The influence of near-bed hydrodynamic conditions on cold-water corals in the Viosca Knoll area, Gulf of Mexico

Deep-Sea Research I 60 (2012) 32–45

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The influence of near-bed hydrodynamic conditions on cold-water corals in the Viosca Knoll area, Gulf of Mexico F. Mienis a,n, G.C.A. Duineveld b, A.J. Davies c, S.W. Ross d, H. Seim e, J. Bane e, T.C.E. van Weering b,f a

MARUM—Center for Marine Environmental Sciences, University of Bremen, Leobenerstraße, 28359 Bremen, Germany Netherlands Institute for Sea Research (NIOZ), P.O. Box 53, 1790 AB, Den Burg, The Netherlands c School of Ocean Sciences, Bangor University, Menai Bridge, LL59 5AB, Wales, United Kingdom d University of North Carolina-Wilmington, Center for Marine Science, 5600 Marvin Moss Lane, Wilmington, NC 28409, USA e University of North Carolina-Chapel Hill, Department of Marine Sciences, 3202 Venable Hall, Chapel Hill, NC 27599-3300, USA f VU University, Faculty of Earth and Life Sciences, Boelelaan 1085, 1081 HV Amsterdam, The Netherlands b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 June 2011 Received in revised form 6 October 2011 Accepted 20 October 2011 Available online 31 October 2011

Near-bed hydrodynamic conditions were recorded for almost one year in the Viosca Knoll area (lease block 826), one of the most well-developed cold-water coral habitats in the Gulf of Mexico. Here, a reeflike cold-water coral ecosystem, dominated by the coral Lophelia pertusa, resembles coral habitats found off the southeastern US coast and the North East Atlantic. Two landers were deployed in the vicinity and outside of the coral habitat and measured multiple near-bed parameters, including temperature, salinity, current speed and direction and optical and acoustic backscatter. Additionally, the lander deployed closest to the coral area was equipped with a sediment trap that collected settling particles over the period of deployment at 27 day intervals. Long-term monitoring showed, that in general, environmental parameters, such as temperature (6.5–11.6 1C), salinity (34.95–35.4) and current speed (average 8 cm s  1, peak current speed up to 38 cm s  1) largely resembled conditions previously recorded within North East Atlantic coral habitats. Major differences between site VK 826 and coral areas in the NE Atlantic were the much higher particle load, and the origin of the particulate matter. Several significant events occurred during the deployment period beginning with an increase in current speed followed by a gradual increase in temperature and salinity, followed by a rapid decrease in temperature and salinity. Simultaneously with the decrease in temperature and salinity, the direction of the current changed from west to east and cold and less turbid water was transported upslope. The most prominent event occurred in July, when a westward flow lasted over 21 days. These events are consistent with bottom boundary layer dynamics influenced by friction (bottom Ekman layer). The Mississippi River discharges large quantities of sediment and dominates sedimentation regimes in the area. Furthermore, the Mississippi River disperses large amounts of terrestrial organic matter and nutrients, resulting in increased primary productivity, whereby marine organic matter is produced that will sink to the seafloor and can serve as food for the cold-water corals and associated species. As a result mass fluxes from the sediment trap were higher (1120–4479 mg m  2 day  1) than those observed in the North East Atlantic and were highest during periods of westward-flow, which corresponded to warm turbid water. During eastward-flow, colder and less turbid water was pushed upslope, resulting in lower mass fluxes. Trap samples had a low CaCO3, high organic carbon content and high C/N ratios, suggesting a fluvial origin. The high sediment load in the water column can be a limiting factor for coral growth, especially since the corals can be smothered with sediment. However, eastward-flows provided periods of relatively clearer water that can remove sediment from the coral area and allow corals to expel sediment from their polyps. Around Viosca Knoll food supply comes from two possible sources. During April and June several fluorescence peaks were observed near the seabed, showing the arrival of phytodetritus in the area. Furthermore, a consistent diel vertical migration of zooplankton was observed that might provide an additional food source. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Gulf of Mexico Viosca Knoll Lophelia pertusa Environmental constraints Ecosystem engineers

1. Introduction n

Corresponding author. Tel.: þ49 42121865584. E-mail address: [email protected] (F. Mienis).

0967-0637/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2011.10.007

Large reefs and carbonate mounds, dominated by the coldwater coral Lophelia pertusa are found on the continental margins

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

of the North Atlantic Ocean (o2000 m water depth) (De Mol et al., 2002; Foubert et al., 2008; Kenyon et al., 2003; Lindberg and Mienert, 2005; Neumann et al., 1977; Paull et al., 2000; Ross and Quattrini, 2007; Van Weering et al., 2003b). Over the last decade, L. pertusa has been recognised as a key species that creates complex habitats. These habitats are often characterised by a high biodiversity of associated species (Henry and Roberts, 2007; Jensen and Frederiksen, 1992; Lessard-Pilon et al., 2010) and are hotspots for carbon cycling (Van Oevelen et al., 2009). Occurrences of L. pertusa in the western North Atlantic have mainly been observed on the continental margin at depths between 300 and 800 m. Mound, lithoherm and ridge structures covered with L. pertusa were found off the southeastern United States and in the Gulf of Mexico (GoM) (Brooke and Schroeder, 2007; Neumann et al., 1977; Paull et al., 2000; Reed et al., 2006; Ross and Nizinski, 2007). In the GoM, L. pertusa coral thickets mainly occur growing on top of authigenic carbonates (i.e., De Soto slope) or on ship wrecks (Brooke and Schroeder, 2007; Cordes et al., 2008; Moore and Bullis, 1960; Schroeder, 2002). Reef or mound structures have been rarely found in the GoM, except on the West Florida Slope and in the Viosca Knoll area (VK 826), which has been considered the most extensive L. pertusa habitat found so far in the GoM (Brooke and Schroeder, 2007; Davies et al., 2010). In general, scleractinian cold-water corals are found at depths between 50 and 2000 m, in regions that contain hard substrata on which to settle, in waters with temperatures between 4 and 12 1C and a salinity of 35, within a density envelope of 27.35–27.65 kg m  3 (Davies and Guinotte, 2011; Dullo et al., 2008; Freiwald, 2002). Additionally, recent studies in the Northeast Atlantic have firmly established a positive relationship between strong near-bed currents, often generated by internal waves and the presence of cold-water corals (Davies et al., 2009; Frederiksen et al., 1992; Mienis et al., 2007). In the Northeast Atlantic, internal waves that are related to tidal variability can act as a pump, which draws surface and intermediate waters that often include surface-derived organic matter to depth (Davies et al., 2009; Mienis et al., 2007; White et al., 2005). Strong currents can prevent cold-water corals from being buried by sediment and may also resuspend recently deposited food particles. In cold-water coral areas in the Northeast Atlantic, where a pronounced diurnal tidal current is present, food particles may become repeatedly available for capture by corals and associated fauna (Davies et al., 2009; Mienis et al., 2009). A similar food supply mechanism was proposed for the cold-water coral areas in the Straits of Florida and on the Florida-Hatteras slope, where strong currents accelerated by the steep flanks of mounds supply enhanced nourishment relative to surrounding areas (Mullins et al., 1981; Neumann et al., 1977; Paull et al., 2000). The VK 826 knolls (29110.00 N/ 881 1.00 W) are elevated features found between 450 and 550 m water depth in the GoM. In the area of VK 826, surveys with remotely operated and human occupied vehicles have documented two distinct cold-water coral habitats. On top of the knolls, L. pertusa forms thickets, which grow on eroded authigenic carbonate blocks and slabs (Davies et al., 2010). These authigenic carbonate blocks and slabs have been formed through the precipitation of calcium carbonate as the by-product of microbial oxidation of hydrocarbons (Formolo et al., 2004). The remaining hard substrata are often colonised by a diverse assemblage of filter feeders including L. pertusa (Cordes et al., 2008; MacDonald et al., 2003; Schroeder, 2002). Video images from VK 826 have also shown that chemosynthetic communities consisting of vestimentiferan worms and mussels are currently present or have been present in close proximity to cold-water corals (Cordes et al., 2008; Davies et al., 2010), but no trophic linkage has been established between the coral and seep communities (Becker et al., 2009). A well-developed reef

