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A bottom-landing water sampling system for the benthic boundary layer
259
Netherlands Journal of Sea Research 34 (4): 259-266 (1995)
A BOTTOM-LANDINGWATER SAMPLING SYSTEM FOR THE BENTHIC BOUNDARY LAYER A.J. BALE and C.D. BARRETT NERC Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL 1 3DH, UK
ABSTRACT A novel water sampling device which enables vertical profiles of water samples to be obtained within the benthic boundary layer in shelf sea waters is described. A maximum of ten samples spread over 2 m immediately above the seabed can be obtained on each deployment, The design of the sample bottles minimizes disturbance to particle aggregates and positive displacement sampling ensures that the samples are representative of the environment. Suspended-solids profiles sampled in the benthic boundary layer over a 15-hour period at a station in the English Channel are presented to demonstrate the utility of the system.
Key words: water sampler, benthic boundary layer, benthic lander
1. INTRODUCTION Measurements of the transport of materials across the sediment-water interface and the relationship of these exchanges to physical factors which influence mobilization and deposition are essential requirements of models which aim to describe the transport and fate of geochemical weathering products and pollutant materials in natural waters. The mobilization of dissolved and particulate materials from bed sediments, whether through physical or biological disturbance, can exert a significant impact on the nature and quality of overlying water. This can be clearly observed in estuaries where the effect of resuspension of bed sediments and pore water under the influence of tidal currents can be detected throughout the water column (Morris et aL, 1982; Ackroyd et aL, 1986). In continental shelf waters the influence of resuspension processes may not be significant in the surface water although input from the bed can often be inferred from water column profiles (Bale & Morris, 1996). For many constituents, increasing dilution and dispersion with distance from the seabed can quickly make them undetectable. Not surprisingly, the most pronounced signals are found in the region closest to the bed. However, this environment is virtually impossible to sample with any confidence using conventional oceanographic sampling apparatus lowered
from a ship because the motion of the vessel does not allow sufficient vertical resolution to be achieved and sediment can be disturbed if the sampler touches the seabed in an uncontrolled manner. In order to obtain samples which allow the flux of material to and from the sediments to be determined quantitatively, a sampler was required which would meet specific requirements: 1) The height of the samples need to be accurately registered relative to the sediment surface and thus the sampler should be vertical, within limits. That is, it would be necessary to know that the sampler was not oriented at a large angle from vertical or, in the worst case, lying on its side. 2) The system should not sample material disturbed from the bed by the apparatus itself. 3) The sampling system should have no dead volume that would allow contamination of samples by surface waters. 4) The sample containers and their mode of filling should avoid contamination of the sample and should collect representative suspensions and, for studies of particle characteristics, with minimal disruption to particle aggregates. This means that the sampler should not cause extra turbulence or excessive deceleration or acceleration of water velocities. The criteria for achieving this last requirement are outlined in Sternberg et aL (1986) after Sundborg (1956), who stated: 1) The mean intake velocity should be approximately equal to the flow velocity. 2) The intake should be pointed into the flow and pro-
260
A.J. BALE & C.D. BARRETT
Plate 1. Shows the bottom-landing sampler suspended in the A-frame of a researchvessel just prior to deployment. Note the off-centre lifting points which keep the mercury tilt switches within the control cylinder open-circuit until the frame lands on the seabed and also orientates the sampling bottles into the current during deployment. trude upstream from any zone of disturbance caused by the mounting brackets. 3) The sampler should fill smoothly without sudden inrush or gulping. 4) The sampler should not be contaminated by water or sediment accumulated in the intake prior to sampling.
