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An instrument for sampling water from the benthic boundary layer
ContinentalShelfResearch,Vol. 14, No. 7/8,pp. 871-882,1994 Copyright~ 1994ElsevierScienceLtd Printedin GreatBritain.All rightsreserved 0278-4343/94$7.00+ 0.00
Pergamon
An instrument for sampling water from the benthic boundary layer LAURENZ THOMSEN,* GERHARD
GRAF,?VOLKER
M-ARTENS~ a n d E p i c STEv,~q§
(Received 28 February 1992; accepted 29 Ju/y 1992) Abstract--The BIOPROBE system, an instrumented tripod is described. It collects water samples and time-series data on physical and geological parameters within the benthic boundary layer in the deep sea at a maximum depth of 4000 m. For biogeochemical studies, four water samples of 15 i each can be collected between 5 and 60 cm above the sea-floor. BIOPROBE contains three thermistor flow meters, three temperature sensors, a transmissiometer, a compass with current direction indicator and a bottom camera system. During R.V. Meteor Cruise 17 in July 1991, BIOPROBE was successfully used on the continental slope of the western Barents Sea at a depth of 1370 m. Analyses of dissolved and particulate matter in layers of 10, 15, 25 and 40 cm above the seafloor showed strong vertical gradients.
INTRODUCTION
OCEANOGRAPHICstudies of flow conditions and suspended sediment movements in the" bottom nepheloid layer have increased significantly in recent years. Instrumented tripods with flow meters, transmissiometers, optical backscatter sensors (OBS), in situ settling cylinders and programmable camera systems have often been used in marine environments (CACCHIONE and DRAKE, 1979; STERNB~R6etal., 1986; KINEKEand STEmqBERG,1989; MCCAVE, 1991). These instruments were deployed to study suspended sediment dynamics in the benthic boundary layer and were able to collect small water samples (1-2 l) at given distances from the sea-floor. Despite these studies there is little information about the organic content and biogenic composition of particles within these different water layers. The processes that laterally distribute and sort the material falling out of the water column, are as important as vertical settling in determining the availability of this material as food. As benthic organisms are generally forced to rely on advective sources for their food s u p p l y (LABARBARA, 1984; MILLERetal., 1984) they are dependent on the fluid medium for transport of this material and thus can influence these transport processes (JUMARSand NOWELL, 1984a; ECKMANN and NOWELL, 1984; GRANT, 1983; MILLER et al., 1984). To examine biogeochemical processes in the near-bottom water, larger water samples (8-20 1) have to be collected. As recent studies indicate, physical and biological
*SFB 313, Heinrich Hecht Platz 10, 2300 Kiel 1, Germany. ? G E O M A R , Wischhofstr. 1-3, 2300 Kiel 14, Germany. :~Institut fiir Meereskunde, Dfisternbroker-Weg 4, 2300 Kiel 1, Germany. §SFB 313, Heinrich Hecht Platz, 10, 2300 Kiel 1, Germany. 871
872
L. THOMSENet aL
resuspension leads to an increase of particulate and dissolved components in the nearbottom water. Final sedimentation depends furthermore on the benthic population, which can actively incorporate particles into the sediment by biodeposition as well as resuspend material into the water column by bioentrainment (GRAF, 1992; THOMSEN and GRAF, submitted). Bottom water samplers for biological studies were developed and used by WELLERSHAUS(1973) for the deep sea and by EVERSaERC(t990) for shallow waters whereas the "Eversberg" sampler was developed with multi-directional water inlets. RITZRAUand GRAF(1992) used a modified "Eversberg,' sampler for the study of reSuspended bacteria in the near-bottom water due to stormy weather conditions in the "Kiet Bight',. This paper provides a description of an instrument system for both physical and biogeochemical studies. The instrument uses a combination of current meters, a transmissiometer, a compass, a camera system and four modified "Eversberg" samplers of 15 I volume each for the study of the temporal and spatial characteristics of dissolved and particulate matter in the benthic boundary layer under the influence of the benthic population. Following the well,known instrument system GEOPROBE, which has successfully been used for studies of suspended sediment dynamics (CAccHIONEand DRAKE, 1979; STERNBERGet al., t986), the new instrument system was named BIOPROBE (Biological Processes Bottom Environmental). BIOPROBE has: :been used on the continental slope of the western Barents Sea~ The resulting measurements represent a few data sets documenting benthos-bottom water interaction in the benthic boundary layer in detail. INSTRUMENTATION The BIOPROBE system consists of eight components (Fig. 1): tripod, water samplers, water-pump, compass, camera system, transmissiometer or OBS, flow meters and electronic control unit with data logging. The components are independently housed to provide flexibility in mounting capabilities for different experimental designs. Sensor operation, data recording and water sampling are synchronized. The individual sensors are summarized in Table 1. Tripod
The tripod consists of an upper instrument cage, three legs, three horizontal strength members and three metal-plate footpads. Stainless steel was used throughout for strength, corrosion resistance and to avoid a deflection of the compass. The frame is modular and can be assembled on the working deck of a research vessel at sea. When fully assembled. the tripod is 1.8 m high and measures about 3 m between the base of each leg. Total weight of the structure (in air) is about 200 kg. Water samplers
The water sampling system consists of four cylindrical Polyethylene (PE/bottles with a volume of 15 i each. Below the bottles are mounted PE tubes which enlarge to a cone (diameter 8 cm) with a 2 mm wide horizontal slit as water inlet. A ball valve is located between each water inlet and tube. The water inlets are located 10, 15,25 and 40 cm off the sea-floor. The tubes are separated from each other by 40 cm. Small tubes with different
An instrument for sampling water from the benthic boundary layer
873
T
el~
180 cm bott
~ter
Fig. 1. The BIOPROBE instrument system containing the stainless steel tripod, water samplers, accu-housing with electronic control unit, camera with strobe flash unit, flow meter and transmissiometer.
Table 1.
Sensor (No.) Current meters (3) Temperature Transmissiometer OBS (4 possible) Camera/flash Data logger Instrument housing Electronic control (3) Akku pack Pump Charger
Summary of individual sensors
Type thermistor thermistor visible light backscatter 35 mm strobe D-log 32kB platine ¢ 17 x 35 cm Timer NiCad 10 amp h 12 VDC .12 VDC D-peak
Manufacturer ADM Elektronik ADM Elektronik Sea-Tech D&A instrument Benthos inc. ADM Elektronik Scholz Ing. Scholz Ing. Scholz Ing. Scholz Ing. Scholz Ing.