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250 m/day

BOBO

Blue Reef BOBO Main knolls

ALBEX

ALBEX 250 m/day

Fig. 1. Multibeam bathymetry (contour interval 20 m) of the Viosca Knoll (VK 826) and the surrounding area showing the lander locations and a CTD transect (line) carried out across the cold-water coral habitats. Inset shows the Gulf of Mexico and the general hydrographic pattern of the Loop Current. Vector plots show the displacement of water (m per day) at both lander locations.

structure (Blue Reef) is situated at the eastern edge of a pockmark 1 km northwest of the VK 826 knolls (Fig. 1) (Davies et al., 2010; Schroeder, 2007). Density of L. pertusa is greater at this site than on the main knoll of VK 826 (Davies et al., 2010). In this study we present long-term measurements (349 days) of near-bed and water column hydrodynamic conditions of two sites within the VK 826 cold-water coral area in the GoM, expanding upon the short-term benthic lander deployments reported by Davies et al. (2010). One benthic lander was deployed in the direct vicinity of the cold-water corals, while the other lander was deployed outside the cold-water coral area. Two different sites were chosen to measure small scale near-bed environmental variability. The complicated circulation patterns in the GoM make long-term measurements important in order to define oceanographic patterns that are relevant for the development and persistence of cold-water coral ecosystems. This study aimed at defining the seasonal variability in quantity and quality of particle flux to the VK 826 coral community in relation to the hydrography of the area. These findings are compared with methodologically similar studies that have been conducted in coral habitats in the eastern North Atlantic. 1.1. Geo-environmental setting The GoM is a semi-enclosed basin with a broad shelf and an irregular continental slope with numerous canyons, knolls, basins and mounds (Sager et al., 1999). The GoM can be described as an active/passive margin due to salt tectonics. Salt movements have caused the slope to be broken by numerous faults, which act as conduits for the migration of hydrocarbons (Sager et al., 2003). As a result the northern GoM contains hundreds of active hydrocarbon seeps (MacDonald et al., 1990). The sediment in the northern GoM mostly consists of clays. Sediment transported by the Mississippi–Atchafalaya River system dominates sedimentation patterns near VK 826 and consists of non-carbonate terrigenous material (Balsam and Beeson, 2003; Morse and Beazley, 2008). The Mississippi– Atchafalaya River system discharges large quantities of sediments and disperses terrestrial organic matter and nutrients. The major river discharge of the Mississippi typically occurs in April.

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Terrestrial organic matter can be directly transported to the seafloor, but an increased amount of organic matter and nutrients can also stimulate primary productivity at the ocean surface, whereby marine organic matter is produced that can eventually sink to the seafloor (Jochens and DiMarco, 2008; Walker et al., 2005). Wind appears to be the dominant forcing factor influencing primary productivity in the northern GoM during other peaks. The major peaks in surface chlorophyll a concentrations southeast of the Mississippi River system usually occur during July and are driven by a reversal of the prevailing wind direction to the southsouthwest, which forces river water eastwards, creating upwelling (Martı´nez-Lo´pez and Zavala-Hidalgo, 2009; Walker et al., 2005).

temperature variations of  0.8 1C over 5–11 h periods, while a high frequency oscillation (20–30 min periods) in temperature was recorded by three different sensors at the ALBEX lander site, which was deployed on the main VK 826 knoll (Davies et al., 2010). Current speeds (1 m above the bottom) ranged between 6.7 and 20.6 cm s  1 (Davies et al., 2010). The Blue Reef and the main knolls at VK 826 were situated in an oxygen minimum zone (2.6–2.8 ml l  1) and no fluorescent particles were observed during the short-term measurement period. However, an upward looking acoustic doppler current profiler (ADCP) recorded a diel vertical migration in the volume scattering strength up to 180 m above bottom (mab), likely related to migrating nekton or zooplankton (Davies et al., 2010).

1.2. Physical oceanographic setting 2. Methods Circulation in the GoM is driven by a number of physical processes, including the Loop Current (LC), the anti-cyclonic rings shed from the LC, cyclonic eddies associated with the LC and LC rings, both local and remote winds and tropical storms (Hofmann and Worley, 1986; Sturges et al., 2010; Sturges and Kenyon, 2008; Teague et al., 2006). The LC enters the GoM through the Yucatan channel and exits via the Straits of Florida. The name of the current changes to the Florida Current within the Straits of Florida, and then it becomes known as the Gulf Stream as it continues its journey along the continental slope of the southeastern United States and into the open Atlantic Ocean. This entire system is commonly known as the Gulf Stream System, which forms the western boundary current of the North Atlantic subtropical gyre (Sheinbaum et al., 2002). The water in the LC is primarily North Atlantic Central Water (NACW), which is characterised by low oxygen concentrations (2.5–3.5 ml l  1) and a temperature and salinity range of 8–19 1C and 35.1–36.7, respectively. Subtropical Underwater (SUW) is typically found below the mixed layer and is characterised by a salinity maximum of 36.5 (Jochens and DiMarco, 2008). The LC brings Antarctic Intermediate Water (AAIW) into the GoM between 500 and 1000 m water depth, which is characterised by a salinity minimum of 34.9 and high nutrient concentrations (Jochens and DiMarco, 2008; Sturges, 2005). An oxygen minimum at about 600 m depth reveals the presence of Tropical Atlantic Central Water (TACW) (Morrison et al., 1983). Intermittently, a deep south-bound counter-flow flows from the GoM through the Yucatan Channel into the Caribbean Sea (Hofmann and Worley, 1986; Sheinbaum et al., 2002; Sturges and Kenyon, 2008). Periodically the LC extends northwards towards the Mississippi delta. This northward penetration of the LC into the eastern GoM undergoes a cycle that is variable in period, but typically takes from 6 to more than 12 months (Sturges, 2005). Over the course of several months, the LC penetrates farther northward into the GoM, the path of the current becomes unstable, and a large anti-cyclonic LC ring gradually separates from the main LC and drifts to the west. Ring separation intervals are most frequent at 6, 9 and 11.5 months, but some 18 month intervals have also been observed (Auladell et al., 2010; Sturges et al., 2010). Near the VK 826 study area, which is situated north of the LC most of the time, long-term measurements showed that currents tended to flow along isobaths. Sub-inertial westward propagating waves with periodicities of 16–21 days and wavelengths of 500 km have been observed between 200 and 500 m depth. Current fluctuations at these periods, which are coherent over the slope through much of the water column, are likely a form of coastally trapped waves (Hallock et al., 2009). Recent measurements from a short-term benthic lander deployment in October 2008 on VK 826 defined two processes causing variability in temperature and salinity, but failed to pick up longer circulation patterns (Davies et al., 2010). Internal waves caused