Although Sholkovitz (1970) identified similar requirements and solved the sampling problem for deep ocean basin studies with a free landing and recovery vehicle, his apparatus was not suitable for the dynamic shelf environment where sampling often
A BOTTOM-LANDINGWATER SAMPLER
needs to be undertaken rapidly in order to follow changes which occur with time, e.g. over tidal cycles. Sholkovitz's bottom lander obtained a profile of water samples close to the seabed but his sample bottles relied on passive flushing and diffusion to obtain representative samples and required a 3-h equilibration time on the seabed; this sampler could take a minimum of 5 hours to deploy and recover. Since that work, numerous workers have contrived sampling systems which attempted to meet the Sundborg (1956) criteria with various degrees of success. These systems have included pumping via swivelled nozzles into flexible plasma bags (Sternberg et aL, 1986, 1991), pressurized chambers with inlet 'snorkels' set at specific depth intervals (Eversberg, 1990) and similar bottles pre-filled with fresh water which was exchanged with sample by use of an onboard pump (Thomsen et al., 1994). While many of these approaches, and others, had some advantages, none of the systems appeared to fully meet the requirements outlined above and virtually all of the systems used pumps or restricted orifices that could break up aggregate particles. Furthermore, none of the existing systems were capable of providing vertical sampling resolution that was compatible with electronic and optical measuring systems for determining physical parameters in the benthic boundary layer. In order to overcome these shortcomings, we have constructed a tethered, bottom-landing apparatus for use in shelf sea waters which, when lowered to the seabed, will collect a profile of ten, 4-dm 3 samples using a syringe action to fill the bottles relatively quickly. Sampling is initiated by a programmable control system which employs self-contained sensors and timers and is completely independent of the vessel once deployment has commenced. 2. DESCRIPTION 2.1. THE LANDER FRAME The complete system is shown in Plate 1 and consists of a stainless steel frame in the form of a truncated, 2.4-m-high pyramid with a 2-m square base and a 1-m square top. Two vertical masts located within the centre of the frame carry an array of ten horizontally mounted sampling bottles. The central axes of the bottom and top bottles are 10 and 212.5 cm, respectively, above the sediment surface. When the bottles are evenly spaced the vertical resolution is 22.5 cm. A control cylinder which 'senses' the frame attitude and initiates the bottle closing mechanism after a preset delay is mounted on the top of the frame. Each of the four frame legs is fitted with a 30 x 30 cm square plate to prevent the frame sinking into the bed. When the bottles are oriented into the flowing current, the feet are 1 m off to each side of the bottle opening and only 0.6 m upstream. Thus disturbances of the seabed by the feet do not influence the
261
samples taken by the lower bottles. If required, up to three 5 kg weights can be added to each foot in order to ballast the frame, e.g. in strong tidal currents. The dimensions of the frame and its form were determined by the need to provide the most stable possible platform which would protect the bottles and the control unit from damage during deployment whilst still being of a practical size to transport and handle. 2.2. ACTIVE SAMPLING BOTTLES The bottles which have been designed for this application incorporate a number of features which are intended to maximize the integrity of the sample. Each bottle consists of a 650 mm length of 100 mm diameter cast acrylic tube. For particular applications, other material could be substituted, e.g. metal for trace organic sampling or polytetrafluoroethylene (PTFE or 'Teflon(R)') lining for trace metal sampling. An end cap machined from polyvinyl chloride (PVC) is fitted to each end of the cylinder using O-ring seals (Plate 2). The PVC piston within the sample bottle cylinder has two loosely fitting but expandable PTFE piston rings which seal the piston whilst providing a low friction sliding surface. The profile of the piston matches the internal profile of the inlet end cap and also protrudes slightly through the opening with the piston in the cocked position. This means that the sample container has no dead volume when the bottles are being deployed through the water column. The piston is actuated by three 3 mm diameter, elastic shock cords. In the cocked position, the piston is retained against the pull of the shock cords by a spring clip which fits a groove machined around the circumference of the part of the piston which protrudes through the inlet. A sealing system which is taken from the end closure of a 1.7 dm3 National Institute of Oceanography water sampling bottle is located on the inlet end cap. This has a hemi-spherical butyl rubber cap which is mounted on a short arm. When cocked the arm is retained by a latch but, when released, the arm swings through 70 ° under the influence of a coil spring to seal the syringe inlet. The inlet is 60 mm diameter and is profiled so that the cap seals efficiently but also so that sample can be drawn through with minimal shear disruption to particle aggregates. This is helped by the large diameter of the opening and the relatively slow progress of the piston. The filling time is marginally, over 3 seconds in still water so that drawing 4 dm ° through the 60-mm-diameter openin9 is equivalent to a flow velocity of 0.47 m-sec" which corresponds to typical NW European shelf sea tidal currents. However, the fill rate increases slightly in fast flowing water due to the increased pressure on the piston. Thus particles tend to experience slightly greater than ambient velocities when sampling at low current speeds and approximately similar to ambient conditions at higher current
262
A.J. BALE & C.D. BARRETT
Plate 2. Close-up views of the active water bottle. Upper picture: shows the large sample inlet with the sealing mechanism in the open position. Lower picture: the base end cap with the pulley wheels which carry the elastic shock cords that operate the piston and the lever which releases the sealing arm when the piston reaches the end of its travel. The piston can be seen through the transparent walls of the bottle cylinder.