Model No.
25 cm No. 108D 372/383 VoMar Typ 1 VoMar Typ 1 VoMar Typ 1 VoMar Typ 1 VoMar Typ 1 VoMar Typ 1
i n t e r n a l d i a m e t e r s are located inside the s a m p l i n g t u b e s , b e t w e e n the w a t e r inlet a n d the valve to create a logarithmical i n c r e a s i n g i n t a k e velocity a p p r o x i m a t e l y e q u a l to the flow velocities. T h e w a t e r inlets (Fig. 2) are m u l t i - d i r e c t i o n a l l y p o i n t e d into the a p p r o a c h i n g flow. T h e i n s t r u m e n t cage is located well a b o v e the b o u n d a r y layer s a m p l i n g section, at least 40 cm a b o v e the highest w a t e r inlet.
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L. TnOMSEN et al.
to the pump
15 mm
T
lO mm
-
k~em
_~ 100cm I 2mm 3cm
ball v alve
c~~ne
t I
Fig. 2. Schematic drawing of one watersampler and one multidirectional water inlet (EvERSB~RG, 1991), modified with inner tubings (A) inner tubings with different diameter; (B) cone; (C) water inlet.
Pump The bottles are connected to a centrifugal pump (12 V, 60 W, 15 1 min -1, magnetically coupled, pulse-width controlled, internal pressure compensating) in the upper part of the instrument cage. During the sampling procedure fresh water in the four bottles is replaced by the near-bottom water. Pumping time is long enough to attain a total exchange of water in all four bottles. The pump-timer is calibrated by the addition of fluorescent stain into the fresh water before a test-deployment. Once the bottles are filled, the ball valves close by gravitation and pressure when the instrument is brought on board. SensoFs
The B I O P R O B E system uses up to eight sensors which have been integrated into a measuring system. They are controlled by a master electronics unit. The individual sensors are summarized in Table 1. Light transmission is measured by a deep-sea transmissiometer (Sea-tech) at a distance of 40 cm above the sea-floor, transmitted over a 25 cm path. The flow measurement system consists of three thermistor flow meters ( A D M electronics, Kiel, Germany) mounted in a vertical array. The sensors are located at a distance of 15, 20 and 65 cm to the sea-floor. They are mounted onto a stainless steel rod which touches the seabed and which is also used to calculate the penetration depth of the device. The thermistor (hot bead) flowmeters (modified after VO~EL, 1981) can be used to measure flows from about 0.5 to 50 cm s -1 with a spatial resolution of 1 mm. They consist
An instrumentfor samplingwaterfromthe benthicboundarylayer
875
of stainless steel rods with plastic tips containing two bead thermistors each. Tips are filled with epoxy resin, embedding the thermistors. The bead thermistors, 0.9 mm in diameter at the top of the tips are used as the sensing thermistors, while glass-encapsulated beads serve as the temperature compensators. The sensors have been calibrated in an arrangement similar to that described by Vo~EL (1981) where cooled water from a Frigomix 1497 refrigerated circulator emerged from a long pipe (1.2 m length; 0.98 cm radius) at rates which are measured with the aid of an overflow into a graduated cylinder. The thermistor current meters are also used as temperature sensors to provide information on the thermal gradients above the sea-floor. Estimations of the mixing properties within the bottom nepheloid layer can be derived from these data as well as the shear velocity measured with these current-temperature sensors. The flow direction is obtained through photographs of a Benthos deep sea compass with a vane. Photographs of the compass and the sea-floor are made by a Benthos deep sea camera with flash. The Benthos camera can be programmed to take pictures at fixed time intervals (typically once every rain). Black and white pictures (50 ASA, 35 ram) from the bottom camera are developed onboard.
Sensor control and power Control electronics, data logger and power supply are located within one housing inside the instrument cage of BIOPROBE. The electronics provide synchronized power to sensors, pump and data logger. The programmable choices are: delay time before first sample (0-30 min/0-167 h); pumping time (3--40 min); bottom water characteristic profile with sensors ( - 2 4 h); sensor operating time (delay/duration, 0-168 h, extendable). Electrical power to operate sensors and pump is provided by 90 Ni-Cd cells with delta peak charger capable of producing a total of 10 amp h -1 at 12 VDC output. BIOPROBE can be triggered to operate via: (1) bottom contact switch (Benthos); (2) one conductor cable; or (3) with the help of a two-channel electronic timer,
Data logging Data logging is accomplished with an ADM electronic eprom. This logger is self contained and can sample up to eight input channels over a wide variety of sampling modes; an internal clock synchronizes data logging with concurrent measurements made by the BIOPROBE instrumentation. After recovery, data are transmitted to a data reading unit via a RS 232 serial cable.
Launch and recovery system The normal launch technique involves lowering the instrument system to the sea-floor using a simple wire cable (10 mm diameter, 200 g m -1 in water). Two buoys (Benthos, 25 kg net buoyancy each) are tied to the wire 30 and 80 m above the instrument respectively. The approach and position on the sea-floor of BIOPROBE is observed by means of a pinger which is attached to the wire 30 m above the instrument system. Immediately after touching down, 50 m more of the wire are added so that the wire is slack enough to prevent disturbance by ship movements. The pinger and the additional cable are suspended by the two buoys. Before sampling, a waiting period of 4-12 min is necessary to allow material
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resuspended during deployment to drift away with the current. The navigation systems of the research vessel and the pinger control display unit help to keep position directly above the instrument system. BIOPROBE can also be triggered via one conductor cable. It can be directed via a short time mooring. With the help of the sufficient electronic power supply, the data logger and extendable sensor operation time, BIOPROBE can be used for incubation experiments and longtime deployments. Due to short deployment times (e.g. 120 min for 2500 m water depth), replicate sampling for the investigation of biological processes is possible. RESULTS AND DISCUSSION
During R.V. Meteor Cruise 17 in July 1991, BIOPROBE was successfully used on the continental slope of the western Barents Sea at a depth of 1370 m. The BIOPROBE sampling scheme for one experiment was set as follows: delay time before collection of first sample (6 rain); pumping time (8 min); profile with sensors of bottom water characteristics (15 min); sensor operating time (delay/duration, 15 min). The launch technique involved lowering the instrument system to the sea-floor using a simple wire cable. The Benthos camera was programmed to take a picture once every minute. The device lodged itself at a sediment depth of approximately 2 cm. The bottom current direction was northwesterly. The highly abundant macrofauna at the site was dominated by sedentary polychaetes and ophiurids (mainly Myriochele spp., Ophiocten spp. JUTERZENKA, personal communication: Fig. 3). Polychaete tubes showed a significant alignment to the current direction. The sediment consists of silty clay (70% wt -< 6.3/~m, 5% wt -> 63/~m~ BLAOME, 1992). Water content of sediment was 50-70%. Unfortunately one flow meter was damaged during first deployment. Figure 4 shows a velocity profile and light transmission measured with the BIOPROBE instrument system at levels of 65 and20 cm above the sea-floor. Bottom shear velocity, u., is plotted in the lower part of this figure, assuming a logarithmic velocity profile, C/
U.
