Hydrodynamic conditions in a cold-water coral mound area on the Renard Ridge, southern Gulf of Cadiz

Journal of Marine Systems 96–97 (2012) 61–71

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Hydrodynamic conditions in a cold-water coral mound area on the Renard Ridge, southern Gulf of Cadiz F. Mienis a,⁎, H.C. De Stigter a, H. De Haas a, C. Van der Land a, 1, T.C.E. Van Weering a, b a b

Netherlands Institute for Sea Research (NIOZ), Department of Marine Geology, P.O. Box 53, 1790 AB Den Burg, The Netherlands VU University Amsterdam, Faculty of Earth and Life Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

a r t i c l e

i n f o

Article history: Received 24 June 2011 Received in revised form 13 January 2012 Accepted 7 February 2012 Available online 14 February 2012 Keywords: Cold-water coral mounds Renard Ridge Hydrodynamic conditions Tidal currents Bottom landers

a b s t r a c t Near-bed hydrodynamic conditions obtained by bottom landers on the Renard Ridge are presented complemented with a data set from repeated CTD casts. On the Renard Ridge cold-water coral mounds were discovered in the last 10 years. Unlike cold-water coral habitats known from the Norwegian and Irish margins, these mounds are not covered with living corals. Mounds are located near the boundary between North Atlantic Central Water and Antarctic Intermediate Water. Mediterranean Water was present at greater depth, but was not observed in the vicinity of the Renard Ridge. Near-bed temperature and current speed reflect a baroclinic semi-diurnal tidal motion, causing vertical watermass movements up to 100 m and temperature fluctuations up to 1.2 °C. Average current speed was 8.8 cm s− 1, while occasionally peak current speeds up to 30 cm s− 1 occurred on top of the Renard Ridge. Tidal currents force the formation of up to 300 m thick bottom nepheloid layers. Near-bed hydrodynamic conditions around the mounds fit in the range for cold-water coral occurrences as described in literature. However, at present coral growth seems restricted by the low near-bed current speeds, the low surface productivity in a well stratified water column and the high near-bed load of fine sediment particles. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Over the past decades it has been demonstrated that cold-water coral ecosystems are widely distributed along the margins of the Northeast Atlantic Ocean (e.g. Freiwald and Roberts, 2005; Roberts et al., 2006). Here cold-water corals either occur as single colonies, patches or as kilometre-long reef and mound structures. On the Norwegian margin mainly reef-like structures are found (Freiwald et al., 1997; Lindberg and Mienert, 2005), kilometre-long and wide mound structures are present on the Rockall Trough margins and in the Porcupine Seabight (De Mol et al., 2002; Van Weering et al., 2003b), while in the Mediterranean Sea mainly isolated coral patches have been observed (Taviani et al., 2005). Cold-water coral mound occurrences are also reported from subtropical latitudes off the coast of NW Africa, on the Mauritanian (Colman et al., 2005; Eisele et al., 2011) and Angolan margins (Le Guilloux et al., 2009). The Gulf of Cadiz likely links coral occurrences as reported from the southern part of the NE Atlantic Ocean with coral occurrences on the Northern European margins (Frank et al., 2011; SchröderRitzrau et al., 2003). In the Gulf of Cadiz cold-water corals were

⁎ Corresponding author at: Center for Marine Environmental Sciences (MARUM), University of Bremen, Leobenerstraße, 28359 Bremen, Germany. Tel.: +49 42121865657. E-mail address: [email protected] (F. Mienis). 1 Present address: University of Edinburgh, Grant Institute, The King's Buildings, West Mains Road, Edinburgh EH9 3JW, Scotland, United Kingdom. 0924-7963/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2012.02.002

found along the Spanish as well as the Moroccan margins on the flanks of mud volcanoes (Diaz-del-Rio et al., 2003; Pinheiro et al., 2003), encrusting carbonate chimneys or growing on top of authigenic carbonate slabs (hardgrounds), on diapiric ridges, fault escarpments and mound structures (Diaz-del-Rio et al., 2003; Wienberg et al., 2009). Reef forming cold-water corals in the Gulf of Cadiz mainly occur between 500 and 1000 m water depths. Mound structures with an average height of 15 m above the surrounding seabed have been discovered on the Renard Ridge, a topographic elevation bounded by the Pen Duick Escarpment (Foubert et al., 2008; Van Rooij et al., 2011; Wienberg et al., 2009), which forms part of the El Arraiche mud volcano field on the Moroccan continental margin (Van Rensbergen et al., 2005b) (Fig. 1). Unlike cold-water coral mounds on the margins of the Rockall Trough and in the Porcupine Seabight (Huvenne et al., 2005; Mienis et al., 2006; Van Weering et al., 2003a), mounds in the southern Gulf of Cadiz are at present not covered with a dense live coral cover (Foubert et al., 2008; Wienberg et al., 2009). However, bottom samples showed that during glacial periods a thriving living coral cover was present (Wienberg et al., 2010). Studies on the hydrodynamic controls in thriving cold-water coral habitats have established that cold-water corals and associated species mainly occur in environments with strong currents often related to internal waves, which prevents living corals from smothering by sediment, but also increase the (food) particle supply (Dorschel et al., 2007; Duineveld et al., 2007; Mienis et al., 2007; White et al., 2005). On elevated structures like ridges and mud volcanoes in the

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Portugal Spain 38

M06-07 21

MU ML

AI

36

M05-24

M05-25 M05-23

CTD BOBO yo-yo 35°19'

LdT MV

Gulf of Cadiz

AC

EM06-05

Morocco

16 15 14 13

34 -10

-8

-6

M05-12

EM06-02

km 0 100

35°18'

