloor provinces are made based on the computed grey scale levels for further analyses based on the angular backscatter strength. Three major parameters are utilized to classify four different seafloor provinces of the Porcupine Seabight by employing a semi-empirical method to analyse multibeam angular backscatter data. The predicted backscatter response which has been computed at 20° is the highest for the mound areas. The coefficient of variation (CV) of the mean backscatter response is also the highest for the mound areas. Interestingly, the slope value of the buried mound areas are found to be the highest. However, the channel seafloor of moderate backscatter response presents the lowest slope and CV values. A critical examination of the inter-channel areas indicates less variability within the estimated three parameters. Financial support of this study was granted by the European Commission Fifth Framework Project GEOMOUND (contract no. EVK3-CT-1999-00016).

Environmental changes and growth history of a cold-water carbonate mound (Propeller Mound, Porcupine Seabight)

On- and off-mound sediment cores from Propeller Mound (Hovland Mound province, Porcupine Seabight) were analysed to understand better the evolution of a carbonate mound. The evaluation of benthic foraminiferal assemblages from the off-mound position helps to determine the changes of the environmental controls on Propeller Mound in glacial and interglacial times. Two different assemblages describe the Holocene and Marine Isotope Stage (MIS) 2 and late MIS 3 (∼31 kyr BP). The different assemblages are related to changes in oceanographic conditions, surface productivity and the waxing and waning of the British Irish Ice Sheet (BIIS) during the last glacial stages. The interglacial assemblage is related to a higher supply of organic material and stronger current intensities in water depth of recent coral growth. During the last glaciation the benthic faunas showed high abundances of cassidulinid species, implying cold bottom waters and a reduced availability of organic matter. High sedimentation rates and the domination of Elphidium excavatum point to shelf erosion related to sea-level lowering (∼50 m) and the progradation of the BIIS onto the shelf. A different assemblage described for the on-mound core is dominated by Discanomalina coronata, Gavelinopsis translucens, Planulina ariminensis, Cibicides lobatulus and to a lower degree by Hyrrokkin sarcophaga. These species are only found or show significantly higher relative abundances in on-mound samples and their maximum contribution in the lower part of the record indicates a higher coral growth density on Propeller Mound in an earlier period. They are less abundant during the Holocene, however. This dataset portrays the boundary conditions of the habitable range for the cold-water coral Lophelia pertusa, which dominates the deep-water reefal ecosystem on the upper flanks of Propeller Mound. The growth of this ecosystem occurs during interglacial and interstadial periods, whereas a retreat of corals is documented in the absence of glacial sediments on-mound. Glacial conditions with cold intermediate waters, a weak current regime and high sedimentation rates provide an unfavourable environmental setting for Lophelia corals to grow. A Late Pleistocene decrease is observed in the mound growth for Propeller Mound, which might face its complete burial in the future, as it already happened to the buried mounds of the Magellan Mound province further north.

Sexual reproduction in three hermaphroditic deep-sea Caryophyllia species (Anthozoa: Scleractinia) from the NE Atlantic Ocean

The reproductive biology and gametogenesis of three species of Caryophyllia were examined using histological techniques. Caryophyllia ambrosia, Alcock 1898, C. cornuformis, Pourtales 1868, and C. sequenzae, Duncan 1873, were collected from the Porcupine Seabight and Rockall Trough in the NE Atlantic Ocean. These three ahermatypic solitary corals inhabit different depth ranges: C. cornuformis – 435–2000 m, C. sequenzae – 960–1900 m, and C. ambrosia – 1100–3000 m. All three species are hermaphroditic. Hermaphroditism in these species was found to be cyclical, with only one sex of gametes viable in any individual at any point in time, although gametes of both sexes were found together within a single mesentery. Once the viable gametes are spawned, the next sex of gametes continues to grow until mature, and so gametogenesis is a continuous cycle. Oocytes and spermacysts in all species increased in density towards the actinopharynx. Maximum fecundity for C. sequenzae was 940 oocytes per polyp, and for C. ambrosia 2900 oocytes per polyp. Fecundity could not be established for C. cornuformis. In all three species, individuals were asynchronous within populations, and production of gametes was quasi-continuous throughout the year. All species are hypothesised to have lecithotrophic larvae owing to their large oocyte sizes (C. cornuformis max – 350 μm; C. sequenzae max – 430 μm; C. ambrosia max – 700 μm). Both the average oocyte size and fecundity increased in species going down the depth gradient of the NE Atlantic.

Thérèse Mound: a case study of coral bank development in the Belgica Mound Province, Porcupine Seabight

High-resolution seismic profiles, swath bathymetry, side-scan sonar data and video imageries are analysed in this detailed study of five carbonate mounds from the Belgica mound province with special emphasis on the well-surveyed Thérèse Mound. The selected mounds are located in the deepest part of the Belgica mound province at water depths of 950 m. Seismic data illustrate that the underlying geology is characterised by drift sedimentation in a general northerly flowing current regime. Sigmoidal sediment bodies create local slope breaks on the most recent local erosional surface, which act as the mound base. No preferential mound substratum is observed, neither is there any indication for deep geological controls on coral bank development. Seismic evidence suggests that the start-up of the coral bank development was shortly after a major erosional event of Late Pliocene–Quaternary age. The coral bank geometry has been clearly affected by the local topography of this erosional base and the prevailing current regime. The summits of the coral banks are relatively flat and the flanks are steepest on their upper slopes. Deposition of the encased drift sequence has been influenced by the coral bank topography. Sediment waves are formed besides the coral banks and are the most pronounced bedforms. These seabed structures are probably induced by bottom current up to 1 m/s. Large sediment waves are colonised by living corals and might represent the initial phase of coral bank development. The biological facies distribution of the coral banks illustrate a living coral cap on the summit and upper slope and a decline of living coral populations toward the lower flanks. The data suggest that the development of the coral banks in this area is clearly an interaction between biological growth processes and drift deposition both influenced by the local topography and current regime.