The BENGAL programme: introduction and overview

Progress in Oceanography 50 (2001) 13–25 www.elsevier.com/locate/pocean

The BENGAL programme: introduction and overview D.S.M. Billett *, A.L. Rice Southampton Oceanography Centre, Waterfront Campus, Empress Dock, Southampton SO14 3ZH, UK

Abstract BENGAL (High-resolution temporal and spatial study of the BENthic biology and Geochemistry of a north-eastern Atlantic abyssal Locality) was a three-year multidisciplinary study of the abyssal benthic boundary layer in the northeast Atlantic. The aim of BENGAL was to determine how the seabed community and the geochemistry of the sediments change seasonally in response to a highly seasonal input of organic matter from the overlying water column. It did this by organising an intensive sampling programme on 14 research cruises over a two-year period. This introductory paper sets the scene for the subsequent scientific contributions. It describes the study area, sampling strategy and techniques, and provides a brief overview of the contributions to the BENGAL Special Volume.  2001 Elsevier Science Ltd. All rights reserved.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2.

Study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.

Sampling strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4.

Overview of the contributions to the BENGAL Special Volume . . . . . . . . . . . . . . . . . . . . . . . . . .

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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Appendix A. BENGAL Partner Institutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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* Corresponding author. Tel.: +44 23 80596354; fax: +44 23 80596247. E-mail address: [email protected] (D.S.M. Billett). 0079-6611/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 1 ) 0 0 0 4 6 - 5

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1. Introduction BENGAL (High-resolution temporal and spatial study of the BENthic biology and Geochemistry of a north-eastern Atlantic abyssal Locality) was a three-year multidisciplinary study of how the abyssal benthic boundary layer (BBL) in the northeast Atlantic responds to, and modifies, the incoming material flux to the seafloor. In particular, BENGAL set out to determine how the geochemistry of abyssal sediments and the characteristics of the BBL community change seasonally in response to a highly seasonal input of organic matter from the overlying water column. The BBL in this study was interpreted as including the top 30 cm of the sediment and to extend 100 m up into the overlying water column. When the BENGAL project was proposed, much had already been discovered about seasonal change at the study site on the Porcupine Abyssal Plain chosen for BENGAL (see below), but this information had been gathered in an ad hoc fashion over a number of years. The different seasons had been sampled in different years, building up a rather fuzzy picture of the pattern of seasonal change. The BENGAL study proposed to concentrate data and sample collection within a single 12-month period to avoid the complications created by interannual variations. BENGAL set out to quantify and characterise the downward fluxes of particulate material through the water column and its arrival on the seabed, using a range of observational techniques including, timeseries sediment traps, marine snow cameras, benthic lander systems, long-term moorings and time-lapse photography. The fate of the incoming particle flux once it had arrived in the BBL, and how the benthic community interacted with it, were addressed by analysing radioisotopes, the organic and inorganic chemistry of core samples, the gut contents of the fauna, and sediment profile images, and by conducting in situ incubation experiments. We studied how the seasonal fluctuations in particle flux effected the composition and activity of all faunal size classes in the benthic boundary layer food web, including microbiota, protozoan and metazoan meiofauna, macrofauna, megafauna, fish and near-bottom zooplankton, micronekton and scavengers. Studies were also conducted on the trophic relationships between these various components of the benthic community. In order to set the BENGAL results in a longer time frame, the long-term sedimentary record at the site was investigated, which involved an understanding of calcite preservation/dissolution processes and the use of biogenic silica as a proxy for past climate change. Rice, Gage, Lampitt, Pfannkuche, and Sibuet (1998) set out the rationale for the BENGAL project. Deep-ocean sediments are a vast sink for carbon from both natural and anthropogenic sources. It is therefore crucial to understand the processes that drive carbon cycling at the seafloor and facilitate its permanent burial. During the mid-1990s, when the BENGAL project was being formulated, many aspects of the structure and functioning of the deep-sea benthic ecosystem were poorly understood. For instance, it was difficult to estimate material flows within benthic ecosystems because the seasonal variations in respiration in different taxa and size classes were unknown. Knowledge of the biomass of the various faunal size classes and functional groups was inadequate and it was uncertain how deposit feeders alter the organic composition of material, and what was their relative importance in recycling organic matter. Although BENGAL was unlikely to resolve all these uncertainties, a prime objective was to achieve a better spatial and temporal resolution of the various mechanisms that control the transformation of material deposited on the seafloor in the abyssal BBL. Rice et al. (1998) devised an 8-point plan to address some of these outstanding gaps in our knowledge of deep-sea BBL processes. These were: 1. To monitor the quantitative and qualitative temporal and spatial variations in the particulate flux to the BBL over a full seasonal cycle at a site influenced by marked seasonal changes in organic flux from the overlying water column. 2. To monitor the hydrodynamic regime within the BBL. 3. To obtain in situ measurements of mass solute fluxes across the sediment/water interface, covering at least four seasons in a single year.