PDE

EM06-01

EM06-08

M05-02

M05-01

M04-07

Plain

Gemini MV 35°17'

M06-06

35°16'

M06-01

6°50'

6°48'

6°49'

6°47'

6°46'

6°45'

Fig. 1. Multibeam map (contour interval 20 m) of part of the El Arraiche mud volcano field showing the Pen Duick Escarpment, the Lazarillo de Tormes (LdT) and Gemini mud volcano. Mounds covered with fossil cold-water corals are present on the edge of the escarpment and on the edge of the gully of the Gemini mud volcano. BOBO lander positions (stars), CTD casts (polygon) and CTD yo-yo station (circle) are indicated. Inset shows the location of the research area in the Gulf of Cadiz and the general circulation pattern of the Atlantic Inflow (AI), Mediterranean Outflow Water (MU = Mediterranean Upper Water, LW = Mediterranean Lower Water), the Azores Current (AC), NASW/NACW (black arrows) and NADW (block arrows).

Gulf of Cadiz, corals could have benefited from the interaction of the local topography with the hydrography, which accelerates the water flow (Frederiksen et al., 1992; Genin et al., 1986). Even though the surface water circulation in the southern Gulf of Cadiz forms an important part of the North Atlantic sub-tropical gyre, the region near the Moroccan margin is under-sampled and under-studied with regards to hydrodynamic conditions compared to the northern part of the Gulf of Cadiz. First results on the hydrodynamic conditions have been published by Alves et al. (2011) and Van Rooij et al. (2011). In this study we report results on near-bed hydrodynamic conditions retrieved from benthic landers that were deployed for periods of several days up to a year on the Renard Ridge, on top of the mounds and on the plain below the Pen Duick

Escarpment (Fig. 1, Table 1). In addition water column studies were carried out with a CTD to measure the water column structure during cruises with the RV Pelagia in August 2004, May 2005 and October 2006 (Fig. 1). Subsequently, the near bed hydrodynamic conditions on the Moroccan margin are compared with the conditions as found in other mound and reef areas, such as the Irish and Norwegian margin. 1.1. Regional setting The Gulf of Cadiz is an embayment of the NE Atlantic Ocean, which is bounded to the NE and SE by the coasts of Spain and Morocco, respectively (Fig. 1) (Machín et al., 2006). The Gulf of Cadiz is underlain

Table 1 Overview of BOBO lander deployments. Station

Location

Latitude

M04-09 M05-01 M05-02 M05-02 M06-01 M06-06 M06-07 EM06-08 EM06-08

On ridge Plain below PDE On ridge On ridge Plain below PDE Gemini MV Lazarilo de Tormes MV Gulley off Gemini Gulley off Gemini

35°18.00 35°17.29 35°17.69 35°17.70 35°16.02 35°16.89 35°19.08 35°18.79 35°18.79

Longitude ′N ′N ′N ′N ′N ′N ′N ′N ′N

6°47.00 6°47.79 6°47.26 6°47.28 6°49.99 6°45.33 6°46.38 6°45.50 6°45.50

′W ′W ′W ′W ′W ′W ′W ′W ′W

Depth

Deployment

Recovery

Days

545 641 498 526 685 422 498 608 608

19 21 21 02 24 06 07 26 28

21 Aug. 04 02 Jun. 05 01 Jun. 05 24 Sep. 06 Drift 24 Oct. 06 30 Apr. 07 27 Oct. 06 05 Nov. 06

3 13 12 479 4 18 174 2 8

Aug. 04 May 05 May 05 Jun. 05 Sep. 06 Oct. 06 Nov. 06 Oct. 06 Oct. 06

F. Mienis et al. / Journal of Marine Systems 96–97 (2012) 61–71

by the boundary between the African and Eurasian plates and has a very complex and active tectonic history (Maldonado et al., 1999). The geological setting is determined by the presence of an accretionary wedge formed by the westward motion of the Gibraltar arc during the middle Miocene. As a consequence of increased subsidence a large olistostrome complex formed during the late Miocene (Medialdea et al., 2004). Compression of plastic sedimentary units of the olistostrome complex and the Gibraltar accretionary wedge, resulted in the formation of diapiric structures and expulsion of fluids manifested at the surface by the presence of fluid seeps and mud volcanoes (Leon et al., 2006; Medialdea et al., 2009; Pinheiro et al., 2003). The El Arraiche mud volcano field, situated on the Moroccan margin, comprises 8 mud volcanoes of varying size (e.g. the Gemini and Lazarillo de Tormes mud volcanoes), which cluster around two sub-parallel seafloor ridges, the Vernadsky and Renard Ridge (Van Rensbergen et al., 2005a, 2005b). The Renard Ridge is situated south of the main Olistostrome complex and is characterised by the presence of thick Late Miocene/Pliocene sedimentary units that are offset by large and active normal faults. The Pen Duick Escarpment, which bounds the Renard Ridge and is situated between 450 and 600 m water depths, is up to 65 m high and has a slope gradient between 15 and 25°. Fifteen mound structures with an average elevation of 15 m above the surrounding seafloor and covered by fossil coldwater corals have been found on top of the Renard Ridge (Foubert et al., 2008; Van Rooij et al., 2011). Below the ridge, an open plain is found, dipping to the west (Fig. 1). Living cold-water corals are almost absent in the area except for some isolated colonies. Sediment cores collected from the mound structures and the slope of the Pen Duick Escarpment revealed layers with abundant coral debris of glacial age, indicating that in the past corals thrived in the area (Wienberg et al., 2009, 2010). At present most of the coral debris (mainly Lophelia pertusa, Madrepora oculata and Dendrophylia sp.) is covered with a centimetres-thick layer of hemipelagic mud (De Haas and Mienis, 2005; Wienberg et al., 2009).