A BOTTOM-LANDING WATER SAMPLER
velocities. The base end cap fitting is machined so that, at the end of its travel, the piston fits exactly into an internal O-ring which seals the base of the syringe. A port in the base with the same cross-sectional area as the sample inlet allows water behind the piston to be vented during sampling. This balances the water pressure across the piston rings and eliminates contamination of the sample by surface water which fills the volume behind the piston when the sampler is first submerged. On reaching the end of its travel, the piston releases the latch holding the inlet seal via a mechanical linkage and closes the bottle. The bottles are secured to brackets on the lander frame using stainless steel over-centre clips and dowel pins to provide precise location. To decant the sample, the bottle is removed from the mount and opened, when vertical, by re-latching the sealing arm into the open position. 2.3. THE CONTROL SYSTEM The control system is housed within a PVC pressure cylinder machined from heavy-duty PVC pipe. This cylinder was designed for a working depth of 200 m but for deeper waters a stronger cylinder could be substituted. A 12 Volt, dry cell, rechargeable battery and electric motor with a torque output of 6 Nm at 4 rpm are housed within the cylinder. The motor is operated by a circuit linked to a number of sensors. The first sensor is a sea water switch so that the motor can only be operated when the system is in the sea. The second sensor is an array of mercury tilt switches which are arranged so that they only operate the circuit if the orientation of the frame is within 5 ° of vertical. Linked to this is a timing circuit where delay options of 30, 60, 120 or 240 seconds can be pre-selected to allow time for disturbed bed sediment to disperse. Thus, when the various sensor criteria are met, the control unit initiates a time delay after which a vertical shaft running through plain journals in each of the sample bottle mounts is rotated once by the motor and the clips are withdrawn allowing the pistons to move under the influence of the shock cords. On deck the seawater switch is open circuit and the control unit is inoperative. During deployment the frame is suspended at an angle of >40 ° from vertical using an off-centre lifting point (see Plate 1). Only when the frame settles on the seabed and becomes upright and the tilt switches detect that the frame is within 5 ° of upright is the delay timer (typically 2 min) initiated. 2.4. DEPLOYMENT To collect a profile of samples the bottles are cocked by latching the seal in the open position and pushing the piston to the limit of its travel using a metal rod
263
whereupon the piston is retained against the pull of the shock cords by fixing the spring clip in place. When the ship is positioned the apparatus is lowered to the seabed. Because of the off-centre lifting points employed to keep the tilt switches open circuit until the apparatus settles on the seabed, the bulk of the frame and sampling bottles are suspended behind the pivot point. As the ship tends to be head to tide for deployments, the frame is automatically oriented by the drag generated by the combination of current flow and vessel movement such that the bottle openings are pointed directly into the flowing currents. Small flotation units are attached to the rope at 10 and 20 m from the frame so that the rope cannot fall onto the frame and snag the bottles or disturb bottom sediment in the vicinity of the sampling frame. In order to minimize the amount of rope which must be paid out and also to prevent the frame from being pulled along or over when it is on the bed, the vessel needs to be stationary relative to the bed which may require moving ahead to offset currents. Under some wind conditions this can be difficult since the vessel will have leeway as well as motion through the water to stem the tide. For the most satisfactory results a vessel with dynamic positioning capability would be ideal. Another alternative may be to anchor if water depth is not too great although this should be avoided if at all possible because of the potential for disturbing sediment upstream of the sampling apparatus. Fortunately, the frame only needs to be on the bed for 2.5 min (using the two minute delay interval) and, except under extreme conditions, movement of the ship can generally be accommodated