1 -
K
z In
zo
The average values for the upper and lower flow meters were 28.3 and 22.5 cm srespectively. Intake velocity at all sampling tube openings was30-cm s -1. Bottom shear velocity ranged from 1.7 to 2.4 cm s- 1. Current velocity data depict the existence of bursts of flow velocity, which have been measured exactly with the thermistor-flow meters. Figure 5(a)-(c) shows the oxygen, chlorophyll and particulate organic matter [POM] concentration analyses of the water samples and demonstrates the results of Barents Sea sediment exposed to a flow of u , = 1.7-2.4 cm s -1. Oxygen content was approx. 7.12 mi 1-1 up to 25 cm above the sea-floor, with higher values of 7.24 ml I- 140 and 500 cm above the sea-floor. The chlorophyll content of the bottom water decreased above the sea-floor from 0.13 to 0.01 gg 1-1. Particulate organic matter content of the bottom water increased above the sea-floor up to 40 cm from 0.41 to 0.56 mg 1- 1with a lower value of 0.45 mg I- 1 at 500 cm above the sea-floor. Computed horizontal flux profiles of chlorophyll and POM depict flux maxima of the two organiccomponents at different heights above the sea-floor. Data suggest that hydrodynamic sorting of the organic fraction occurs within the benthic boundary layer producing flux profiles, which are "top heavy" for the lighter fraction (organic debris) and "bottom heavy" for the fraction with higher settling rates (phytodetri-
An instrument for sampling water from the benthic boundary layer
Fig. 3. Current flow direction and penetration depth of the BIOPROBE device, obtained through photographs of a Benthos deep sea compass with current direction indicator a: penetration depth was 2 cm. Compass pointing to magnetic north is about 7 cm in diameter.
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=
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=
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=
=
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.~ 8 8 , 9 88,8-
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3230. 0 _go
>
65 cm
2826.
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20 cm
22"
O
20
111 Fig. 4.
Velocity profile and light transmission measured with the B I O P R O B E instrument system at levels of 65 and 20 cm above the sea floor.
@Horizontal flux ot Chl. ~ lOOcm'2dl| 0 0,1 0,2 0,3
~500:
@Horizontal flux oi POM [mg 100c.M12s1] 0 0,5 1 1,5 o ~ B
100"
f
zsl 3. 0
7 7~2 Oxygen [ml/I]
7.
0
011 0,2 0 Chlorophyll [Itg/I]
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0,5 1 0 POM [mg/I]
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Fig. 5. Vertical profiles of oxygen, chlorophyll (Chl) and particulate organic matter (POM) concentrations and horizontal flux of Chl and POM at Sta. 384 (water depth 1370 m). Velocity profile was estimated using data of two thermistor flow-meters at 20 and 65 cm height above seafloor respectively. Average current velocity at 20 cm was 22.5 cm s -1 and at 65 cm height, 28.3 c m s - 1.
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L. THOMSEN et al.
tus). Aggregated phytodetritus, which is associated with chlorophyll and originates in surface waters has high settling velocities of 0.1-0.2 cm s -1 (ALLDRE~Eand SILVER,1988) in the water. Once concentrated and further aggregated on the sediment surface by currents experimental estimated settling rates of the fluff rise up to 0.35-1.2 cm s -1 (LAMprrr, 1985). At the Barents Sea site, decreasing concentrations of chlorophyll above the sea-floor support the results cited above. Concentrations of POM increased above the sea-floor and these particles with low settling rates tend to follow streamlines in flow. remain suspended longer and hence the state of the boundary layer becomes more important in their dispersion. The phenomenon of hydrodynamic sorting is shown by MuSCnENHF_JM (1987) with the help of flume-experiments and further discussed by THOMSEN and Ge,A~ (submitted) for continental margin environments. Low oxygen concentrations within the 10-25 cm water layer suggest that oxygen consumption is enhanced within the near-bottom water due to the influence of the filter-feeders. The bottom shear velocity was high and erosion of the sediment consisting of silty clay on the station could be expected (WEaNEa etal., 1987). The high abundance ofmacrofauna at the site (1200 sedentary polyctiaetes per m2. feeding height 3-4 cm. JUTERZENKA. personal communication) appears to elevate sediment cohesion and the roughness length Zo. The biological influence on sediment-dynamic effects can induce both erosion and compaction of sediments (JUMARS and NOWELL, 1984b: MCCAVE, 1984; THISTLEet al.. 1991) and will be one of the main topics for future research. Although the BIOPROBE system is versatile, several limitations should be considered in planning field experiments with this device. Sampling errors will arise from differences in ambient flow velocity relative to the design velocity of the intake water inlets. Accelerations and decelerations of water at the tube entrance causes under- and oversampiing of suspended sediment and should be minimized. MUSCHENHEIMet aL (1986) suggest that matching velocity at sampling tube opening to within 50% of the ambient flow velocity will avoid serious artifacts. This is evident for samples taken immediately above the seafloor (0-10 cm) within the logarithmic layer, where gradients in flow velocity are very high. Suspended sediment sizes of 150 gm can cause sampling errors of up to 7% while suspended sediment sizes of 62 ~m can cause sampling errors of up to 1.6% due to intake velocities differing from the real boundary flow by 50% (STERNBERGet at.. 1986). Intake velocity during the experiment was matched to within 10-40% of the ambient flow velocity and coulter counter analyses of suspended sediment samples collected during a deployment showed a mean particle size of 45 #m, but aggregate formation occurs in the deep sea and these large flocs are fragile and notoriously hard to sample without breaking (EISMA. 1986; HoNJo et al., 1984; KRANKand MILLIGAN.1985). They will break up in the shear field of any water inlet. Results of KINEKEand STERNBERG(1989) using the empirical relationship relating aggregate size to settling velocity presented by GraBs (1985) are encouraging in the sense that using sizes based on in situ photographs of suspended particulates produces a settling velocity distribution close to that of a settling cylinder Data suggest that BtOPROBE should be modified with a deep sea particle-camera system. However, in spite of the known limitations, BIOPROBE is capable of documenting the spatial and temporal characteristics of the suspended sediment field and the biological relevant particulate and dissolved matter within the benthic boundary layer. Acknowledgements--The authors wish to thank the crew of R.V. Meteor who recovered the instrument system after a 12 h search when it was lost during a 6 h deployment. The instrument was dredgedwith a 10 km wire with
An instrument for sampling water from the benthic boundary layer
881
only one flow meter being damaged. We thank D. Boesch, S. Brodie-Cooper and W. Ziebis for constructive criticism of the manuscript. This is publication No. 168 of the Sonderforschungsbereich 313, Kiel University.