1.2. Oceanographic setting The general near-surface circulation in the Gulf of Cadiz is anticyclonic and should be considered in relation to the northeastern Atlantic circulation. The water mass representing the upper 100 m of the water column is North Atlantic Superficial Water (NASW), while between 100 and 600 m North Atlantic Central Water (NACW) is present (Criado-Aldeanueva et al., 2006, 2009). NACW is characterised by an almost linear decrease in temperature (16–12.5 °C) and salinity (36.25–15.5) (Alves et al., 2011). Surface water temperatures vary from 16.5 °C in winter to 22.0 °C in summer with maximum temperatures during August (Vargas et al., 2003). The summer season is characterised by a strongly stratified upper water layer until 150 m water depth (Machín et al., 2006). The Gulf of Cadiz is connected with the Mediterranean Sea via the Strait of Gibraltar. Through the Strait of Gibraltar less saline Atlantic surface water flows from the Gulf of Cadiz into the Mediterranean Sea, while in opposite direction saline dense Mediterranean Water (MW), which consists of a mixture of Levantine Intermediate Water and Western Mediterranean Deep Water, flows as a bottom water layer into the Gulf of Cadiz (Fusco et al., 2008; Hopkins, 1999; Serra et al., 2005). As denser MW water descends down the continental slope, it entrains overlying NACW (Johnson and Stevens, 2000), feeding the Atlantic inflow through the Strait of Gibraltar (Johnson and Stevens, 2000; Vargas et al., 2003). The Azores Current (AC), which is a recirculation arm of the Gulf Stream, is a meandering jet that flows from the Azores towards the Strait of Gibraltar and its strength and variability are directly related to the magnitude of water mass exchange through the Strait of Gibraltar (Pelegrí et al., 2005; Rogerson et al., 2004; Volkov and Fu, 2010).

63

In the Gulf of Cadiz, MW is found between 700 and 1400 m water depths, which is characterised by a positive temperature and salinity anomaly. Under the influence of the Coriolis force the MW veers north-westward after passing the Strait of Gibraltar. The main core divides due to topographic steering into two major cores: the Mediterranean Upper Water (MU) flowing north and the Mediterranean Lower Water (ML), flowing in a westward direction, with a small branch flowing into the southern part of the Gulf of Cadiz (Fusco et al., 2008). MW strongly interacts with the seafloor and strongly influences sedimentation processes (Hernandez-Molina et al., 2003; Sierro et al., 1999). At several locations irregularities in the seabed topography promote the generation of MW anticyclonic vortices, socalled meddies (Ambar et al., 2008; Serra et al., 2005). Most meddies are directed south-westward from Cape St. Vicente. However, occasionally meddies have been observed near the Moroccan margin between 750 and 1500 m (Carton et al., 2002). In the southern Gulf of Cadiz at depths around 600 m a core of Antarctic Intermediate Water (AAIW) can be recognised, flowing northward between the base of NACW and top of MW, which is characterised by a salinity minimum (b35.6), high nutrient and low oxygen concentrations and a potential density of 27.5 (σθ). In the Gulf of Cadiz AAIW, which has the same density as the MW, can undergo mixing and entrainment with MW. This rapidly decreases the salinity of the MW. North Atlantic Deep Water (NADW) is present below 1500 m, recirculating in a northward direction along the continental margin (Van Aken, 2000). 2. Methods 2.1. CTD observations The locations of the Conductivity–Temperature–Depth (CTD) and lander stations were selected on the basis of multibeam maps and extensive video and photo surveys, collected during cruises with the RV Pelagia in 2004, 2005 and 2006 (Fig. 1). Water column profiling was performed with a CTD/Rosette system equipped with an SBE-9 Seabird CT sensor. Additionally a Seapoint optical backscatter sensor, a Seatech sensor to measure light transmission, a Wetlabs sensor to measure chlorophyll concentrations and a Chelsea Fluorometry sensor were mounted on the frame. The sampling rate was 24 Hz and CTD data were acquired using the Seasave Win 32 software version 5.28c. On the seabed plain below the Pen Duick Escarpment and on the Renard Ridge, yo-yo stations were carried out, where the CTD was lowered and raised at the same location for a 12–24 hour period without leaving the water, to trace fluctuations, including tidal variability. CTD transects were made along the cold-water coral mounds and across the escarpment. Sampling stations were situated 2 km apart and all data were collected for one day. Water samples were taken with the CTD Rosette system from the surface water (chlorophyll maximum), in water with low turbidity values between 100 and 200 m depths and from the bottom nepheloid layer (BNL) between 500 and 700 m water depths. From each water depth, 5 l of water was filtered over pre-weighed polycarbonate filters (47 μm). The filters were rinsed with demineralised water to remove salt, dried and weighed again to determine the total amount of suspended matter in the water column. 2.2. Short and long term near-bed hydrodynamic observations Free falling Bottom Boundary (BOBO) landers, designed by NIOZ (Van Weering et al., 2000), were deployed at selected locations to obtain short and long term records of near bottom current velocity (speed and direction), particle flux, temperature, salinity and optical and acoustic backscatter (Fig. 1). Landers were deployed for variable duration of several days up to a year at a number of positions near

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F. Mienis et al. / Journal of Marine Systems 96–97 (2012) 61–71

deployments sediment trap bottles were filled with filtered seawater without additive, while during long term deployments bottles were filled with a solution of mercury chloride (1 g l − 1) buffered with borax (2 g l − 1) in seawater. After recovery of the landers, sediment trap samples were filtered over pre-weighed polycarbonate filters, rinsed with demineralised water, dried and weighed. The content was studied using a light microscope.