2.1. Benthic landers Two benthic landers were deployed in the VK 826 area in October 2008 during a cruise with the NOAA ship Nancy Foster and were retrieved in September 2009 by the R/V Seward Johnson. Two contrasting sites were selected to study local long-term near-bed conditions in an area with corals and an area without coral cover. An ALBEX lander was deployed at 481 m water depth in an area with fine sediments (near the deployment site of the BOBO lander in Davies et al. (2010)) without corals and was situated between the main knoll and the reef-like structure known as Blue Reef. The BOBO lander was positioned at 480 m depth in an area close to the Blue Reef (Fig. 1). Both landers were deployed for almost a year (349 days) to obtain long-term records of near-bed environmental conditions. The ALBEX lander consisted of an aluminium tripod (Duineveld et al., 2004), equipped with a 3D acoustic Nortek-AS current meter, a Seabird SBE 16þ CT sensor measuring temperature and conductivity, a Wetlabs combined optical backscatter sensor and fluorometer to measure the turbidity and amount of fluorescent particles in the water column and a Teledyne RDI 300 kHz upward looking ADCP to measure ambient currents in the water column. All instruments were mounted in the frame at 1 mab, except the ADCP, which was mounted at 2 mab. The BOBO lander consisted of an aluminium tripod frame that extended 4 mab, allowing a clear area underneath the lander for current velocity and acoustic backscatter measurements (Van Weering et al., 2000). The BOBO lander was equipped with a downward looking Teledyne RDI 1200 kHz ADCP mounted at 2 mab measuring current velocity and acoustic backscatter in 5 cm bins, an SBE 16þ CT sensor mounted at 3 mab, a Wetlabs combined fluorescence and optical backscatter sensor and a NIOZ designed pan and tilt camera recorded video images of the seabottom in six different positions every 48 h. All equipments on both landers were programmed to record at 15 min intervals, except the upward looking ADCP, which recorded observations every 2.5 min. A daily moving average was calculated for all optical backscatter, acoustic backscatter and fluorescence data from the BOBO and ALBEX landers. 2.2. CTD CTD profiles were made on 12 October 2008 using a SBE 911 þ profiler deployed from the Nancy Foster. CTD profiles used in this study and by Davies et al. (2010) were recorded along a transect across the cold-water coral area with stations at 500 m spacing (Fig. 1). 2.3. ADCP data processing Upward-looking ADCP data from the ALBEX lander were binned at 4 m intervals. Echo intensities were converted to backscatter

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

coefficient (Sv) (Deines, 1999). Short-term ensembles (2.5 min) were subjected to quality control to remove outliers prior to averaging. Current values for any ensemble without at least one 4-beam solution or that relied on more than two 3-beam solutions were removed. Velocities where backscatter values exceeded  45 dB were also removed. Once all corrections were applied, the data were averaged into mean hourly values. The spatial (vertical) and temporal structure of the currents is represented using empirical orthogonal function (EOF) analysis (e.g. Preisendorfer, 1988) and follows the development in Edwards and Seim (2008). The technique is used to estimate the most common vertical modes of the horizontal currents in the overlying circulation.

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2.5. Sediment sampling Sediment samples were taken at the location of the BOBO lander with a cylindrical box corer, which was designed and constructed by NIOZ, with a height of 55 cm and a diameter of 30 cm. The box corer was sub-sampled by inserting a PVC liner into the sediment. The top 1 cm slice of the sub sample was measured for 210Pb activity, total organic carbon and nitrogen, biogenic silica and stable C and N isotopes.

3. Results 3.1. Seabed and watermass characteristics

2.4. Sediment trap sampling The BOBO lander was equipped with a Technicap PPS4/3 sediment trap with the aperture (0.05 m2) at 4 m above bottom. The sediment trap was equipped with a rotating carousel of 12 bottles (250 ml). Each bottle collected material for 27 days, and samples were preserved in a pH buffered HgCl2 solution in seawater, which was collected from the deployment site (Bonnin et al., 2002). After retrieval of the lander all sediment trap samples were stored at 4 1C until further analyses. All sediment trap samples were filtered with a 1 mm sieve to remove zooplankton when present. Samples were split with a rotor splitter and two splits of each sample were rinsed with demineralised water to remove salts and HgCl2. All samples were freeze dried, after which total matter was weighed and mass fluxes calculated. Total organic carbon and nitrogen of the samples were analysed with a Heraeus CHN-O-Rapid element analyser at the ¨ Department of Geosciences at the University of Bremen (Muller et al., 1994). For the analysis of total organic carbon 3.5–5 mg of dried sample was weighed in silver crucibles and wetted with a few drops of ethanol to suppress foaming during acidification. Samples were decalcified with 6 M HCl and dried on a hotplate at 80 1C. Biogenic silica was measured with the automated leaching ¨ method described by Muller and Schneider (1993). To determine seasonal deposition patterns of fresh particles, the stable carbon and nitrogen isotope composition of bulk material in the trap samples were analysed (10 mg of sample) using an elemental analyser-isotope ratio mass spectrometer (EA-IRMS). Standards that were used are atmospheric N2 (air) and a d13C ¼0.3% vs V-PDB. Concentrations of chlorophyll a and its derivatives (phaeophorbides, phaeophytines) were determined with reverse-phase HPLC according to the method outlined by Witbaard et al. (2000). Briefly, the Waters HPLC system consisted of a 600E controller, a 996 photodiode array in line with a 470 Fluorescence detector. Peak areas of chlorophyll a were converted into concentrations using conversion factors determined with standards (DHI, Denmark). Derivates were quantified using the extinction coefficients from Jeffreys et al. (1997). We used the P ratio of chlorophyll-a/ phaeopigments as an indicator for freshness of phytodetritus (Lavaleye et al., 2002). The 210Pb activity of sediment trap samples was used as an indicator of the relative proportion of freshly settled and resuspended material. Fresh settling material has a high 210Pb signal, whereas older resuspended material has lower values due to radioactive decay. Low 210Pb values of sediment trap samples thus indicate trapping of a higher amount of resuspended material. The 210Pb specific activity was determined by alpha spectrometry from 210Po, which was extracted from the sample by leaching with concentrated HCl. 210Po collected on silver disc via spontaneous deposition was counted using a Canberra alpha detector (Boer et al., 2006).