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4. To undertake a quantitative analytical description of the composition and activities of all benthic faunal size classes of the BBL food web over a full year. 5. To describe the temporal kinetics of organic matter, both within the sediment and within the guts of benthic deposit feeders, through observation and experiment, and to relate these data to the abundance and distribution of micro- and meio-faunal organisms. 6. To improve our understanding of the complex relationships between the activity of the benthic fauna and its imprint on the sedimentary record using a combination of radioisotope distributions within the sediment and biological studies. 7. To establish the relationship of all of the above to carbon burial using a variety of proxies, including opal and lipid biomarkers and a mix of radio-tracers covering a range of half lives. 8. To reconcile the divergent geochemical and biological approaches to seafloor carbon cycling by developing a three-dimensional diagenetic model for the BBL system. During the 1980s and early 1990s it had become clear that sea surface production and deep-sea benthic ecosystems were much more closely coupled than had previously been thought. Large seasonal fluctuations in the downward fluxes of particulate material had been detected (Deuser & Ross, 1980; Honjo, 1982; Wefer, 1989). In some areas, the peak in seasonal deposition of material was so great, that it generated an almost continuous carpet of phytodetritus over much of the seabed in summer months (Billett, Lampitt, Rice, & Mantoura, 1983; Lampitt, 1985; Rice et al., 1986; Smith, Kaufmann, & Baldwin, 1994; Smith et al., 1996; Gutt, Starmans, & Dieckmann, 1998). This massive depostion was particularly evident on the Porcupine Abyssal Plain (Thiel et al., 1989; Rice, Thurston, & Bett, 1994) and was a major reason for selecting the area for the BENGAL project. Paradoxically, during the BENGAL project, no such carpet of phytodetritus accumulated during the summer months despite marked seasonal pulses in downward fluxes being evident in material collected by sediment traps (Lampitt et al., 2001). The scope of the BENGAL project was, therefore, broadened to consider the reasons for this inconsistency and to study the longer-term flux variations on the Porcupine Abyssal Plain, using data collected from the earlier national and European-funded observational programmes to provide a decadal time series. The aim of this introductory paper is to set the scene for the subsequent scientific contributions. It describes the study area, sampling strategy and techniques, and provides a brief overview of the contributions to the BENGAL Special Volume. The BENGAL partner institutions are listed in Appendix A.

2. Study area The study area was located in the middle of the Porcupine Abyssal Plain (c. 4850 m depth) about 270 km southwest of Ireland. Its central location was at 48°50⬘N 16°30⬘W (Fig. 1). This locality had been sampled previously between 1989 and 1994, during other EU-funded projects (Rice et al., 1993; Rice, 1995). It was chosen because it is a relatively flat area, remote from both the continental slope to the east and the mid-ocean ridge to the west, and so is unlikely to be influenced by strong downslope or advective processes. The site lies between two other important sampling localities in the NE Atlantic; firstly 47°N 20°W, in the foothills of the Mid-Atlantic Ridge, which was studied extensively during the JGOFS North Atlantic Bloom Experiment (NABE) (Ducklow & Harris, 1993; Lampitt et al., 2001), and secondly the European continental margin southwest of Ireland, which was the focus of the EU-funded Ocean Margin EXchange programmes (OMEX I and II) (van Weering, McCave, & Hall, 1998; Joint & Wassmann, 2001). Details of the hydrography, primary productivity and upper ocean mixed layer dynamics are presented by Lampitt et al. (2001). The depth of winter mixing of the upper water column is about 500 m, leading to significant seasonal fluctuations in primary productivity and fluxes of organic matter to the seabed (Rice et al., 1994). The sediment at the site is a calcareous ooze with a median grain size of 8 to 8.6 µm (Rice,

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Fig. 1. Chart showing the location of the BENGAL site in the middle of the Porcupine Abyssal Plain (NE Atlantic Ocean) and inset showing details of the BENGAL sampling area. Sampling locations for lander experiments are shown in boxes and numbered 1 to 5. MAC, Module Autonome de Colonisation. MAP, Module Autonome Pluridisciplinaire. For details of equipment—see text.