26

24

Thermocline Waters

22

3. Results 3.1. Water mass characteristics CTD profiles collected during cruises carried out with the RV Pelagia in 2004, 2005 and 2006 showed a seasonal thermocline in the upper 150 m and the presence of North Atlantic Central Water (NACW) between 200 and 600 m water depths (Fig. 2). Surface water temperatures varied during the season and were 19 °C in May 2005, 24 °C in August 2004 and 21.5 °C in October 2006, respectively. At the depth where cold-water coral mounds were found on the Renard Ridge (between 530 and 580 m depths), a temperature range of 10.7–11.8 °C, a salinity range of 35.56–35.65 and a potential density range of 27.15–27.25 (kg m − 3) were observed (Fig. 2). CTD stations on the plain below the Pen Duick Escarpment showed the influence of Antarctic Intermediate Water (AAIW) around 600 m water depth, as shown by a salinity minimum of 35.55 (Van Aken, 2000).

18

16

14

NACW 12

Mound Occurence AAIW

10 35.4

35.6

35.8

3.2. Yo-yo CTD stations and CTD transects

36

36.2

36.4

36.6

Salinity Fig. 2. T-S plot of CTD profiles recorded during cruises in August 2004 (black), May 2005 (light grey) and October 2006 (grey). NACW: North Atlantic Central Water, AAIW: Antarctic Intermediate Water. The corresponding potential density anomalies (kg m− 3) are plotted every 0.5 units (dashed lines).

and on top of the cold-water coral mounds, on top of the Gemini and Lazarillo de Tormes mud volcano and on the seabed plain below the Pen Duick Escarpment and on the Renard Ridge (Table 1). Landers were equipped with downward facing RD Instruments 1200 kHz Acoustic Doppler Current Profilers (ADCP) mounted at 2 m above the bottom in the centre of the frame, measuring current speed, acoustic backscatter and current direction in 5 cm bins at 15minute intervals. In this manuscript data from a bin at 1 m above the bottom are presented. An SBE-16 CT sensor was mounted in the frame at 3 m above the bottom, measuring temperature and salinity. Two Seapoint optical backscatter sensors were mounted in the frame at 1 and 3 m above the bottom, which provide a relative measure of the amount of backscatter intensity. A Technicap PPS 4/3 sediment trap with a carrousel of 12 bottles was mounted in the frame with the aperture (0.05 m 2) at 4 m above the bottom. During short

°C 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10

Depth (m)

-200 -300 -400 -500

-100

36.55 36.5 36.45 36.4 36.35 36.3 36.25 36.2 36.15 36.1 36.05 36 35.95 35.9 35.85 35.8 35.75 35.7 35.65 35.6 35.55

-200

Depth (m)

-100

Vertical baroclinic motions of water masses above the Renard Ridge were clearly reflected by the undulating patterns of isotherms and isohalines observed at a 24-hour yo-yo CTD station (M04-07, Fig. 3), which was recorded in August 2004 on the plain below the escarpment and a 12-hour yo-yo recorded in October 2006 on top of the Renard Ridge. The amplitude of vertical water displacement was up to 50 m at intermediate depth, increasing up to 100 m around 500 m water depth. At 650 m depth on the plain below the Pen Duick Escarpment temperature fluctuations were 0.5 °C. The largest variability in temperature (1 °C) and salinity (0.2) occurred between 200 and 300 m, which is an interval with a steep downward decrease in temperature, salinity and density with depth. Both in 2004 and 2006, a 50 to 100 m thick intermediate nepheloid layer (INL) was observed between 200 and 300 m water depths (Figs. 3 and 4). The INL changed depth and thickness during the semi-diurnal tide along with the vertical shifts in the isotherms and isohalines. An up to 300 m thick bottom nepheloid layer (BNL) occurred on the plain below the escarpment (Fig. 3), while a 100 m thick BNL was present on top of the ridge. The BNLs reached up to the depth of the cold-water coral mound occurrences, which have their summits at around 550 m depth. The BNLs changed thickness depending on the semi-diurnal tidal cycle.

-300 -400 -500 -600

-600 21.00

3.00

Time (h)

9.00

FTU

-100

0.32 0.3 0.28 0.26 0.24 0.22 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0

-200

Depth (m)

Temperature(oC)

20

-300 -400 -500 -600

21.00

3.00

Time (h)

9.00

21.00

3.00

9.00

Time (h)

Fig. 3. Contour plots of temperature, salinity and optical backscatter of a 24-hour yo-yo CTD station carried out in May 2004 on the open plain below the Pen Duick Escarpment.

F. Mienis et al. / Journal of Marine Systems 96–97 (2012) 61–71

16

15

14

13

12

16

15

14

13

12

16

15

14

13

12

12:27

14:08

15:32

16:21

17:55

12:27

14:08

15:32

16:21

17:55

12:27

14:08

15:32

16:21

17:55

°C

-100

36.55 36.5 36.45 36.4 36.35 36.3 36.25 36.2 36.15 36.1 36.05 36 35.95 35.9 35.85 35.8 35.75 35.7 35.65 35.6 35.55

18

-200

-200

17 16

-300

-300

15 14 13

-400

-400

12 11

-500

-500

10

-600

FTU

-100

-100 19

Depth (m)

65

0.32 0.3 0.28 0.26 0.24 0.22 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0

-200

-300

-400

-500

-600

-600

Fig. 4. Contour plots of temperature, salinity and turbidity of a CTD transect made across the escarpment in May 2005. Station (situated 2 km apart) numbers and time of deployments are indicated on the horizontal axis. The depth of the mound occurrences is indicated in the profile.