by paying out the rope. 3. PERFORMANCE During a series of ten deployments over a 15-h period at a station in the English Channel we were able to observe the performance of the apparatus using a self-contained underwater video camera and lighting unit attached to the frame. With this apparatus we were able to check the operation of the piston release mechanism in situ and determine that the bottles were not mis-firing before they reached the bed. On some deployments, with the video camera oriented towards the feet of the frame, we could observe the degree of bed disturbance and determine whether the programmed delay interval was sufficient to allow disturbed sediment to be flushed away. In practice, sediment disturbed by the feet quickly drifted away but, more importantly, passed down each side of the frame away from the bottle openings. We were also able to observe that the frame was stable on the bed in current velocities of 1 m.sec "1 and monitor its behaviour during recovery. We found that a rope length of five times the water depth generally allowed us to achieve stable frame deployments although recovery of the frame after a lot of rope had been paid out tended to result in the frame being tipped over
264
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and pulled along on its side, sledge-like, before it was lifted out of the water. On a relatively smooth seabed this was not a problem as the bottles remained attached and protected. However, it might become problematic if the bottom was very irregular. A thorough echo sounder and, if possible, video camera survey of the intended sampling site would be an essential prerequisite for interfacial sampler deployments on unfamiliar ground. Examples of profiles obtained from one series of measurements during this fieldwork at different tidal states (Fig. 1) clearly show the resolution obtainable with this sampler. At slack water the suspended sediment concentration (SSC) ranged from 0.6 to 1.0 mg-dm "3 and values were uniform through the profile even though the potential for contamination of sample by material disturbed from the bed by the lander feet was greatest at this time. The consistency of the values from sample to sample gave a large measure of confidence in the analytical measurements of SSC during this work. At maximum current velocity there was a clear increase in SSC throughout the profile with concentrations reaching 5 to 6 mg-dm "3 in the lower 1 m. When the SSC values obtained from all ten profiles taken over a 15-h period were contoured using a proprietary contouring package (Fig. 2) the results clearly showed that SSC changes were closely related to changes in current velocity at the seabed
which implies that sampling artefacts are not a problem with this instrument. Current velocity measured 1.5 m above the bed with an electromagnetic current meter mounted on a heavily weighted seabed frame is given in the box below the contoured data in Fi£1.2. The recorded velocities ranged from 0 m.sec" at slack water to 0.8 m'sec -1 at mid4ide. Only one data set is presented here to illustrate the utility of this sampling apparatus as the synthesis of the data sets collected during this study will be reported elsewhere. Although there are always risks in drawing conclusions from one set of measurements, the coherent nature of the data collected with this apparatus can allow some general observations to be made. Periods of maximum near bed SSC appear to be related to the accelerating period of the tidal cycle and maximum SSC seems to occur when velocities reach approximately 0.4 m.sec -1 to 0.5 m.sec -1. SSC appears to decrease slightly at periods of maximum velocity and this may suggest that all the easily mobilized superficial sediment has been eroded by the time this velocity has been reached and/or that local SSC is reduced due to more efficient mixing of eroded material higher into the water column with increased current velocity. SSC declines uniformly as energy levels decrease to give uniformly tow values throughout the profile at slack tide when current velocities are minimal. This suggests that the majority of material in this type of environment settles very rapidly, which is broadly consistent with previous observations in a marine environment in which fine, slowly settling material is quickly aggregated and/or otherwise incorporated into organic-rich aggregates due to biological activity and its by-products (Jago et aL, 1993). 