Billett, Thurston, & Lampitt, 1991). The sedimentation rate is 苲3.5 cm ky⫺1 (Rice et al., 1991). The surficial sediment has a total organic carbon (TOC) content of about 0.35%, although a slightly higher value of 0.45% was recorded at the start of the BENGAL project in September 1996 (Rabouille, Witbaard, & Duineveld, 2001). The C:N ratio varies between 4.8 and 7.8 (Santos, Billett, Rice, & Wolff, 1994).

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3. Sampling strategy The sampling for the BENGAL project was carried out during fourteen research cruises (Table 1). Six of these were devoted entirely to the BENGAL project and formed the main sampling effort over a twoyear period (September 1996–October 1998). The first cruise, RRS Discovery cruise 222 in September 1996 (Rice, 1996), was envisaged as a preliminary cruise to provide baseline data, to test and develop sampling methodologies and to deploy long-term moorings for gathering background data for the main BENGAL sampling period (March 1997–March 1998). The subsequent cruises, RRS Discovery cruise 226 (March 1997; Rice, 1997), RRS Discovery cruise 229 (July 1997; Bett, 1997) and RRS Discovery cruise 231 (March 1998; Rice, 1998), were timed to cover a complete seasonal cycle. A cruise scheduled for l’Atalante in September 1997 had to be cancelled at short notice, but we were able to conduct a reduced work programme during a short cruise on RRS Challenger (October 1997; Billett, 1998). The savings allowed us to mount an additional cruise on RRS Discovery in September 1998 (Sibuet, 1999), which extended the BENGAL sampling programme to cover two annual cycles rather than the single cycle originally planned. Specific details of the sampling methodologies and the samples taken are described in detail in the succeeding contributions, but here we present a brief overview. The locations of the observations are presented in Fig. 1, which demonstrates the precautions taken to avoid interference between them. Sediment samples were taken with a variety of corers including a multiple corer (Barnett, Watson, & Connelly, 1984) (for sediment geochemistry, microbiology and meiofauna), USNEL Mk 2 type spade box corer (Hessler & Jumars, 1974), sometimes with a vegematic subsampler insert (for macrofauna), and a 2 m Kasten Corer (for radionuclide studies). The majority of the multiple core, box core and Kasten core samples were collected within a 3 km radius of the BENGAL central locality. Other core samples were collected close to where the various benthic chamber landers were deployed, and also where otter trawl samples were obtained ‘out of area’ (see Billett et al., 2001). In situ images of profiles through the upper few centimetres of the sediment were obtained using a Sediment Profile Imager (SPI) (Grehan, McKillen, & Keegan, 1998). Large benthic fauna and fish were collected with a semi-balloon otter trawl (OTSB, Merrett & Marshall, 1981), a ‘chalut a` perche’ beam trawl and an acoustically monitored epibenthic sledge (Rice, Aldred, Darlington, & Wild, 1982). The benthopelagic fauna was sampled with a double MOCNESS multiple Table 1 List of cruises on which BENGAL work was undertaken Vessel name

Cruise No.

R.R.S. Discovery R.R.S. Discovery F.S. Meteor R.R.S. Discovery F.S. Meteor F.S. Meteor R.R.S. Discovery R.R.S. Discovery R.R.S. Challenger R.R.S. Discovery F.S. Meteor R.R.S. Discovery R.V. Pelagia R.R.S. Discovery

217 222 (1) 36/4 222 (2) 36/5 36/6 226 229 135 231 42/2 236 123 237

Project

Start date

27/09/95 27/07/96 20/08/96 BENGAL 29/08/96 07/09/96 BENGAL/BIGSET 09/10/96 BENGAL 12/03/97 BENGAL 02/07/97 BENGAL 15/10/97 BENGAL 28/02/98 BENGAL/BIGSET 17/07/98 ALIPOR 23/08/98 OMEBEN 02/09/98 BENGAL 24/09/98

ALIPOR

End date

Principal scientist

Institute

22/10/95 26/08/96 05/09/96 24/09/96 06/10/96 03/11/96 10/04/97 31/07/97 30/10/97 30/03/98 22/08/98 22/09/98 18/09/98 08/10/98

Dr. Dr. Dr. Dr. Dr. Dr. Dr. Dr. Dr. Dr. Dr. Dr. Dr. Dr.