REFERENCES
ALLDREI~3EA. L. and M. W. SILVER(1988) Characteristics, dynamics and significance of marine snow. Progress in Oceanography, 20, 41-82. BLAUMEF. (1992) Hochakkumulationsgebiete am norwegischen Kontinentalhang: Sedimentologische Abbiider Topographie-geffihrter Str6mungsmuster. Dissertation, University of Kiel, 153 pp. CACCmONE D. A. and D. E. DRAKE (1979) A new instrument system to investigate sediment dynamics on continental shelves. Marine Geology, 30, 299-312. ECrO~ANJ.E. and A. R. M. NOWELL(1984) Boundary skin friction and sediment transport about an animal tube mimic. Sedimentology, 31,851-862. EISMAD. (1986) Flocculation and de-flocculation of suspended matter in estuaries. Netherlands Journal of Sea Research, 20, 183-199. EVERSnERG U. (1990) A new device for sampling water from the benthic boundary layer. Helgoliinder Meeresuntersuchungen, 44, 329-334. GraBS R. J. (1985) Estuarine flocs: Their size, setting velocity and density. Journal of Geophysical Research, 90, 3249-3251. GRAF G. (1992) Benthic pelagic coupling: A benthic view. Oceanography and Marine Biology Annual Review, 30, 149-190. GRANT J. (1983) The relative magnitude of biological and physical sediment reworking in an intertidal community. Journal of Marine Research, 41,673--689. HONJOS., K. W. DOHERTY,Y. C. AGRAWALand V. L. ASPER(1984) Direct optical assessment of large amorphous aggregates (marine snow) in the deep ocean. Deep-Sea Research, 31, 67-76. JUMARSP. A. and A. R. M. NOWELL(1984a) Fluid and sediment dynamic effects on marine benthic community structure. American Zoology, 24, 45-55. JUMARSP. A. and A. R. M. NOWELL(1984b) Effects of benthos on sediment transport: difficulties with functional grouping. Continental Shelf Research, 3, 115-130. KINEKE G. C. and R. W. STERNBERG(1989) The effect of particle settling velocity on computed suspended sediment concentration profiles. Marine Geology, 90, 159-174. KnANCK K. and T. G. MILLmAN (1985) Origin of grain size spectra of suspension deposited sediment. GeoMarine Letters, 5, 61--66. LAEARaERA M. (1984) Feeding currents and particle capture mechanisms in suspension feeding animals. American Zoology, 24, 71-84. LAMPrrr R.S. (1985) Evidence for seasonal deposition of detritus to the deep-sea floor and its subsequent resuspension. Deep-Sea Research, 32, 885-879. McCAW I. N. (1984) Size spectra and aggregation of suspended particles in the deep ocean. Deep-Sea Research, 31,329-352. McCAVE I. N. and T. F. GRoss (1991) In-situ measurements of particle settling velocity in the deep sea. Marine Geology, 99, 403--413. MILLER D. C., P. A. JUMARSand A. R. M. NOWELL(1984) Effects of sediment transport on deposit feeding: scaling arguments. Limnology and Oceanography, 29, 1202-1217. MUSCHENHEIMD. K., J. GRANTand E. L. MILLS(1986) Flumes for benthic ecologists: theory, construction and practice. Marine Ecology Progress Series, 28, 185-196. MUSCHENHEIMD. K. (1987) The role of hydrodynamic sorting of seston in the nutrition of a benthic suspension feeder, Spio setosa (Polychaeta: Spionidae). Biology and Oceanography, 265-288. RrrZRAU W. and Gv.AvG (1992) Increase of microbial biomass in the benthic turbidity zone BTZ of Kiel Bight, Baltic Sea, after resuspension by a storm event. Limnology and Oceanography, 37(5), 1081-1086. STERNaER6 R. E., R. V. JOHNSON, D. A. CACCHIONEand D. E. DRAKE (1986) An instrument system for monitoring and sampling suspended sediment in the benthic boundary layer. Marine Geology, 71,187-199. TreSTLE D. and S. C. ERTUAN(1991) The fauna of the HEBBLE site. Patterns in standing stock and sediment dynamic effects. Marine Geology, 99,413--423. THOMSEN L. and G. GRAF (subm.) Benthic boundary layer characteristics of the continental margin of the western Barents Sea.
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VOGELS. (1981) Life in movingfluida. The physical biology of flow. Princeton University Press, 352 pp. WEANERF., S. R. McLeAN, U. V. GRA~'ZSTE~,~,H. ERLEb'KEUSER,M. SARWrdErN,U. SCHAU~R,G. UNSOLD,E. WALGER and R. WrrrsTocr (1987) Sedimentary Records of benthic processes. In: Seawater-sediment interactions in coastal waters. J. RUMOHR.E. WALGERand B. ZEITZSCHEL,editors, Lecture Notes on coastal and Estuarine Studies, 13, Springer, pp. 162-263. WELLERSHAUSS. (1973) A new method for collecting near-bottom water in the deep sea. "'Meteor" Forschungsergebn. Reihe A, 13, 50-57.