12.8 M06-06, 422 m 12.6 12.4 12.2 12 11.8 11.6 11.4 7-Oct 8-Oct

9-Oct

the plain below the Pen Duick Escarpment a BNL up to 100 m thick was present (Fig. 4). 3.3. Short-term near-bed hydrodynamic conditions A distinct semi-diurnal tidal cycle in temperature and salinity was also observed in the near-bottom layer during short term lander deployments (Fig. 5, Table 1). The two parameters co-vary at the stations on top of the Gemini mud volcano (M06-06, 422 m) and on the mounds on the Renard Ridge (M05-05, 526 m), due to the fact that both show a downward decreasing gradient at that depth interval. At 685 m water depth on the plain below the escarpment (M06-01),

10-Oct

11-Oct

12-Oct

35.9 35.85 35.8 35.75 35.7 35.65 35.6 35.55 13-Oct

Salinity

Temperature (oC)

A CTD transect across the cold-water coral mounds from the ridge towards the plain below the escarpment was recorded in May 2005 (Fig. 4). In contrast to August 2004, a layer with increased optical backscatter values was observed in the surface waters at 50 m water depth, which corresponded to a chlorophyll maximum. Elevated optical backscatter values were not observed near the water surface in August, nor in October. A dipping INL of 100 m thick was found between 250 and 400 m water depths, and was also found in the yo-yo CTD station. The dipping of the nepheloid layer can be attributed to the fact that all CTD casts were recorded during one tidal cycle, during which the INL moved about 100 m vertically. A BNL up to 50 m thick was observed on the Renard Ridge itself. On

Date (days) 35.65

11.4 35.6 11.2 11 22-Jun

23-Jun

24-Jun

25-Jun

26-Jun

27-Jun

Salinity

Temperature (oC)

11.6 M05-02, 498 m

35.55 28-Jun

Date (days) 35.66 35.64 10.7 35.62 10.6

Salinity

Temperature (oC)

10.8 M06-01, 685 m

35.6 25-Sep

26-Sep

27-Sep

28-Sep

Date (days) Fig. 5. Lander records of temperature and salinity measured at three different depths in the research area. All stations show the presence of a semi-diurnal tidal cycle. At station M06-06 (summit Gemini) and M05-05 (mound summit) temperature and salinity co-vary. Temperature and salinity anti-correlate at station M06-01 (plain below Pen Duick Escarpment).

110

0.28

100

0.24

90

0.2

80

0.16 0.12

70

0.08 11.6

60 25 20

11.4

15 10

11.2

5 11 22-Jun

currentspeed (cms-1)

Temperature (oC)

0.32

Acoustic Backscatter

F. Mienis et al. / Journal of Marine Systems 96–97 (2012) 61–71

Optical backscatter

66

0 23-Jun

24-Jun

25-Jun

26-Jun

27-Jun

28-Jun

Date (days) Fig. 6. Lander records of station M05-02 showing temperature (3 mab), current speed (1 mab), acoustic backscatter (1 mab) and turbidity (1 mab) as indicated by optical backscatter. Peak current speeds up to 30 cm s− 1 were measured. Changes in current speed occur after temperature maxima or minima have been reached.

temperature decreased and salinity increased (35.63), showing opposite patterns. Likely this increase in salinity represents the mixing of cooler and fresher AAIW with MW (Fig. 2). At the shallowest station M06-06 the temperature varied over 1.2 °C and the salinity over 0.15. At the deeper station M05-02 the temperature fluctuated over 0.4 °C and the salinity over 0.04. At the deepest station M06-01 temperature and salinity fluctuations were smallest, 0.2 °C and 0.02 respectively (Fig. 5). Variations in current speed appear also related to the baroclinic tidal motions (Fig. 6). A slow increase in temperature, salinity and current speed is followed by a sharp drop in temperature and salinity, indicating an asymmetric wave pattern. Peak current speeds are found right after the drop in temperature after which the current speed shows a fast decrease. Tidal peak current speeds were up to 20.2 cm s − 1 on top of the Gemini mud volcano (M06-06), while the

M06-06, 422 m

20

120

15 100 10 80

5 0

60 8-Oct

12-Oct

M05-02, 498 m

140

20

120

15 100 10 80

5 0 22-Jun 25

Current speed (cm s-1)

10-Oct

23-Jun

24-Jun

25-Jun

M05-01, 641 m

26-Jun

60 27-Jun 140

20

120

15

100

10 80

5 0 22-May

Acoustic backscatter

Current speed (cm s-1)

25

Acoustic backscatter

140

24-May

26-May

Acoustic backscatter

Current speed (cm s-1)

25

average current speed was 7.6 cm s − 1 (Fig. 7). On top of the ridge (M05-02), peak current speeds were higher up to 25 cm s − 1, while the average current speed was 8.8 cm s − 1. On the plain below the Pen Duick Escarpment (M05-01) peak current speeds and the average current speed were 24 and 8 cm s − 1, respectively. Around the cold-water coral mounds on the ridge (M05-02) and on the top of Gemini mud volcano (M06-06), peaks in optical and acoustic backscatter corresponded to peaks in current speed, likely indicating resuspension of bottom sediments. Acoustic backscatter values varied around 80 dB (Fig. 7). Optical backscatter values on the plain below the Pen Duick Escarpment were more than 10 times higher than the optical backscatter values measured on the Renard Ridge. Acoustic backscatter values vary on the plain around 100 dB and were much higher than the values measured on the Renard Ridge and on Gemini mud volcano. Also a large increase in acoustic

60 28-May

Fig. 7. Lander records of current speed and acoustic backscatter measured at station M06-06 (summit Gemini), M05-02 (mound summit) and M05-01 (plain below Pen Duick Escarpment). Acoustic backscatter values were highest on the plain below the Pen Duick Escarpment.