SOC U. Aberdeen GEOMAR SOC GEOMAR GEOMAR SOC SOC SOC SOC GEOMAR U. Aberdeen NIOZ IFREMER

R.S. Lampitt I.G. Priede G. Graf A.L. Rice Ch. Hemleben O. Pfannkuche A.L. Rice B.J. Bett D.S.M. Billett A.L. Rice O. Pfannkuche I.G. Priede G. Duineveld M. Sibuet

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opening/closing net and environmental sensing system (Wiebe et al., 1985). Photographic imaging of the seafloor was undertaken using the epibenthic sledge and the Wide Angle Seabed Photography (WASP) vehicle (Huggett, 1990). These operations were all conducted within a 37 km radius of the central BENGAL site, mainly across a large expanse of flat seabed to the west of the central locality. Necrophages were studied using baited traps of several different designs and a baited time-lapse camera system (‘Bathysnack’). A sediment trap mooring was deployed in October 1995, following previous deployments at the JGOFS site, with three Parflux sediment traps (Honjo & Doherty, 1988) set at depths of 1000 m, 3000 m and 4700 m (c. 100 m above the seabed) (Lampitt et al., 2001). Another sediment trap was set at 3 m above the seabed on the Module Autonome Pluridisciplinaire (Vangriesheim, Springer, & Crassous, 2001). A Bathysnap time-lapse camera system (Lampitt & Burnham, 1983) was deployed in the same general area as the sediment traps in order to record seasonal changes in the appearance of the seabed (Bett, Malzone, Narayanaswamy, & Wigham, 2001; Lampitt et al., 2001). There were several long-term lander deployments principally using two systems, the Module Autonome de Colonisation (MAC) (Desbruye`res, Bervas, & Khripounoff, 1980) which was used to study the colonisation of sediment under varying levels of organic enrichment, and the Module Autonome Pluridisciplinaire (MAP) which was equipped with current meters, thermistor chain, seabed camera, sediment trap and nephelometer (Vangriesheim et al., 2001). A variety of short-term landers were used to measure benthic fluxes across the sediment–water interface. These included the Netherlands Institute for Sea Research (NIOZ) free-fall system (Witbaard, Duineveld, van der Weele, Berghuis, & Reyss, 2000), the GEOMAR benthic chamber lander, and the Go¨ teborg benthic chamber lander (Tengberg et al., 1995). These lander experiments were conducted at five localities within a 15 km radius of the central BENGAL position (Fig. 1). Water samples were taken throughout the water column with a 12×10 L water bottle rosette mounted on a CTD. On some occasions, a marine snow profiling camera (MSP) was used in conjunction with the CTD deployments. In addition to the sediment trap samples, particulate material in the water column was collected using Stand Alone Pumps (SAPs). Samples of water and particulate material were taken very close to the seabed using a Bottom Water Sampler (BWS) (modified after Thomsen, Graf, Martens, & Steen, 1994). Multiple coring, box coring, trawling and the CTD deployments were undertaken during all the dedicated BENGAL cruises. The summary of the deployments of major pieces of equipment used during the BENGAL programme is given in Table 2.

4. Overview of the contributions to the BENGAL Special Volume As expected, there were considerable seasonal fluctuations in the downward particle fluxes during the BENGAL period (Lampitt et al., 2001), with peak fluxes occurring during mid-summer. However, there was surprisingly little seasonal variation in the gross composition of the sedimentary flux, although some inter-annual differences were evident. Seasonal changes in composition were apparent, however, in specific organic compounds, notably labile lipids (Kiriakoulakis et al., 2001) and phytopigments (Fabiano et al., 2001). Kiriakoulakis et al. (2001) analysed the organic composition of the sinking particles in detail and conclude that during the peak fluxes the organic material had a higher proportional content of labile, fatty acids and low molecular weight alcohols. Micro-organisms were contributing about 2% to the Particulate Organic Carbon (POC) flux and were significantly correlated with DNA fluxes (Vanucci et al., 2001). Overall there was a clear seasonal signal in the supply of organic matter to the abyssal seafloor, as had been anticipated. However, inter-annual variability in the downward particle fluxes was evident over a longer period (Lampitt et al., 2001). Combining sediment trap data with outputs of an upper ocean model, Lampitt et al. (2001) show that despite the inter-annual variability, no consistent long-term trend was