Pergamon
An instrument for sampling water from the benthic boundary layer LAURENZ THOMSEN,* GERHARD
GRAF,?VOLKER
M-ARTENS~ a n d E p i c STEv,~q§
(Received 28 February 1992; accepted 29 Ju/y 1992) Abstract--The BIOPROBE system, an instrumented tripod is described. It collects water samples and time-series data on physical and geological parameters within the benthic boundary layer in the deep sea at a maximum depth of 4000 m. For biogeochemical studies, four water samples of 15 i each can be collected between 5 and 60 cm above the sea-floor. BIOPROBE contains three thermistor flow meters, three temperature sensors, a transmissiometer, a compass with current direction indicator and a bottom camera system. During R.V. Meteor Cruise 17 in July 1991, BIOPROBE was successfully used on the continental slope of the western Barents Sea at a depth of 1370 m. Analyses of dissolved and particulate matter in layers of 10, 15, 25 and 40 cm above the seafloor showed strong vertical gradients.
INTRODUCTION
OCEANOGRAPHICstudies of flow conditions and suspended sediment movements in the" bottom nepheloid layer have increased significantly in recent years. Instrumented tripods with flow meters, transmissiometers, optical backscatter sensors (OBS), in situ settling cylinders and programmable camera systems have often been used in marine environments (CACCHIONE and DRAKE, 1979; STERNB~R6etal., 1986; KINEKEand STEmqBERG,1989; MCCAVE, 1991). These instruments were deployed to study suspended sediment dynamics in the benthic boundary layer and were able to collect small water samples (1-2 l) at given distances from the sea-floor. Despite these studies there is little information about the organic content and biogenic composition of particles within these different water layers. The processes that laterally distribute and sort the material falling out of the water column, are as important as vertical settling in determining the availability of this material as food. As benthic organisms are generally forced to rely on advective sources for their food s u p p l y (LABARBARA, 1984; MILLERetal., 1984) they are dependent on the fluid medium for transport of this material and thus can influence these transport processes (JUMARSand NOWELL, 1984a; ECKMANN and NOWELL, 1984; GRANT, 1983; MILLER et al., 1984). To examine biogeochemical processes in the near-bottom water, larger water samples (8-20 1) have to be collected. As recent studies indicate, physical and biological
*SFB 313, Heinrich Hecht Platz 10, 2300 Kiel 1, Germany. ? G E O M A R , Wischhofstr. 1-3, 2300 Kiel 14, Germany. :~Institut fiir Meereskunde, Dfisternbroker-Weg 4, 2300 Kiel 1, Germany. §SFB 313, Heinrich Hecht Platz, 10, 2300 Kiel 1, Germany. 871
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L. THOMSENet aL
resuspension leads to an increase of particulate and dissolved components in the nearbottom water. Final sedimentation depends furthermore on the benthic population, which can actively incorporate particles into the sediment by biodeposition as well as resuspend material into the water column by bioentrainment (GRAF, 1992; THOMSEN and GRAF, submitted). Bottom water samplers for biological studies were developed and used by WELLERSHAUS(1973) for the deep sea and by EVERSaERC(t990) for shallow waters whereas the "Eversberg" sampler was developed with multi-directional water inlets. RITZRAUand GRAF(1992) used a modified "Eversberg,' sampler for the study of reSuspended bacteria in the near-bottom water due to stormy weather conditions in the "Kiet Bight',. This paper provides a description of an instrument system for both physical and biogeochemical studies. The instrument uses a combination of current meters, a transmissiometer, a compass, a camera system and four modified "Eversberg" samplers of 15 I volume each for the study of the temporal and spatial characteristics of dissolved and particulate matter in the benthic boundary layer under the influence of the benthic population. Following the well,known instrument system GEOPROBE, which has successfully been used for studies of suspended sediment dynamics (CAccHIONEand DRAKE, 1979; STERNBERGet al., t986), the new instrument system was named BIOPROBE (Biological Processes Bottom Environmental). BIOPROBE has: :been used on the continental slope of the western Barents Sea~ The resulting measurements represent a few data sets documenting benthos-bottom water interaction in the benthic boundary layer in detail. INSTRUMENTATION The BIOPROBE system consists of eight components (Fig. 1): tripod, water samplers, water-pump, compass, camera system, transmissiometer or OBS, flow meters and electronic control unit with data logging. The components are independently housed to provide flexibility in mounting capabilities for different experimental designs. Sensor operation, data recording and water sampling are synchronized. The individual sensors are summarized in Table 1. Tripod
The tripod consists of an upper instrument cage, three legs, three horizontal strength members and three metal-plate footpads. Stainless steel was used throughout for strength, corrosion resistance and to avoid a deflection of the compass. The frame is modular and can be assembled on the working deck of a research vessel at sea. When fully assembled. the tripod is 1.8 m high and measures about 3 m between the base of each leg. Total weight of the structure (in air) is about 200 kg. Water samplers
The water sampling system consists of four cylindrical Polyethylene (PE/bottles with a volume of 15 i each. Below the bottles are mounted PE tubes which enlarge to a cone (diameter 8 cm) with a 2 mm wide horizontal slit as water inlet. A ball valve is located between each water inlet and tube. The water inlets are located 10, 15,25 and 40 cm off the sea-floor. The tubes are separated from each other by 40 cm. Small tubes with different
An instrument for sampling water from the benthic boundary layer
873
T
el~
180 cm bott
~ter
Fig. 1. The BIOPROBE instrument system containing the stainless steel tripod, water samplers, accu-housing with electronic control unit, camera with strobe flash unit, flow meter and transmissiometer.
Table 1.
Sensor (No.) Current meters (3) Temperature Transmissiometer OBS (4 possible) Camera/flash Data logger Instrument housing Electronic control (3) Akku pack Pump Charger
Summary of individual sensors
Type thermistor thermistor visible light backscatter 35 mm strobe D-log 32kB platine ¢ 17 x 35 cm Timer NiCad 10 amp h 12 VDC .12 VDC D-peak
Manufacturer ADM Elektronik ADM Elektronik Sea-Tech D&A instrument Benthos inc. ADM Elektronik Scholz Ing. Scholz Ing. Scholz Ing. Scholz Ing. Scholz Ing.
Model No.