F. Mienis et al. / Journal of Marine Systems 96–97 (2012) 61–71

backscatter was observed on 27 May 2005, which cannot be related to an increase in current speed. No correlation was observed between current speed, temperature, optical and acoustic backscatter on the plain below the Pen Duick Escarpment. 3.4. Long-term near-bed hydrodynamic conditions

T (oC)

Long-term deployments of BOBO landers in May 2005–June 2006 (M05-02, on mound) and November 2006–April 2007 (M06-07, on the Lazarillo de Tormes mud volcano) revealed intra-annual fluctuations in temperature, current speed and acoustic backscatter (Fig. 8). The average current speed at station M05-02 on the Renard Ridge was 8.8 cm s − 1. Up to 90% of the time current speeds were below 15 cm s − 1, while peak currents speeds up to 30 cm s − 1 were occasionally present (1% of the measurements) (Fig. 9). Acoustic backscatter values appeared to increase with increasing current speed. The strongest currents (above 15 cm s − 1) had a NNE direction, while weak currents (below 5 cm s − 1) were directed to the NW. The residual currents at station M05-02 and M06-07 were directed to the NE as illustrated by the progressive vector plots (Fig. 10).

12.4 12 11.6 11.2 10.8 10.4

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Over the year the average near-bed temperature (3 m above the bottom) at station M05-02 on the Renard Ridge was 11.3 °C, the average salinity 35.54 and the average potential density 27.15 (kg m − 3). Spring and neap tidal cycles can be observed in the current and temperature record. During neap tidal cycles temperature fluctuations were low and average current speeds decreased. During spring tidal cycles the average current speed increased as well as acoustic backscatter values, e.g. during August 2005 and March 2007 (Fig. 8). A prominent feature in both long term records was a temperature maximum between October and January (Fig. 8).

3.5. Transport of water Progressive vector plots of the displacement (in km) relative to the positions of the landers show the main direction and magnitude of the displacement of water at the different locations. The general flow of water on the Renard Ridge and on Gemini mud volcano is in a north-eastward direction (Fig. 9). On the plain below the Pen Duick Escarpment the largest displacement is to the NW and SE. In

M05-02

Acoustic Backscatter

120 100 80 60

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Nov-06

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Mar-07

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120 100 80 60

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40 30 20 10 0

Fig. 8. Time series of temperature, current speed and acoustic backscatter (1 mab) measured by bottom landers in two consecutive years. A prominent feature recorded in both years is a temperature maximum (indicated by the arrows) from October till January. Black lines show the daily moving average.

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M06-07 %

50 25

35° 91’

0

LdT

M04-09

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%

%

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%

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EM06-08

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M06-06

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0 500 m day

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-1

-6° 45’

Fig. 9. Progressive vector plots of the long term deployments near the Pen Duick Escarpment. Units are in km.

this area as well as in the gully around the Gemini mud volcano the water is likely steered by the topography (Fig. 9).

3.6. Suspended particulate matter During cruises in 2004 and 2005 water samples were taken from the surface (chlorophyll maximum), clear (100–200 m) and bottom water layers (500–700 m). Highest SPM values (0.015–0.05 mg l − 1) were within the BNLs as observed at the yo-yo stations. High values (0.015–0.04 mg l − 1) were also observed in the chlorophyll maximum layer in May 2005, while lowest values (0.01 mg l − 1) were found at water depths around 200 m (Fig. 11).

Samples collected with a sediment trap during short-term deployments on top of the Gemini mud volcano and on the plain showed that mass fluxes in the area were high (47–215 g m − 2 day − 1). Only 1% of the collected material consisted of particles >63 μm. The main components of this coarse fraction were planktonic foraminifera (>50%), followed by benthic foraminifera, quartz grains and radiolarians.

0

200

2400

Depth(m)

South-North

2000

M05-02

1600

400

1200 800

M06-07 400

600

0 0

200

400

600

West-East Fig. 10. Bathymetric map of the study area (contour interval 5 m) showing the water displacement as calculated from current speed and direction (in m day− 1) and the frequency distribution of current speed as measured at 1 mab at the different lander locations.

0

0.01

0.02

0.03

0.04

SPM (mgl-1) Fig. 11. Graph showing the amount of SPM (mg l− 1) versus water depth.