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Table 2 Inventory of gear and the number of deployments made during BENGAL and BENGAL-related cruises TRAWLS Chalut a` Perche Beam Trawl Semi-Balloon Otter Trawl SOC Epibenthic Sledge

16 24 2

LANDERS IFREMER MAP IFREMER MAC Go¨ teborg Lander (old design) Go¨ teborg Lander (new design) NIOZ Lander GEOMAR FFB Lander GEOMAR FFR Lander

2 6 6 6 17 5 8

SEDIMENT CORERS Multicorer Plain USNEL Box Corer Vegematic USNEL Box Corer Kasten Corer GEOMAR Box Grab Megacorer NIOZ Circular Box Corer NIOZ Square Box Corer

134 72 22 9 2 3 2 1

PHOTOGRAPHIC GEAR Wide Angle Seabed Photography Bathysnap Bathysnack Sediment Profile Imager Agassiz Video Trawl GEOMAR Fototrawl

16 5 1 7 2 4

WATER-COLUMN SAMPLERS CTD with Water Bottle Multisampler CTD with Marine Snow Profiler Bottom Water Sampler (BWS) Stand Alone Pump Moored Stand Alone Pump

96 22 17 11 2

SEDIMENT TRAPS SOC Sediment Trap Array NIOZ Sediment Trap Array OTHERS MOCNESS (Zooplankton sampler) MSN (Multiple closing net) SOC DEMAR (Amphipod Trap) VET (Amphipod Trap) RK (Amphipod Trap)

7 6

13 8 12 4 2

evident. An observation that is particularly important in relation to some of the observed changes in the benthic community which are described below. Long-term measurements were made of near-bed currents and particle concentrations using an autonomous lander, and these are related to the characteristics of particles collected in the Bottom Nepheloid Layer (Vangriesheim et al., 2001). The resuspension of smaller particles by tidal currents was clearly evident in the nephelometer data, but the resuspension of large particles during the summer months, which was apparent in the material collected in the near-bed sediment traps, could not be correlated to observed variations in current speed and direction. The authors consider that the supply of fresh material, and feeding and sediment re-working by the megafauna, may be influencing this resuspension of the larger particles. An indication of the possible role of bottom-feeding organisms in resuspending material in the BBL was also seen in the lipid composition of material collected close to the seabed (Kiriakoulakis et al., 2001). Varnavas, Panagiotaras, and Wolff (2001) discuss other biogeochemical processes at the sediment–water interface. Several of the papers catalogue changes in various components of the benthic community during the BENGAL period. We had expected to see a response to the seasonal deposition of phytodetritus, at least by the smaller size classes. Despite the intense sampling regime, no temporal changes were detected even in the bacteria (Eardly, Carton, Gallagher, & Patching, 2001). This may have been the result of a mismatch between the timing of the cruises and the deposition of fresh phytodetritus, but it may also have been related to the marked increase in reworking of the phytodetritus by megafaunal species during the BENGAL period (Bett et al., 2001). Rather than seeing seasonal fluctuations, the faunal studies generally indicate that there were much longer term changes taking place scales at the site during BENGAL. There was an apparent increase in the abundance of opportunistic species (Vanreusel et al., 2001), with different taxa