The BOBO lander was deployed on the eastern edge of Blue Reef at 480 m, a dense cold-water coral structure, which has developed on top of a knoll on the edge of a pockmark (Fig. 1). The summit of Blue Reef is situated at 470 m, while the slopes of the reef consist of several metres high ridges that are mainly covered by the structure forming coral L. pertusa. Video data recorded with human occupied vehicles and Remotely Operated Vehicles showed that at some sites coral colonies were covered with a thin cover of muddy sediments. Other coral taxa, such as Alcyonacea and Antipatharia, were also common in the area. The gullies between the ridges and lower flanks of the Blue reef were characterised by coral debris and dense anemone fields. Observations from the seafloor with the camera mounted on the BOBO lander and sediment samples taken with the boxcore showed fine clay-rich sediment with burrows and Actiniaria. Furthermore, video records showed that large fluffy aggregates were floating near the seafloor during the deployment. A conspicuous increase in the amount of these aggregates was observed within images recorded between 12 and 14 April 2009, when a brown layer of aggregates settled on the seafloor (Fig. 2). The ALBEX lander was located in a low relief area (near the deployment site of the BOBO lander in Davies et al. (2010)) between the Blue Reef and the main knoll of VK 826 (Fig. 1), which was characterised by a fine sandy seafloor, also showing burrows and cnidarians. CTD profiles from a transect across the VK 826 area (Davies et al., 2010), showed relatively low near-surface salinity (upper 30 m), which was likely related to the mixing of oceanic water with fresher river and continental shelf waters. Subtropical underwater (SUW), characterised by a salinity maximum, was present at around 100 m depth, (Fig. 3A). NACW occupied depths below the salinity maximum and reached around 600 m depth (Jochens and DiMarco, 2008). Near 600 m depth low salinity values indicated the probable mixing of NACW with AAIW. AAIW, brought into the GoM by the LC, is characterised by a salinity minimum and density (sy) between 27.3–27.5 kg m  3 (Jochens and DiMarco, 2008). Plotting the long-term BOBO lander data onto the temperature and salinity plots showed that throughout the deployment period, the cold-water coral area was regularly influenced by the AAIW (Fig. 3B). 3.2. Overlying circulation The data of the upward looking ADCP, which was mounted on the ALBEX lander, contained very few missing velocity estimates (o0.1%) and some outliers within 30 m directly above the lander. One elevation (26 m above the bottom) was anomalous and removed from subsequent analysis. Valid current estimates were consistently reported at 130 m above the seafloor. Based on the depth-averaged current measured by the upward-looking ADCP, the lower 130 m of the water column had a predominantly east–west flow, with the variance ellipse oriented within a degree of true east and exhibiting nearly rectilinear flow (standard deviations, su ¼16 cm s  1

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F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

Fig. 2. Images (5 and 14 April 2009) from the pan and tilt camera on the BOBO lander. An increased amount and deposition of aggregates on the seafloor was observed on 12–14 April 2009.

A

Temperature (°C) 5

10

15

20

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34.8 35.2 35.6 36 36.4 36.8 25

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Occurence of cold-water corals

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5 34

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Salinity Fig. 3. (A) Temperature and salinity plots of CTD casts carried out during a cruise in 2008. The depth interval of the cold-water coral occurrences on VK 826 is marked. (B) Temperature-salinity plot of CTD data, red dots show CT data from the two landers. Water masses indicated: NACW (North Atlantic Central Water), AAIW (Antarctic Intermediate Water), SUW (Subtropical underwater). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and sv ¼0.03 cm s  1 in the east and north components, respectively). An EOF decomposition of the currents in the lower 115 m indicated a single mode, explaining almost all of the current variability (92% of the observed variance). The vertical structure of this mode portrays a bottom boundary layer, with maximum speeds at 78 m above the bottom, decreasing towards the seafloor to half the maximum speed at 6 m above bottom (Fig. 4). The mean flow direction and the character of its temporal variability changed between the first half of the deployment (October 2008–March 2009) and second half of the deployment (April 2009–September 2009). Mean currents, measured by the upward looking ADCP, flowed eastward at 5 cm s  1 in the first half when the variability was dominated by along-isobath oscillations with a weekly periodicity that had typical peak-to-peak velocities of approximately 30 cm s  1. Mean currents were

westward at 7 cm s  1 during the second half of the deployment, and the variability was characterised by a longer periodicity along-isobath oscillations, including a strong and prolonged westward flow from early June to mid-July 2009. Typical peakto-peak velocities were 40–60 cm s  1. No significant barotropic tidal motions were observed. Backscatter at the ALBEX site showed a clearly defined diurnal cycle, in agreement with previous observations (Davies et al., 2010). This cycle was highly repeatable and is visualised in Fig. 5 as a phase average over 12 days in March, 2009. A descending limb of elevated backscatter appeared at 0700 local time and coincided with a roughly 3 dB increase in Sv in the lowest 50 m of the water column. An ascending limb formed near the bottom at approximately 1700 local time and Sv fell to its background levels by 1900 local time.

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The direction of the long-term net water displacement differed between the two landers sites. On the ALBEX site the net year long displacement was to the southwest, whilst the net displacement on the BOBO site was directed to the southeast during the first half of the year and to the southwest during the second half of the deployment.

120

height above bottom (m)

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3.4. Near-bed currents and variability

80

60

40

20

0 0.1

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mode 1 amplitude (m s-1) Fig. 4. Amplitude of the vertical structure of the first EOF mode of the upward looking ADCP, which was mounted on the ALBEX lander.

180 -52

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Fig. 5. Phase-averaged backscatter strength of the upward looking ADCP, which was mounted on the ALBEX lander (Sv in dB relative to 1 mPa at 1 m) over a 12 days period in March 2009. Note the morning descending and evening ascending limb of elevated backscatter strength.

3.3. Transport of water Vector plots of displacement relative to the lander position show the direction and magnitude of the transport of water at both lander locations during the deployment period (ellipses in Fig. 1). The directions of the water transport at the BOBO location were mainly towards the east and west, similar to the overlying currents. At the ALBEX site the largest transport was towards the southeast and west. The south-eastward water transport was likely due to topographical steering of the water flow between the two main knolls.