25 cm No. 108D 372/383 VoMar Typ 1 VoMar Typ 1 VoMar Typ 1 VoMar Typ 1 VoMar Typ 1 VoMar Typ 1
i n t e r n a l d i a m e t e r s are located inside the s a m p l i n g t u b e s , b e t w e e n the w a t e r inlet a n d the valve to create a logarithmical i n c r e a s i n g i n t a k e velocity a p p r o x i m a t e l y e q u a l to the flow velocities. T h e w a t e r inlets (Fig. 2) are m u l t i - d i r e c t i o n a l l y p o i n t e d into the a p p r o a c h i n g flow. T h e i n s t r u m e n t cage is located well a b o v e the b o u n d a r y layer s a m p l i n g section, at least 40 cm a b o v e the highest w a t e r inlet.
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to the pump
15 mm
T
lO mm
-
k~em
_~ 100cm I 2mm 3cm
ball v alve
c~~ne
t I
Fig. 2. Schematic drawing of one watersampler and one multidirectional water inlet (EvERSB~RG, 1991), modified with inner tubings (A) inner tubings with different diameter; (B) cone; (C) water inlet.
Pump The bottles are connected to a centrifugal pump (12 V, 60 W, 15 1 min -1, magnetically coupled, pulse-width controlled, internal pressure compensating) in the upper part of the instrument cage. During the sampling procedure fresh water in the four bottles is replaced by the near-bottom water. Pumping time is long enough to attain a total exchange of water in all four bottles. The pump-timer is calibrated by the addition of fluorescent stain into the fresh water before a test-deployment. Once the bottles are filled, the ball valves close by gravitation and pressure when the instrument is brought on board. SensoFs
The B I O P R O B E system uses up to eight sensors which have been integrated into a measuring system. They are controlled by a master electronics unit. The individual sensors are summarized in Table 1. Light transmission is measured by a deep-sea transmissiometer (Sea-tech) at a distance of 40 cm above the sea-floor, transmitted over a 25 cm path. The flow measurement system consists of three thermistor flow meters ( A D M electronics, Kiel, Germany) mounted in a vertical array. The sensors are located at a distance of 15, 20 and 65 cm to the sea-floor. They are mounted onto a stainless steel rod which touches the seabed and which is also used to calculate the penetration depth of the device. The thermistor (hot bead) flowmeters (modified after VO~EL, 1981) can be used to measure flows from about 0.5 to 50 cm s -1 with a spatial resolution of 1 mm. They consist
An instrumentfor samplingwaterfromthe benthicboundarylayer
875
of stainless steel rods with plastic tips containing two bead thermistors each. Tips are filled with epoxy resin, embedding the thermistors. The bead thermistors, 0.9 mm in diameter at the top of the tips are used as the sensing thermistors, while glass-encapsulated beads serve as the temperature compensators. The sensors have been calibrated in an arrangement similar to that described by Vo~EL (1981) where cooled water from a Frigomix 1497 refrigerated circulator emerged from a long pipe (1.2 m length; 0.98 cm radius) at rates which are measured with the aid of an overflow into a graduated cylinder. The thermistor current meters are also used as temperature sensors to provide information on the thermal gradients above the sea-floor. Estimations of the mixing properties within the bottom nepheloid layer can be derived from these data as well as the shear velocity measured with these current-temperature sensors. The flow direction is obtained through photographs of a Benthos deep sea compass with a vane. Photographs of the compass and the sea-floor are made by a Benthos deep sea camera with flash. The Benthos camera can be programmed to take pictures at fixed time intervals (typically once every rain). Black and white pictures (50 ASA, 35 ram) from the bottom camera are developed onboard.
Sensor control and power Control electronics, data logger and power supply are located within one housing inside the instrument cage of BIOPROBE. The electronics provide synchronized power to sensors, pump and data logger. The programmable choices are: delay time before first sample (0-30 min/0-167 h); pumping time (3--40 min); bottom water characteristic profile with sensors ( - 2 4 h); sensor operating time (delay/duration, 0-168 h, extendable). Electrical power to operate sensors and pump is provided by 90 Ni-Cd cells with delta peak charger capable of producing a total of 10 amp h -1 at 12 VDC output. BIOPROBE can be triggered to operate via: (1) bottom contact switch (Benthos); (2) one conductor cable; or (3) with the help of a two-channel electronic timer,
Data logging Data logging is accomplished with an ADM electronic eprom. This logger is self contained and can sample up to eight input channels over a wide variety of sampling modes; an internal clock synchronizes data logging with concurrent measurements made by the BIOPROBE instrumentation. After recovery, data are transmitted to a data reading unit via a RS 232 serial cable.
Launch and recovery system The normal launch technique involves lowering the instrument system to the sea-floor using a simple wire cable (10 mm diameter, 200 g m -1 in water). Two buoys (Benthos, 25 kg net buoyancy each) are tied to the wire 30 and 80 m above the instrument respectively. The approach and position on the sea-floor of BIOPROBE is observed by means of a pinger which is attached to the wire 30 m above the instrument system. Immediately after touching down, 50 m more of the wire are added so that the wire is slack enough to prevent disturbance by ship movements. The pinger and the additional cable are suspended by the two buoys. Before sampling, a waiting period of 4-12 min is necessary to allow material
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resuspended during deployment to drift away with the current. The navigation systems of the research vessel and the pinger control display unit help to keep position directly above the instrument system. BIOPROBE can also be triggered via one conductor cable. It can be directed via a short time mooring. With the help of the sufficient electronic power supply, the data logger and extendable sensor operation time, BIOPROBE can be used for incubation experiments and longtime deployments. Due to short deployment times (e.g. 120 min for 2500 m water depth), replicate sampling for the investigation of biological processes is possible. RESULTS AND DISCUSSION
During R.V. Meteor Cruise 17 in July 1991, BIOPROBE was successfully used on the continental slope of the western Barents Sea at a depth of 1370 m. The BIOPROBE sampling scheme for one experiment was set as follows: delay time before collection of first sample (6 rain); pumping time (8 min); profile with sensors of bottom water characteristics (15 min); sensor operating time (delay/duration, 15 min). The launch technique involved lowering the instrument system to the sea-floor using a simple wire cable. The Benthos camera was programmed to take a picture once every minute. The device lodged itself at a sediment depth of approximately 2 cm. The bottom current direction was northwesterly. The highly abundant macrofauna at the site was dominated by sedentary polychaetes and ophiurids (mainly Myriochele spp., Ophiocten spp. JUTERZENKA, personal communication: Fig. 3). Polychaete tubes showed a significant alignment to the current direction. The sediment consists of silty clay (70% wt -< 6.3/~m, 5% wt -> 63/~m~ BLAOME, 1992). Water content of sediment was 50-70%. Unfortunately one flow meter was damaged during first deployment. Figure 4 shows a velocity profile and light transmission measured with the BIOPROBE instrument system at levels of 65 and20 cm above the sea-floor. Bottom shear velocity, u., is plotted in the lower part of this figure, assuming a logarithmic velocity profile, C/
U.