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(foraminifera, opheliid polychaetes and other groups of polychaetes) responding to the changes on the seafloor at different rates (Gale´ ron et al., 2001). The various components of the sediment community exhibited changes in their vertical distribution in the sediment with time, perhaps in response to the apparent impoverishment of the surface sediment layers and the effects of disturbance and bioturbation. An opportunistic foraminiferan species (Gooday & Alve, 2001) was found that shows remarkable similarities to a closely related species inhabiting the shelf and upper slope habitats. This implies that there are close ecological and morphological parallels between these species that inhabit environments that are widely separated bathymetrically. Long-term change is perhaps most clearly seen in the abundances of the invertebrate megafauna and in the identity of the dominant taxa. Comparing the samples collected during BENGAL with those collected using identical methods during the early 1990s, significant changes had occurred in the abundances of actiniarians, annelids, pycnogonids, tunicates, and ophiuroids, but particularly, in some of the holothurians (Billett et al., 2001). These changes may have been related to variations in the flux of material to the seafloor (Billett et al., 2001). However, neither the modelling nor the observational studies of the downward particle flux provide any evidence for a large change in particle flux in the mid-1990s that might account for these substantial changes in abundance (Lampitt et al., 2001). The increases in abundance of two holothurian species, Amperima rosea (cover photograph) and Ellipinion molle, by three orders of magnitude, have important implications for organic material cycling at the sediment surface. The increased activity by these two holothurians together with that by small ophiuroids reduced the time taken for the megafauna to rework the entire sediment surface from longer than two years to about one month (Bett et al., 2001)! While the sediment trap data indicate that there were large and significant inputs of particulate material into the BBL during the BENGAL period (Lampitt et al., 2001), almost no signs of phytodetritus appeared in the time-lapse camera sequences. This we interpret as a result of the substantial increase in megafaunal activity. It is also likely that the increase in megafaunal activity was also having a significant impact on sediment geochemistry and hence on the rest of the benthic ecosystem (Ginger et al., 2001). The comprehensive study of the food web structure of the benthic fauna on the Porcupine Abyssal Plain (Iken, Brey, Wand, Voigt, and Junghans, 2001) shows that the holothurian and ophiuroid species that had increased in abundance so dramatically were specialist phytodetritus feeders. Ginger et al. (2001) similarly concludes that the ‘blooming’ holothurian species were removing phytosterols from the surficial sediments (0–5 mm) extremely rapidly. As phytosterols are energetically “expensive” to biosynthesise, Ginger et al. (2001) suggest that their availability may be an important factor controlling the abundance of some deepsea taxa, and may also influence the benthopelagic fauna. Lipids typical of diatoms and dinoflagellates were found in many benthopelagic species, and Bu¨ hring and Christiansen (2001) consider that variations in the wax esters of abyssal benthopelagic copepods are indicative of strong seasonality in the food supply. Studies were also conducted on the nutrition of specific holothurians. Witbaard, Duineveld, Kok, van der Weele, & Berghuis (2001) find that the contents of the guts of Oneirophanta mutabilis contained phytopigment concentrations that are between 5 and 15-times higher than those in surficial sediments collected by multicorer. Nucleic acid concentrations are also up to 80-times greater, which may either come from the holothurian itself (Ginger et al., 2001) or be derived from the bacterial flora of the holothurian’s gut (Witbaard et al., 2001). Roberts et al. (2001) investigate the distribution and activity of enzymes and of bacteria in the digestive tracts of three holothurian species. They observe some inter-specific differences in the activity of certain enzymes, but while bacterial activity varied along the gut within individual holothurians, they observed no seasonal variations in activity. Rabouille et al. (2001) use a time-dependent model with time-variable bioturbation to investigate the effect of phytodetritus deposition on the diagenesis of organic matter. They compare a number of model scenarios. In some, the seabed receives a large deposition of fresh phytodetritus while in others little fresh organic matter is incorporated into the sediment. The first scenario may typify the conditions prevailing