The ALBEX and BOBO landers recorded nearly the same nearbed variability in temperature and salinity, even though they were deployed at different locations. The temperature and salinity co-varied and showed a range over the year of 6.5–11.6 1C and 34.95–35.4, respectively (Fig. 6). Temperatures were lowest during winter (6.5 1C), whilst temperature increased in summer to a maximum of 11.6 1C. Average current speed over the year at the ALBEX and BOBO locations was 8 cm s  1, with peak current speeds of up to 38 cm s  1. In comparison, the upward looking ADCP measured an average current speed of 14 cm s  1 and peak speeds up to 60 cm s  1 over 130 mab (Fig. 6). East–west currents were strongest in the area, corresponding with water flow along isobaths (Fig. 1). For 7% of the time current speeds were above 15 cm s  1 at both lander locations, which is the given threshold to resuspend and transport sediment and aggregates on the western European continental margin (Balsam and Beeson, 2003; Thomsen and Gust, 2000) (Fig. 7). Several events occurred, where a strong predominantly westward flow accompanied by a gradual increase in temperature and salinity, changed into a rapid drop in the temperature and salinity (Fig. 8). The drop in temperature corresponded with an upward transport of water in a predominantly eastward direction. These events had no fixed duration or frequency and could not be linked to a tidal periodicity. However, they occurred more frequently during autumn and winter than in summer (Fig. 6). The most prominent event occurred in July 2009, during which the current speed was unidirectional (westward flow) and lasted for 21 day (Fig. 8). This event corresponded with the highest mass fluxes recorded during the study. The levels of near-bed optical and acoustic backscatter at both lander sites were mainly related to the current direction and strength, which are related to the volume of suspended particles in the water column. With a dominant westward flow, optical and acoustic backscatter values increased, whilst temperatures also increased. Shortly after the reversal of the current to the east, turbidity and backscatter levels declined, as illustrated by significant negative correlations between E–W current speed (E is positive) and acoustic and optical backscatter. This supports the link between a decline in backscatter during periods with eastern (and upwards) currents (r ¼   0.3). Analysing eastward and westward currents separately, acoustic backscatter appears in both cases to be positively correlated with current speed, indicating that resuspension events increased with stronger currents (r ¼0.5). Most likely this relationship was driven by the larger particles (aggregates), such as those observed by the BOBO video camera, especially given that the current meter mounted on the ALBEX is particularly sensitive for larger particles between 160 and 320 mm. Relationships between current speed and optical backscatter (turbidity) were weaker and negative. During periods of eastward flow the increased upslope flow brought clearer water onto the knoll, which was accompanied by a decrease in the background turbidity. Fluorescence values at the study site were generally low, except for two large (April and July 2009) and two small (June and July 2009) excursions. Each fluorescence peak coincided with increased levels of optical and acoustic backscatter. However, the peaks in fluorescence were not linked to either temperature

38

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

J mass (mg m-2day-1)

5000 4000 3000 2000 1000 0 Current speed (cm s-1)

50

Upward looking ADCP

40 30 20 10 0

Current speed (cm s-1)

40

BOBO ALBEX

30 20 10 0 120

Acoustic backscatter (DMA)

110 100 90 80 70

Turbidity (FTU, DMA)

0.3 0.25 0.2 0.15 0.1 0.05

Fluorescence (µg l-1, DMA)

0.2 0.18 0.16 0.14 0.12

Temperature (°C)

12 BOBO ALBEX

10

8

6 Oct-08

Dec-08

Feb-09

Apr-09

May-09

Jul-09

Sep-09

Date Fig. 6. Temperature (BOBO and ALBEX), daily moving average (DMA) of fluorescence, turbidity (DMA), acoustic backscatter (DMA), near bottom current speed, depth averaged current speed from the upward looking ADCP (0–130 mab) and the mass flux (collecting interval 27 days). Both landers recorded exactly the same near-bed variability. Fluorescence values near the seabed were low except for two periods in April and June.

or current speed (Fig. 6), but correlated with an increase in pigments and biogenic silica (see Section 3.5 below). 3.5. Mass fluxes and composition of sediment trap samples Each sediment trap bottle collected material for 27 days, and all samples contained zooplankton. From October to January mass fluxes were high (3.6–4.5 g m  2 day  1), corresponding to several

strong current events, which also correlated with several peaks in acoustic backscatter and turbidity (Figs. 6 and 9). Low mass fluxes (1.2–1.2 g m  2 day  1) occurred in February and March during periods of fluctuating east and westward flows. During March and April 2009 mass fluxes (2.9–3.1 g m  2 day  1) continuously increased, apparently related to prolonged periods of mainly westward flow. However, the mass fluxes began to decrease towards the end of April. The increased mass fluxes in March

50

50

40

40

% Current speed ALBEX

% Current speed BOBO

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

30

20

39

30

20

10

10

0

0 0

5

10

15

20

25

30

0

Current speed (cm s-1)

5

10

15

20

25

30

Current speed (cm s-1)

Acoustic backscatter (DMA)

Direction

Fig. 7. Frequency histograms of current speed (1 m above the bottom) at the BOBO and ALBEX sites. For up to 93% of the time the current speeds were below 15 cm s  1.

360 300 240 180 120 60 0 120 100 80 60

Vertical speed (mm s-1)

40 20 0 -20 current speed (cm s-1)

30 20 10 0

Temperature (°C)

12 10 8 6 11-Jun

26-Jun

11-Jul

26-Jul

10-Aug

25-Aug

Date Fig. 8. Several events were recorded by the BOBO and ALBEX lander, characterised by a gradual increase in temperature and salinity, followed by a fast decrease, which corresponds to upward transport of water in an eastward direction. The most prominent event in July (30 May–30 August) is indicated by the arrow, corresponding to a westward flow, which lasted for 21 days.

40

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

1600 1200 800 400 0

J CaCO3 (mg m-2day-1)

36 30 24 18 12

% CaCO3

210Pb (Bq g-1)

1600 1400 1200 1000 800

12

18

0.5

12

0.25

6

0

0

7 6 5 4 3

200 150 100 50 0

2500

Acoustic backscatter (DMA)

J Mass (mg m-2day-1)

5000

Turbidity (FTU, DMA) Fluorescence (µg l-1, DMA)

J Corg (mg m-2day-1)

0.75

J Ntot (mg m-2day-1)

10

% Corg

% total N

C/N ratio

14

0 125 100 75 0.3 0.2 0.1 0 0.2 0.18 0.16 0.14 0.12 Oct-08

Dec-08

Feb-09

Apr-09

May-09

Jul-09

Sep-09

Date Fig. 9. Sediment trap data of Corg, CaCO3, Ntot, 210Pb, C/N and corresponding fluxes (bar charts). Lander data of acoustic backscatter (DMA), fluorescence (DMA) and turbidity (DMA) are also shown. High mass fluxes were observed during periods of peaks in acoustic backscatter.