0.05

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4. Discussion Contrary to what is observed on mounds and reefs on the Irish and Norwegian margins, where thriving cold-water coral communities are found at present day, the mound structures on the Renard Ridge are largely devoid of living coral cover. Observations of living coral occurrences are relatively sparse despite intensive surveys with remotely operated vehicles and towed camera systems (De Haas and Mienis, 2005; Wienberg et al., 2009). Abundant fossil remains of cold-water corals with a glacial age, which are at present covered with a drape of Holocene mud or partially exposed by the bottom currents, were observed in sediment cores, showing that cold-water corals flourished during glacial periods (Foubert et al., 2008; Van Rooij et al., 2011; Wienberg et al., 2009). Reviews on the global present-day distribution of framework building cold-water corals like L. pertusa and M. oculata by Freiwald (2002) and Davies et al. (2008) defined ranges of temperature, salinity, density, dissolved oxygen concentrations and nutrient concentrations, which are supposed to be favourable for cold-water coral growth. Other studies were directed at the oceanographic conditions favouring surface water productivity and hydrodynamic mechanisms transporting food particles towards the corals (Davies et al., 2009; Duineveld et al., 2007; Frederiksen et al., 1992; White et al., 1998). In the light of these studies and the results of the present study we can put constraints on the factors that have likely caused the decline of cold-water corals on the Renard Ridge in the Gulf of Cadiz. 4.1. Hydrography Cold-water coral mounds on the Renard Ridge, which occur between 480 and 550 m depths, are presently located in the lower part of the NACW near the transition with the AAIW (Fig. 2). Below 600 m water depth on the plain below the Pen Duick Escarpment AAIW was observed in the study area, characterised by a salinity minimum of 35.55 (Machín et al., 2006; Van Aken, 2000). Temperature and salinity values as acquired near the mounds showed no evidence of the influence of MW or meddies. Residual currents recorded on the Renard Ridge during the long-term deployments remained consistently directed NE during the whole year (Fig. 10). Furthermore, the long-term current record did not show reversals that can be attributed to passing meddies. However, periodically mixed water of MW and AAIW can move towards the plain below the PDE, which was observed by the lander deployed at 685 m water depth that showed opposite fluctuations in temperature and salinity, indicating the vicinity of MW at this depth (Fig. 5). AAIW, which has the same density as the MW, can undergo strong mixing and entrainment with MW, decreasing the salinity rapidly (Van Aken, 2000). CTD transects across the Moroccan margin to a depth of 2000 m show the presence of MW at greater water depth (700–1400 m), characterised by an increase in salinity (Alves et al., 2011; Van Aken, 2000). Meddies spinning from the main MW in the northern Gulf of Cadiz have occasionally been observed on the Moroccan margin (Ambar et al., 2008; Carton et al., 2002). These meddies occurred between 750 and 1500 m water depths, which is clearly below the range where cold-water coral mounds were observed on the Renard Ridge. On the Renard Ridge a maximum in the near-bottom water temperature between October and January was recorded in two consecutive years. This prominent seasonal feature can be attributed to two possible mechanisms. The temperature maximum could be related to meandering of the water masses present near the ridge. The Azores Current forms a meandering jet that flows into the Gulf of Cadiz around 35° N. Likely the research area is influenced by this jet (Johnson and Stevens, 2000; Volkov and Fu, 2010), as the residual current and water displacement as measured in the area are in general directed to the NNE, which was also observed by Pelegrí et al.

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(2005). However, at some sites currents are following the general contours and are influenced by the local and regional topography like on the plain below the escarpment and in the gully around the Gemini mud volcano, where water and sediment are likely flushed through the channel in an east and westward direction (Fig. 10). Another mechanism could be the eastward water inflow of NACW towards the coastal regions, which is not restricted to the Strait of Gibraltar, but also occurs along the African coast. The eastward flow exists all year long and recirculates southwest along the slope. This onshore flow seems to be more zonal during winter and intensifies during summer (Machín et al., 2006; Mauritzen et al., 2001). Average near-bed temperature (11.3 °C) and salinity (35.54) values recorded on the mounds during the long term deployment (M05-02) fit in the range for L. pertusa (T, 4–12 °C and S, 35–37) as given by Freiwald (2002) and are 2–3 °C higher than temperatures typically observed in areas with a thriving cold-water coral community on the Irish and Norwegian margins (Mienis et al., 2007; Rüggeberg et al., 2011). Present bottom water temperatures around mounds on the Renard Ridge may thus be regarded as sub-optimal for coral growth, but not limiting since living cold-water corals were observed in even warmer waters in the Mediterranean Sea (13.9 °C) (Correa et al., 2010; Orejas et al., 2009) and on the US margin (15 °C, F. Mienis and G. Duineveld, unpublished data). Salinity values fit well within the range as given by Freiwald (2002). However, average potential density values near the mounds on the Renard Ridge are relatively low (27.14) and do not fit in the potential density envelope of 27.35–27.65 (σθ) as described by Dullo et al. (2008) for L. pertusa in the NE Atlantic. The density range as measured on the Renard Ridge is comparable to values that were measured in the Gulf of Mexico near the Viosca Knoll (VK826), where a living L. pertusa reef-like structure was observed (Davies et al., 2010). 4.2. Near-bed hydrodynamic regime The bottom water hydrodynamic regime on the Renard Ridge is dominated by semi-diurnal water movements, induced by the internal tide. Their presence was demonstrated by the yo-yo CTDs showing large vertical water movements of up to 100 m at 500 m water depths within a single tidal cycle (Fig. 3). Near the seabed these tidal motions were recorded by the landers as semi-diurnal fluctuations in temperature, salinity and current speed (Figs. 5–7). Whereas average current speeds on top of the Renard Ridge were 8.8 cm s − 1 during the long term deployment, instantaneous peak current speeds up to 30 cm s − 1 were recorded occasionally (1% of the measurements). On average current speeds on top of the Renard Ridge were more vigorous than current speeds measured on top of the Gemini mud volcano and on the plain below the Pen Duick Escarpment (Fig. 10). Daily peak current speeds recorded on cold-water coral mounds on the Rockall Bank, in the Porcupine Seabight and near the Mingulay reef on the Scottish margin were much higher; 45–60 cm s − 1, 51 cm s − 1, and 67–81 cm s − 1 respectively (Davies et al., 2009; Dorschel et al., 2007; Mienis et al., 2007). Strong currents are generally assumed to be an essential requirement for a healthy coral community since they advect (fresh) food particles when they are available to the corals and keep particulate matter in suspension, making it available to the suspension feeding community several times (Mienis et al., 2009). In addition strong currents will prevent living corals from smothering by sediment. On the Rockall Bank baroclinic motions, which induce high current speeds, are considered to periodically enhance the particle fluxes near the seabed (Duineveld et al., 2007). Although currents on the Renard Ridge appear not as vigorous as currents on the Irish and Scottish margins they are still high enough to resuspend non-cohesive fine grained particulate matter from the seabed. Bottom nepheloid layers up to 300 m thick on the plain below the Pen Duick Escarpment and up to 100 m thick on the Renard