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at the BENGAL site in 1996, whereas the latter may have typified conditions in 1997 and 1998 as a result of the marked increased megafaunal activity (Bett et al., 2001; Billett et al., 2001). They conclude that the diagenetic system at the BENGAL site might be dominated by the interannual, rather than the seasonal, variability in the organic matter supply. Over longer time periods (⬎100 years), Rabouille et al. (2001) find that the present deposition fluxes of carbonate are out of balance with its rates of burial and dissolution of carbonate in sediments on the Porcupine Abyssal Plain. They conclude that carbonate fluxes have decreased by a factor of three at some period during the last few centuries; perhaps an indication of a regime shift analogous to those observed in the North Pacific (e.g. Beamish, Kim, Terazaki, & Wooster, 1999). A study of the preservation of biogenic silica was undertaken by Ragueneau et al. (2001) in order to refine the use opal as a proxy in palaeoceanographic studies. They calculate that 苲12% of the opal flux deposited on the seabed becomes buried, which is a much greater amount than would be expected by simply comparing the sedimentation rate with the dissolution rate in the upper sediment. They consider that this could be related to variability in the supply of organic matter to the seabed. The papers presented in this Special Volume cover many different processes in the Benthic Boundary Layer in the abyssal NE Atlantic. Some papers demonstrate that recently there have been significant seasonal changes in these processes; these changes are described in some detail because of the sampling intensity achieved during the BENGAL project. However, it is, perhaps, the demonstration that there have been major longer-term changes in BBL processes that has been the most unexpected outcome of BENGAL. These inter-annual changes were the very processes that originally we set out to eliminate by intensive sampling throughout a single year. It is now evident that considerable changes have occurred in the abyssal BBL in the region over a period of a few years. Future studies must consider how short-term observations can be fitted into a scenario of decadal and longer-term variability. It is through the supply of organic matter that the abyssal seafloor is linked intimately to processes at the sea surface. So perhaps it is not surprising that the decadal-scale changes now recognised in processes in the upper ocean, are being transmitted to the deep-sea floor. The changes we have observed provide some insight into a spectrum of longterm fluctuations in the deep ocean environments that previously we were unaware of. While there are no reasons to link them with human activities, they do need to be understood, because as the deep ocean is increasingly exploited by deep fishing and for the utilisation of offshore resources, it will be increasingly important to be capable of identifying unambiguously if and when these activities start to have any impact on the deep ocean.

Acknowledgements

We gratefully acknowledge the hard work of the entire BENGAL project team. It was very rewarding working with all the partners and to have shared so many pleasant cruises with them in the sometimes less than pleasant waters of the NE Atlantic. The project depended on the intensive sampling programme we achieved with 6 cruises over a 2-year period. We therefore would like to thank particularly the officers and crew of RRS Discovery and RRS Challenger for their support at sea and the operational and technical teams of the Natural Environment Research Council’s (NERC) Research Vessel Services (RVS) and the SOC Ocean Technology Division. Finally, we would like to make special mention of the part played by Pam Talbot in administering the BENGAL project, in liasing with all the partners and the European Commission and in preparing the many reports produced during the project. The work was funded, in part, by EC contract MAS-3 950018 under the MAST III programme.

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Appendix A. BENGAL Partner Institutions The BENGAL project involved 17 partners from 9 European countries. Alfred Wegener Institut fu¨ r Polar und Meeresfoschung, Bremerhaven, Germany GEOMAR, Forschungszentrum fu¨ r Marine Geowissenschaften der Christian Albrechts Universita¨ t, Kiel, Germany Go¨ teborg Universitet, Avdelningar fo¨ r Analytisk o Marin Kemi, Sweden Institut Francais de Recherche pour l’Exploitation de la Mer (IFREMER), Plouzane´ , France Institut Oceanographique, Fondation Albert 1er de Monaco, Paris, France Institut Universitaire Europe´ en de la Mer, CNRS, Universite´ de Bretagne Occidentale, Brest, France Irish Marine Data Centre (ISMARE), Marine Institute, Dublin, Ireland (sub-contractor) Laboratoire des Sciences du Climat et de l’Environnement, Unite´ Mixte de Recherche CNRS-CEA, Gifsur-Yvette, France Nederlands Instituut voor Onderzoek der Zee (NIOZ), Texel, The Netherlands Queen’s University, School of Biology and Biochemistry, Belfast, UK Scottish Association for Marine Science (SAMS), Dunstaffnage Marine Laboratory, Oban, UK Southampton Oceanography Centre (SOC), Natural Environment Research Council (NERC), UK (Coordinating Institution) Universita¨ t Hamburg, Institut fu¨ r Hydrobiologie und Fisschereiwissenschaft, Germany Universiteit Gent, Instituut voor Dierkunde, Mariene Biologie, Belgium University College Galway, Martin Ryan Marine Science Institute, Ireland Universita’ degli Studi di Ancona, Faclta’ di Scienze, Cattedra di Biologia Marina, Italy University of Liverpool, Department of Earth Sciences, UK University of Patras, Department of Geology, Greece University of Plymouth, Department of Environmental Sciences, UK (sub-contractor)

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