and April appeared to be related to an increase in primary productivity, which was recorded near the seabed as a peak in fluorescence and the arrival of a large amount of aggregated particles recorded by the camera on the BOBO lander (Fig. 2). The period with the highest mass flux (4.7 g m  2 day  1) corresponded to the event in June 2009 and was correlated with a peak in current speed and a unidirectional westward flow that

lasted for 21 days (Figs. 6 and 9). During periods of westward flow, acoustic and optical backscatter values were high, indicating an increase in the amount of particles in the water column (Table 1), which correlated with a greater amount of material in the sediment trap. Of the total particulate matter collected in the sediment trap 15.6–28.2% was CaCO3. The biogenic silica (SiO2) content varied

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

41

Table 1 Spearman correlation coefficients (r) calculated for measured parameters. 27 day averages, corresponding to the collecting interval of the sediment trap of temperature, acoustic backscatter, current speed and turbidity were used. For values presented in bold, r o0.01; values presented in bold italic font r o0.05. Average DT

Average DT Average turbidity Average acoustic backscatter Average current speed Mass flux (mg day  1/m2) % Corg % CaCO3 % Ntot 210 Pb % SiO2 Pigment flux (mg day  1/m2)

Average turbidity

Average acoustic backscatter

Average current speed

Mass flux % Corg (mg day  1/m2)

% CaCO3

% total N

210

Pb

% SiO2

Pigment flux (mg day  1/m2)

0.729

0.829 0.618

0.059 0.379 0.199

0.665 0.457 0.000

0.762 0.319 0.020

0.342 0.391 0.430

0.829 0.319 0.033

0.557 0.199 0.265

0.914 0.557 0.010

0.308 0.404 0.245

0.226

0.007

0.085

0.042

0.812

0.075

0.746

0.024

0.633

0.075

0.048

0.191

0.059

0.443

0.000 0.391

0.983 0.587 0.914

0.009 0.319 0.003 0.542

0.729 0.880 0.527 0.001 0.366

 0.112 0.070

0.161

0.559

 0.280

0.399

0.140

0.238

0.895

0.378

 0.098 0.301 0.070  0.189  0.035 0.322

0.315 0.273 0.315  0.399 0.189 0.266

 0.657 0.252  0.615  0.350  0.706 0.364

 0.727 0.517  0.594 0.077  0.531  0.105

Table 2 Geochemical data of a core top that was taken at the location of the BOBO lander. Type

Data

Corg (%) N (%) C/N 210 Pb (Bq g  1) d15N (%) d13C (%)

2.66 0.19 12.41 739 6.87  21.34

 0.643 0.154  0.531  0.580  0.406 0.559

 0.245 0.937 0.007 0.713 0.112

 0.273  0.175  0.315  0.049

0.035 0.776 0.203

 0.196  0.818 0.287

The background value of a sediment core taken in the vicinity of the landers showed a 210Pb activity of 738 m Bq g  1 (Table 2).

4. Discussion

between 2% and 11% and organic carbon (Corg) values varied between 3.2% and 6.3%. During periods of high mass fluxes the proportion of Corg was low (3.2–3.8%). During periods of low mass flux, the proportion of Corg increased. Notable increases in Corg (6.7%) and SiO2 (11.4%) also occurred during periods of peak fluorescence (April 2009 and July 2009). Bottle 11 (July 2009) showed the highest peaks in pigments, SiO2, Corg and Ntot values (Fig. 9). Corg proportions were negatively correlated with the monthly averaged current speed (r ¼  0.7) and acoustic backscatter (r ¼ 0.65) (Table 1). When current speed, acoustic backscatter values and mass fluxes increased, Corg proportions decreased. This can be related to dilution due to the presence of resuspended material that had a lower Corg content (background sediment of boxcore 2.66%, (Table 2)). C/N (atomic) ratios varied between 9.7 and 13.7. The highest C/N ratio was observed in March–April 2009, whilst the lowest values occurred in June and July. The latter can be related to the arrival of fresh phytodetritus at the seafloor as indicated by peaks in fluorescence. This was confirmed by pigment analysis of the sediment trap samples, which showed increased fluxes of chlorophyllous pigments during fluorescence peaks in combination with a higher freshness of the collected phytodetritus as indicated by a4100-fold increase in the ratio of P chlorophyll-a/ phaeopigments. The d15N values of the sediment trap had a relatively fresh signature and ranged between 2.98% and 4.51%. An exceptionally high d15N value of 7.1% was measured in June 2009, which corresponds to the highest mass flux (Fig. 10). The d15N of a core top of a sediment core taken at the position of the BOBO lander was 6.87% (Table 2). The d13C values varied between  21.49% and  20.44%, within the typical range of marine organic matter. Total 210Pb activity ranged between 966 and 1530 m Bq g  1. The 210Pb activity was negatively correlated with mass flux (Table 1), indicating that during periods of high mass flux, some resuspended material is collected in the sediment trap (Fig. 9).

4.1. Hydrodynamics This study presents a long-term record of near-bed environmental conditions within the Viosca Knoll VK 826 cold-water coral area in the GoM. Long-term observations allowed for the first time the comparison of the physical conditions in cold-water coral habitats of the GoM with those in the North East Atlantic, using an identical methodology. In the North East Atlantic the cold-water coral species Lophelia pertusa and Madrepora oculata have formed large reef and mound structures (De Mol et al., 2002; Freiwald et al., 1997; Lindberg and Mienert, 2005; Van Weering et al., 2003a). On the Irish margin, these cold-water coral species contributed to the formation of mound structures of up to 380 m high with a thriving living coral cover on the summits of the mounds (Kenyon et al., 2003; Mienis et al., 2006; Wheeler et al., 2007). In the GoM cold-water corals have mainly formed thickets, growing on authigenic carbonate blocks and slabs (Brooke and Schroeder, 2007; Cordes et al., 2008; Davies et al., 2010; Huebscher et al., 2010). Close to VK 826, however, Blue Reef situated near the edge of a pockmark consisted of a dense framework of L. pertusa and resembled structures found off the southeastern United States and in the North East Atlantic (Davies et al., 2010; Ross and Nizinski, 2007; Schroeder, 2007). During short-term measurements on VK 826 temperature fluctuations of up to 2 1C were measured (Brooke et al., 2009; Davies et al., 2010). However, annual temperature (  5 1C) and salinity fluctuations were much larger as shown by this study, demonstrating the importance of long-term monitoring. The near-bed environmental conditions (temperature, salinity, current speed) recorded in the cold-water coral area on Viosca Knoll fit in the range as described for cold-water coral areas on the European margin (Freiwald, 2002) and from niche models (Davies and Guinotte, 2011; Davies et al., 2008). The main differences between the cold-water coral area at VK 826 and cold-water coral habitats in the North East Atlantic are that on VK 826 a tidal motion is absent, dissolved oxygen concentrations are low (Davies et al., 2010) and the particle load (up to 3 mg l  1) is relatively high. Cold-water corals on VK 826 are most of the time bathed in the water masses of the Loop Current (Davies et al., 2010; Jochens and

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

δ15N

42

8 7 6 5 4 3 2 12 8 6

100

4 0

2

200 J pigments (mg m-2day-1)

% SiO2

10 200

0.2

160 0.18

120 80

0.16

40 0

Fluorescence (μg l-1, DMA)

J SiO2 (mg m-2day-1)

300

0.14 Oct-08

Dec-08

Feb-09

Apr-09

May-09

Jul-09

Sep-09

Date 15

Fig. 10. Sediment trap data of pigments, SiO2, d N and fluorescence and corresponding fluxes (bar charts).