1 -
K
z In
zo
The average values for the upper and lower flow meters were 28.3 and 22.5 cm srespectively. Intake velocity at all sampling tube openings was30-cm s -1. Bottom shear velocity ranged from 1.7 to 2.4 cm s- 1. Current velocity data depict the existence of bursts of flow velocity, which have been measured exactly with the thermistor-flow meters. Figure 5(a)-(c) shows the oxygen, chlorophyll and particulate organic matter [POM] concentration analyses of the water samples and demonstrates the results of Barents Sea sediment exposed to a flow of u , = 1.7-2.4 cm s -1. Oxygen content was approx. 7.12 mi 1-1 up to 25 cm above the sea-floor, with higher values of 7.24 ml I- 140 and 500 cm above the sea-floor. The chlorophyll content of the bottom water decreased above the sea-floor from 0.13 to 0.01 gg 1-1. Particulate organic matter content of the bottom water increased above the sea-floor up to 40 cm from 0.41 to 0.56 mg 1- 1with a lower value of 0.45 mg I- 1 at 500 cm above the sea-floor. Computed horizontal flux profiles of chlorophyll and POM depict flux maxima of the two organiccomponents at different heights above the sea-floor. Data suggest that hydrodynamic sorting of the organic fraction occurs within the benthic boundary layer producing flux profiles, which are "top heavy" for the lighter fraction (organic debris) and "bottom heavy" for the fraction with higher settling rates (phytodetri-
An instrument for sampling water from the benthic boundary layer
Fig. 3. Current flow direction and penetration depth of the BIOPROBE device, obtained through photographs of a Benthos deep sea compass with current direction indicator a: penetration depth was 2 cm. Compass pointing to magnetic north is about 7 cm in diameter.
877
879
A n instrument for sampling water from the benthic boundary layer
~ 89,4. 89,3-:"
2 |
=
6 =
=
I> t 10 14 min =
=
=
89,2
E 89,1c
89.
.~ 8 8 , 9 88,8-
34:
3230. 0 _go
>
65 cm
2826.
c-
20 cm
22"
O
20
111 Fig. 4.
Velocity profile and light transmission measured with the B I O P R O B E instrument system at levels of 65 and 20 cm above the sea floor.
@Horizontal flux ot Chl. ~ lOOcm'2dl| 0 0,1 0,2 0,3
~500:
@Horizontal flux oi POM [mg 100c.M12s1] 0 0,5 1 1,5 o ~ B
100"
f
zsl 3. 0
7 7~2 Oxygen [ml/I]
7.
0
011 0,2 0 Chlorophyll [Itg/I]
0,3
0,5 1 0 POM [mg/I]
1,5
Fig. 5. Vertical profiles of oxygen, chlorophyll (Chl) and particulate organic matter (POM) concentrations and horizontal flux of Chl and POM at Sta. 384 (water depth 1370 m). Velocity profile was estimated using data of two thermistor flow-meters at 20 and 65 cm height above seafloor respectively. Average current velocity at 20 cm was 22.5 cm s -1 and at 65 cm height, 28.3 c m s - 1.
880
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tus). Aggregated phytodetritus, which is associated with chlorophyll and originates in surface waters has high settling velocities of 0.1-0.2 cm s -1 (ALLDRE~Eand SILVER,1988) in the water. Once concentrated and further aggregated on the sediment surface by currents experimental estimated settling rates of the fluff rise up to 0.35-1.2 cm s -1 (LAMprrr, 1985). At the Barents Sea site, decreasing concentrations of chlorophyll above the sea-floor support the results cited above. Concentrations of POM increased above the sea-floor and these particles with low settling rates tend to follow streamlines in flow. remain suspended longer and hence the state of the boundary layer becomes more important in their dispersion. The phenomenon of hydrodynamic sorting is shown by MuSCnENHF_JM (1987) with the help of flume-experiments and further discussed by THOMSEN and Ge,A~ (submitted) for continental margin environments. Low oxygen concentrations within the 10-25 cm water layer suggest that oxygen consumption is enhanced within the near-bottom water due to the influence of the filter-feeders. The bottom shear velocity was high and erosion of the sediment consisting of silty clay on the station could be expected (WEaNEa etal., 1987). The high abundance ofmacrofauna at the site (1200 sedentary polyctiaetes per m2. feeding height 3-4 cm. JUTERZENKA. personal communication) appears to elevate sediment cohesion and the roughness length Zo. The biological influence on sediment-dynamic effects can induce both erosion and compaction of sediments (JUMARS and NOWELL, 1984b: MCCAVE, 1984; THISTLEet al.. 1991) and will be one of the main topics for future research. Although the BIOPROBE system is versatile, several limitations should be considered in planning field experiments with this device. Sampling errors will arise from differences in ambient flow velocity relative to the design velocity of the intake water inlets. Accelerations and decelerations of water at the tube entrance causes under- and oversampiing of suspended sediment and should be minimized. MUSCHENHEIMet aL (1986) suggest that matching velocity at sampling tube opening to within 50% of the ambient flow velocity will avoid serious artifacts. This is evident for samples taken immediately above the seafloor (0-10 cm) within the logarithmic layer, where gradients in flow velocity are very high. Suspended sediment sizes of 150 gm can cause sampling errors of up to 7% while suspended sediment sizes of 62 ~m can cause sampling errors of up to 1.6% due to intake velocities differing from the real boundary flow by 50% (STERNBERGet at.. 1986). Intake velocity during the experiment was matched to within 10-40% of the ambient flow velocity and coulter counter analyses of suspended sediment samples collected during a deployment showed a mean particle size of 45 #m, but aggregate formation occurs in the deep sea and these large flocs are fragile and notoriously hard to sample without breaking (EISMA. 1986; HoNJo et al., 1984; KRANKand MILLIGAN.1985). They will break up in the shear field of any water inlet. Results of KINEKEand STERNBERG(1989) using the empirical relationship relating aggregate size to settling velocity presented by GraBs (1985) are encouraging in the sense that using sizes based on in situ photographs of suspended particulates produces a settling velocity distribution close to that of a settling cylinder Data suggest that BtOPROBE should be modified with a deep sea particle-camera system. However, in spite of the known limitations, BIOPROBE is capable of documenting the spatial and temporal characteristics of the suspended sediment field and the biological relevant particulate and dissolved matter within the benthic boundary layer. Acknowledgements--The authors wish to thank the crew of R.V. Meteor who recovered the instrument system after a 12 h search when it was lost during a 6 h deployment. The instrument was dredgedwith a 10 km wire with
An instrument for sampling water from the benthic boundary layer
881
only one flow meter being damaged. We thank D. Boesch, S. Brodie-Cooper and W. Ziebis for constructive criticism of the manuscript. This is publication No. 168 of the Sonderforschungsbereich 313, Kiel University.