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Ridge give evidence of sediment resuspension by bottom currents. Surface sediments consist mostly of fine muddy sediment with a median grainsize of 6–12 μm (Wienberg et al., 2010). Up to 10% of the time current speeds were above 15 cm s − 1. According to resuspension experiments of Thomsen and Gust (2000) tidal currents on the Renard Ridge should be sufficient to resuspend material of a grainsize >100 μm and prevent this fine material from settling, creating the thick bottom nepheloid layers. Particle fluxes (47–215 g m − 2 day − 1) measured from sediment trap samples were significantly higher than particle fluxes (0.2–1.1 g m − 2 day − 1) as measured on the Irish margin and mainly fine-grained particulate matter was trapped (only 1% of the trapped material was >63 μm). The composition of fraction >63 μm of the sediment trap samples, showing the presence of quartz grains and benthic foraminifera, indicates that at least part of the material must be derived from the seafloor. Two different origins of the terrigenous sediment fraction were defined. Sediment with a mean grainsize b6 μm was attributed to hemipelagic processes, while sediment >6 μm was interpreted as eolian dust (Wienberg et al., 2010). An important question that is not answered by our observations is if the resuspended material is of sufficient nutritive value to be favourable to coral growth. On the contrary a continuously present high sediment load in the water column can have a negative effect on coral growth by merely clogging the coral polyps (Brooke et al., 2009; Mortensen, 2001). 4.3. Surface water productivity Located within the subtropical zone of the Atlantic Ocean, the Gulf of Cadiz has a moderately oligotrophic productivity regime, characterised by a modest phytoplankton bloom in late winter/early spring as observed on satellite images (Antoine et al., 1996). This period is followed by very low productivity in the summer and autumn months when the water column is strongly stratified and consequently nutrients in the euphotic layer are depleted. Annual productivity in the area is on the order of 0.3–0.35 g cm − 2 day − 1 (Antoine et al., 1996). CTD data showed that surface water temperatures in the Gulf of Cadiz were 19 °C in May 2005, temperatures reached up to 24 °C and 21 °C in August 2004 and October 2006, respectively (Fig. 2), which is in agreement with data presented by Vargas et al. (2003). A distinct maximum in optical backscatter and fluorescence observed in May 2005 around 50 m water depth indicated enhanced concentrations of phytoplankton at the base of the surface mixed layer. This near-surface layer was absent in August 2004 and October 2006, illustrating the decline in surface water productivity due to decreased mixing between surface water and deeper water layers (Peliz et al., 2007). The oligotrophic productivity regime of the southern Gulf of Cadiz is distinctly different from that of the cold-water coral habitats in the sub-polar higher latitudes, like the Rockall Bank and Porcupine Seabight, where a much higher productivity is observed, on the order of 0.65–0.8 g cm − 2 day − 1 (Antoine et al., 1996; Yoder et al., 1993). Palaeoceanographic reconstructions of surface and bottom water conditions around cold-water coral mounds on the Renard Ridge during the last glacial, on the basis of the analyses of stable isotopes, planktonic foraminiferal assemblages and sortable silt in sediment cores, indicate that during glacial periods bottom currents were stronger in the Gulf of Cadiz. Furthermore, productivity in the area was enhanced by intensified upwelling near the Azores front (Volkov and Fu, 2010) or upwelling in the coastal regions due to stronger west blowing winds (Bertrand et al., 1996). The Azores front is a robust feature of surface circulation in the Atlantic Ocean that remained present in glacial and interglacial times at the same latitude. However, during the LGM and YD the Azores front did penetrate eastward into the Gulf of Cadiz (Rogerson et al., 2004). In addition increased eolian dust transport fertilised the ocean and likely

enhanced the primary productivity (Wienberg et al., 2010). With the transition to Holocene conditions, the productivity in the Gulf of Cadiz diminished and bottom current speeds decreased (Van Rooij et al., 2011; Wienberg et al., 2010). However, currents remained still strong enough to resuspend fine sediment particles from the seabed, possibly smothering the living corals on the mounds and slowly burying them in a layer of fine silty sediment in the course of the Holocene. Acknowledgements We thank the officers and crew of the RV Pelagia and Royal NIOZ staff and technicians for their support during cruise preparations and at sea. Bob Koster was our help during lander operations. We also thank Hendrik van Aken and Jenny Ullgren for helpful discussions. The carbonate mound studies were financially supported by the European Science Foundation/Netherlands Organisation for Scientific Research under contract number 855 01 106 (MiCROSYTEMS). FM was funded through the DFG-Research Center/Excellence Cluster “The Ocean in the Earth System”. HdS acknowledges funding from the European Community's Seventh Framework Programme (FP7/ 2007–2013) under the HERMIONE project, grant agreement no. 226354. We thank Veerle Huvenne and one anonymous reviewer, for their helpful comments and suggestions, which helped to improve the manuscript considerably. References Alves, J.M.R., Carton, X., Ambar, I., 2011. Hydrological structure, circulation and water mass transport in the Gulf of Cadiz. Int. J. Geosci. 2, 432–456. Ambar, I., Serra, N., Neves, F., Ferreira, T., 2008. Observations of the Mediterranean undercurrent and eddies in the Gulf of Cadiz during 2001. J. Mar. Syst. 71, 195–220. Antoine, D., Andre, J.-M., Morel, A., 1996. Oceanic primary production 2. Estimation at global scale from satellite (coastal zone ocean color scanner) chlorophyll. Global Biogeochem. Cycles 10, 57–69. 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