DiMarco, 2008). During the present long-term deployment, coldwater corals near VK 826 were located near the boundary of the NACW and AAIW. In the GoM, AAIW characterised by its salinity minimum, is generally found below 600 m water depth (Hofmann and Worley, 1986; Jochens and DiMarco, 2008; Sturges, 2005), but regularly occurs around VK 826, which is probably related to upslope transport of water. The cold-water coral habitats on VK 826 are situated on a sloping bottom near 480 m water depth. Previous studies have shown that this water depth is characterised by the presence of bottom intensified currents with relatively high velocities (Carnes et al., 2008; Jochens and DiMarco, 2008). During the long-term lander deployments, near-bed peak current speeds of up to 38 cm s  1 were measured, whilst the average current speed was 8 cm s  1. No tidal periodicity was observed and flow was mainly along isobaths in eastward or westward direction as also observed during previous studies in the region (Carnes et al., 2008). Several reoccurring events were observed, characterised by an increase in current speed, followed by a temperature increase. This pattern is consistent with bottom boundary layer dynamics influenced by friction (i.e., a bottom Ekman layer). Eastward overlying flow drives a northward boundary layer flow near the bottom, which produces decreased temperatures through upslope advection of the temperature field. A clear correlation can be observed between the direction of the water flow and the amount of material collected in the sediment trap. During periods of westward flow, large amounts of particulate matter are transported towards the cold-water coral area. This results in increased levels of acoustic and optical backscatter and higher mass fluxes. During periods of eastward flow, colder and clearer water was pushed up the margin. This phenomenon was likely also detected during a CTD transect carried out during the short deployment, when a water layer with low turbidity values was observed around VK 826. Indeed the flow regime during the short deployment was to the east, corresponding to the upward movement of cold and less turbid water towards the cold-water corals (Davies et al., 2010).

In the immediate vicinity of the ALBEX lander, topography alters the local flow pattern, channelling the horizontal current and on occasion producing large vertical velocities. This demonstrates the strong influence of topography in generating small spatial scale variability in the bottom boundary layer flow field. Evidence for this latter effect is seen in the displacement plots (Fig. 1) and in the change in the veering direction of the vertical structure within 20 m of the seafloor (Fig. 4). This structure closely resembles the 5-day mean flow from the short-term deployment (Davies et al., 2010), and is similar to the structure of topographically trapped subinertial waves that are prevalent over the continental slope in the northern GoM (Hallock et al., 2009). During winter the frequency of higher current speed excursions was greater compared to the summer. In winter the current variability was dominated by fluctuations of near weekly periods, while these periods were longer during summer (Fig. 6). 4.2. Particle supply Mass fluxes in this study were ten times less than those described by Brooke et al. (2009). In their study they used a PVC cylinder with the aperture 30 cm above the seafloor, which collected material for 14 months (July 2004–September 2005). It is likely that large amounts of locally resuspended and horizontally advected sediment were captured during their deployment, especially since the aperture of the tube was so close to the seafloor, which resulted in very high mass fluxes. These mass fluxes could not be related to specific events like the hurricanes or other large resuspension events, since all material was collected in one tube. (Brooke et al., 2009). Nevertheless, the mass fluxes as obtained from our sediment trap with the aperture at 4 mab on VK 826 were also high compared to those measured using the same approach on cold-water coral mounds on the Rockall Bank (Table 2). These fluxes and especially the organic carbon fluxes on the VK 826 were two to four times higher than those recorded on the Rockall and Galicia Banks. At these two sites in the North East Atlantic the trap

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

collected mainly resuspended material (Duineveld et al., 2004; Mienis et al., 2009). In contrast, the composition of the trap samples from VK 826, (low CaCO3 content, high organic carbon content and high C/N ratios) suggests a more terrestrial origin (Balsam and Beeson, 2003; Morse and Beazley, 2008). While C/N ratios obtained at 10 mab near coral areas in the North East Atlantic were close to those of marine phytoplankton (Redfield ratio C/N: 6.5) (Kiriakoulakis et al., 2007), C/N values at VK 826 were above 9, which suggests that part of the material is either of terrestrial, likely riverine origin or is degraded resuspended material (Duan et al., 2007). Resuspension was not the main contributor of mass fluxes at VK 826 and the largest proportion of trapped particles appears to have been captured from the water column. We conclude this from the fact that highest C/N value (March–April 2009) did not correspond with the lowest 210Pb values, a high mass flux or period of increased current speed. By contrast, the C/N ratio correlated with peaks in optical and acoustic backscatter. These peaks might be linked to increased river discharge in the area during this period (see below). During the deployment at VK 826, current speeds exceeding 15 cm s  1 (the threshold velocity to resuspend aggregates and sediment (Thomsen and Gust, 2000)) accounted for only 7% of the total observations. In comparison, on the Rockall Bank coral sites, 30% of the current measurements were in excess of 15 cm s  1 (Mienis et al., 2009). Several peaks in current speed at VK 826 (October–December 2008 and June 2009) corresponded with resuspension events recorded by the sediment trap samples containing lower 210Pb values, decreasing Corg content and increasing stable nitrogen isotope values. This demonstrates the dilution of fresh material with older resuspended material. The largest resuspension event occurred during the prolonged period of westward currents in June 2009. Brooke et al. (2009) described the effect of different concentrations of suspended sediment on survival of L. pertusa from the GoM. They found that long exposure (14 days) to concentrations above 100 g l  1 caused mortality. During the year of our observations, supended particulate matter concentrations at 4 mab (average 3 mg l  1) never reached the critical levels as reported by Brooke et al. (2009). Furthermore, corals at VK 826 experience periods of clearer water during eastern flow. This will prevent the corals from burial and suffocation and allows them to expel sediment from the polyps (Brooke et al., 2009; Mortensen, 2001). On the other hand, periods of high particle load will have a positive effect on the extension of the reef structure. The coral framework will create locally low energy environments and sediment particles will baffle between the network of coral branches (De Haas et al., 2009). 4.3. Food supply Corals require nutritious particles to grow and outpace sedimentation rates and to maintain a healthy framework. In general cold-water corals are found in areas where a consistent food supply is present, which can either be high quality surface derived material (phytodetritus) (Becker et al., 2009; Davies et al., 2009; Duineveld et al., 2004; Kiriakoulakis et al., 2004), zooplankton (Dodds et al., 2009) or a combination of both (Van Oevelen et al., 2009). In the GoM hydrocarbon seepage could form a third source of food. Although seep communities are present in the vicinity of the cold-water corals on VK 826 (Cordes et al., 2008), no link has been observed between active seepage and the feeding behaviour or chemical composition of L. pertusa (Becker et al., 2009). The d13C of coral tissue was  21.7 (Becker et al., 2009) and the material collected in the BOBO sediment trap showed an isotopic range between  21.49% and  20.44%, which is the typical range for marine organic matter. These isotopic ranges are similar to those found in samples from the North East Atlantic (Duineveld