REFERENCES
ALLDREI~3EA. L. and M. W. SILVER(1988) Characteristics, dynamics and significance of marine snow. Progress in Oceanography, 20, 41-82. BLAUMEF. (1992) Hochakkumulationsgebiete am norwegischen Kontinentalhang: Sedimentologische Abbiider Topographie-geffihrter Str6mungsmuster. Dissertation, University of Kiel, 153 pp. CACCmONE D. A. and D. E. DRAKE (1979) A new instrument system to investigate sediment dynamics on continental shelves. Marine Geology, 30, 299-312. ECrO~ANJ.E. and A. R. M. NOWELL(1984) Boundary skin friction and sediment transport about an animal tube mimic. Sedimentology, 31,851-862. EISMAD. (1986) Flocculation and de-flocculation of suspended matter in estuaries. Netherlands Journal of Sea Research, 20, 183-199. EVERSnERG U. (1990) A new device for sampling water from the benthic boundary layer. Helgoliinder Meeresuntersuchungen, 44, 329-334. GraBS R. J. (1985) Estuarine flocs: Their size, setting velocity and density. Journal of Geophysical Research, 90, 3249-3251. GRAF G. (1992) Benthic pelagic coupling: A benthic view. Oceanography and Marine Biology Annual Review, 30, 149-190. GRANT J. (1983) The relative magnitude of biological and physical sediment reworking in an intertidal community. Journal of Marine Research, 41,673--689. HONJOS., K. W. DOHERTY,Y. C. AGRAWALand V. L. ASPER(1984) Direct optical assessment of large amorphous aggregates (marine snow) in the deep ocean. Deep-Sea Research, 31, 67-76. JUMARSP. A. and A. R. M. NOWELL(1984a) Fluid and sediment dynamic effects on marine benthic community structure. American Zoology, 24, 45-55. JUMARSP. A. and A. R. M. NOWELL(1984b) Effects of benthos on sediment transport: difficulties with functional grouping. Continental Shelf Research, 3, 115-130. KINEKE G. C. and R. W. STERNBERG(1989) The effect of particle settling velocity on computed suspended sediment concentration profiles. Marine Geology, 90, 159-174. KnANCK K. and T. G. MILLmAN (1985) Origin of grain size spectra of suspension deposited sediment. GeoMarine Letters, 5, 61--66. LAEARaERA M. (1984) Feeding currents and particle capture mechanisms in suspension feeding animals. American Zoology, 24, 71-84. LAMPrrr R.S. (1985) Evidence for seasonal deposition of detritus to the deep-sea floor and its subsequent resuspension. Deep-Sea Research, 32, 885-879. McCAW I. N. (1984) Size spectra and aggregation of suspended particles in the deep ocean. Deep-Sea Research, 31,329-352. McCAVE I. N. and T. F. GRoss (1991) In-situ measurements of particle settling velocity in the deep sea. Marine Geology, 99, 403--413. MILLER D. C., P. A. JUMARSand A. R. M. NOWELL(1984) Effects of sediment transport on deposit feeding: scaling arguments. Limnology and Oceanography, 29, 1202-1217. MUSCHENHEIMD. K., J. GRANTand E. L. MILLS(1986) Flumes for benthic ecologists: theory, construction and practice. Marine Ecology Progress Series, 28, 185-196. MUSCHENHEIMD. K. (1987) The role of hydrodynamic sorting of seston in the nutrition of a benthic suspension feeder, Spio setosa (Polychaeta: Spionidae). Biology and Oceanography, 265-288. RrrZRAU W. and Gv.AvG (1992) Increase of microbial biomass in the benthic turbidity zone BTZ of Kiel Bight, Baltic Sea, after resuspension by a storm event. Limnology and Oceanography, 37(5), 1081-1086. STERNaER6 R. E., R. V. JOHNSON, D. A. CACCHIONEand D. E. DRAKE (1986) An instrument system for monitoring and sampling suspended sediment in the benthic boundary layer. Marine Geology, 71,187-199. TreSTLE D. and S. C. ERTUAN(1991) The fauna of the HEBBLE site. Patterns in standing stock and sediment dynamic effects. Marine Geology, 99,413--423. THOMSEN L. and G. GRAF (subm.) Benthic boundary layer characteristics of the continental margin of the western Barents Sea.
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VOGELS. (1981) Life in movingfluida. The physical biology of flow. Princeton University Press, 352 pp. WEANERF., S. R. McLeAN, U. V. GRA~'ZSTE~,~,H. ERLEb'KEUSER,M. SARWrdErN,U. SCHAU~R,G. UNSOLD,E. WALGER and R. WrrrsTocr (1987) Sedimentary Records of benthic processes. In: Seawater-sediment interactions in coastal waters. J. RUMOHR.E. WALGERand B. ZEITZSCHEL,editors, Lecture Notes on coastal and Estuarine Studies, 13, Springer, pp. 162-263. WELLERSHAUSS. (1973) A new method for collecting near-bottom water in the deep sea. "'Meteor" Forschungsergebn. Reihe A, 13, 50-57.