43

et al., 2007) and fall far outside ranges measured from chemosynthetic communities. Perhaps the most important source of nutrition in the VK 826 coral area is indirectly related to the Mississippi River outflow. The river discharges high amounts of nutrients and terrestrial organic matter, which promotes increased surface productivity in the Northern Gulf of Mexico (Duan et al., 2007; Jochens and DiMarco, 2008; Morse and Beazley, 2008; Walker et al., 2005). Part of the photic zone production reaches the cold-water corals at VK 826 as was observed by several periods of increased fluorescence at 1 mab. These peaks in fluorescence corresponded with an increase in the amount of pigments, Corg, Ntot and biogenic SiO2 in the sediment trap (Figs. 9 and 10). Two distinct periods of increased fluorescence occurred in 2009 near the seabed, both of which can be linked to surface blooms visible on satellite images showing a high productivity area near the study site (Martı´nez-Lo´pez and Zavala-Hidalgo, 2009). One large peak was observed in April 2009 and several small peaks were observed during June and July 2009. The April peak occurred during the annual period of major discharge from the Mississippi River (Duan et al., 2007; Martı´nez-Lo´pez and Zavala-Hidalgo, 2009). Video records from this period showed a marked increase of large fluffy aggregates floating across the area and settling on the seafloor (Fig. 2). The smaller peaks in June and July can be related to changes in wind direction from the east to the southsouthwest, driving river waters eastwards in the eastern GoM and creating upwelling (Jochens and DiMarco, 2008; Walker et al., 2005). Thus, peaks in fluorescence at the seabed at the BOBO lander site in the vicinity of the Blue Reef can be related to surface processes, demonstrating a strong coupling between the photic zone and the coral habitat at 470 m water depth. This was confirmed by the stable nitrogen and carbon isotopes of the trap samples, which indicate the presence of relatively fresh marine organic matter near the seafloor. The stable carbon isotope composition of corals on VK 826 suggests that these species thrive on a surface derived food source (Becker et al., 2009). The d15N values (2.98–4.51) of the suspended particulate organic matter fit in the range of isotope values measured in living L. pertusa tissue as described by Becker et al. (2009) and for coral areas in the NE Atlantic (Duineveld et al., 2007). Several cold-water coral habitats on the European continental margin also appear to be closely coupled to surface productivity albeit via different mechanisms (internal waves, Ekman drainage or hydraulic jumps) (Davies et al., 2009; Kiriakoulakis et al., 2004; Mienis et al., 2007). A second food source for the corals living at VK 826 could be the daily migrating zooplankton that was recorded by the upward looking ADCP in this study, and also during the short deployment by Davies et al. (2010). Limbs in acoustic backscatter moving through the water column were consistent with nocturnal surface feeders seeking shelter at depth during daylight hours. The strength of the backscatter signal varied over the year, but without a record of species composition in the water column it is not possible to say if the changes are due to changes in density of organisms or due to shifts in community composition over time. The stable isotope data from Becker et al. (2009) for L. pertusa colonies showed a wide range, suggesting that the corals may feed across two trophic levels. This suggests that coldwater corals on VK 826 might benefit from the pulses of fresh phytodetritus as well as organic matter derived from the vertical migration of zooplankton or their faeces.

5. Conclusion Results presented from a long-term deployment of two bottom landers in a cold-water coral area in the GoM show that most of

44

F. Mienis et al. / Deep-Sea Research I 60 (2012) 32–45

the environmental near-bed conditions measured on VK 826 strongly resemble conditions as measured using identical methodology in the NE Atlantic cold-water coral sites. However, two main differences were observed at VK 826: 1) a higher particle load in the water column and 2) lower dissolved oxygen concentrations. Both parameters can be limiting factors for coldwater coral growth. The high sediment load was linked to the presence of the Mississippi River outflow that occurs in the vicinity of VK 826. Riverine material increased the amount of nutrients and organic carbon in the water column, making this part of the GoM an area with high primary productivity, which creates marine organic matter that will sink to the cold-water coral area near VK 826. Particle supply in the area is mainly dependent on the current direction. With a westward current, turbid and warm waters provide the corals with particles, whilst during eastward flow cold and less turbid water is pushed up the slope. These periods of clear water could be important for the survival and growth of the corals, since they may provide time for them to expel sediment from their polyps. Furthermore, two possible food sources were observed in the GoM: 1) phytoplankton blooms and 2) a diel vertical migration of zooplankton above the coral area. Corals may feed on both sources as was suggested during previous studies in the GoM and elsewhere. Long-term monitoring in the vicinity of these fragile ecosystems is exceptionally important due to the variable nature of particle supply and environmental conditions. With only shortterm monitoring this variability would be missed, and our understanding of the long-term temporal dynamics of these ecosystems would be much poorer.

Acknowledgements We greatly acknowledge the assistance of captain, crew and technicians on board the NOAA ship Nancy Foster and the R/V Seward Johnson. Bob Koster and Lorendz Boom were our support during lander operations. We also thank Marco Klann for help with the sediment trap analyses. Wim Boer and Rineke Gieles are thanked for their help with the 210Pb measurements. Birgit Meyer-Schack is thanked for her help with the stable isotope measurements. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under the HERMIONE Project, Grant agreement no 226354, MiCROSYSTEMS Project (ESF/NWO Contract number 855 01 106) and CARBONATE Project (ESF/NWO Contract number 855 01 120). FM was funded through the DFGResearch Center/Excellence Cluster ‘‘The Ocean in the Earth System’’. A Winston Churchill Travelling Fellowship funded AJD’s participation. We thank the U.S. Geological Survey (Outer Continental Shelf Ecosystem Studies Program), especially Dr. G. Brewer, for funding (under Cooperative Agreement number 05HQAG0099, subagreement 5099HS0013), which supported cruises and some data analyses. We appreciate the encouragement and support of G. Boland (BOEMRE) during these studies. Nancy Foster ship time was provided by the NOAA Undersea Research Center (NURC) through a Grant to SWR. Erik Cordes provided the multibeam sonar image of VK 826. References Auladell, M., Pelegrı´, J.L., Garcı´a-Olivares, A., Kirwan Jr, A.D., Lipphardt Jr, B.L., Martı´n, J.M., Pascual, A., Sangra , P., Zweng, M., 2010. Modelling the early evolution of a Loop Current ring. J. Mar. Syst. 80, 160–171. Balsam, W.L., Beeson, J.P., 2003. Sea-floor sediment distribution in the Gulf of Mexico. Deep Sea Res. Part I: Oceanogr. Res. Pap. 50, 1421–1444.

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