4. CONCLUSIONS The experience gained during the development of this system and the results of suspended solids analyses from a series of deployments in the English Channel have convinced us that we have constructed a general purpose water and particle sampler for the benthic boundary layer which meets most of the specifications outlined in the introduction to this paper. The lander concept is not new but the design of the bottles employed in conjunction with this lander provides a significant improvement over existing sampling and control systems. The materials used in the construction of this prototype were chosen in relation to sampling suspended particles but could be changed to suit particular applications whilst retaining the same basic mechanical operation. At sea we were able to cycle the sampler at an hourly frequency but this included processing the samples by filtration on to tared GF/F filter papers. If sample processing was quicker or samples were processed independently of the sampling operation or stored for subsequent processing the sampler could be cycled at a
A BOTTOM-LANDING WATER SAMPLER
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Fig. 2. Upper box: contoured representation of all the SSC data obtained from a series of ten profiles at one station taken over a period of 15 h. The contour interval is 0.5 mg.dm3. Lower box: changes in the current velocity measured using an electromagnetic current meter mounted 1.5 m above the seabed during the sampling period. frequency of 15 to 20 min, a speed which is limited mainly by the time taken to remove and decant each bottle before replacing it on the frame and re-setting the bottles.
It is recognized that the relation of sediment erosion and deposition processes to current velocities measured at a non-specific distance from the site of the observations and at only one height above the
266
A.J. BALE & C.D. BARRETT
bed is not ideal. Subsequent developments aim to include the simultaneous measurement of a current velocity profile coincident with the collection of near-bed water samples. This will be achieved with a number of electromagnetic current meters mounted on the lander frame and operated and logged by the lander control system. We also aim to measure and record the direction of the current flow relative to the axis of the bottles. Acknowledgements.--The authors are grateful for the opportunity to participate in the CPB Manche II survey, which formed part of the field programme of the French Programme Nationale d'Oceanographie Cotiere (PNOC) and to the officers and company of the research vessel 'Le Noroit" for their constructive help during the cruise. Tony Bale would like to thank Gerard Thouzeau and co-workers for practical assistance during the survey and for allowing us to use the current meter data obtained during the survey. Development and construction of the interracial sampler was partly funded by the UK Department of the Environment under contract PECD 7/8/215. 5. REFERENCES Ackroyd, D.R., A.J. Bale, R.J.M. Howland, S. Knox, G.E. Millward & A.W. Morris, 1986. Distributions and behaviour of dissolved Cu, Zn and Mn in the Tamar Estuary.--Est, coast. Shelf Sci. 23" 621-640. Bale, A.J. & A.W. Morris, 1996. Organic carbon in suspended particulate material in the North Sea: effect of
mixing resuspended and background particles.---Cont. Shelf Res. (in press). Eversberg, U., 1990. A new device for sampling water from the benthic boundary layer.--Helgol&nder Meeresunters. 44" 329-334. Jago, C.F., A.J. Bale, M.O. Green, M.J. Howarth, S.E. Jones, I.N. McCave, G.E. Mil!ward, A.W. Morris, A.AI Rowden & J.J. Williams, 1993. Resuspension processes and seston dynamics, southern North Sea.--Phil. Trans. R. Soc. Lond. A. 343: 475-491. Morris, A.W., A.J. Bale & R.J.M Howland, 1982. The dynamics of estuarine manganese cycling.--Est. coast. Shelf Sci. 14" 175-192. Shotkovitz, E.R., 1970. A free vehicle bottom-water sampler.--Limnol. Oceanogr. 1,5: 641-644. Sternberg, R.W., R.V. Johnson II, D.A. Cacchione & D.E. Drake, 1986. An instrument for monitoring and sampling suspended sediment in the benthic boundary layer.--Mar. Geol. 71: 187-199. Sternberg, R.W., G.C. Kineke & R. Johnson, 1991. An instrument for profiling suspended sediment, fluid, and flow conditions in shallow marine environments.--Cont. Shelf Res. 11: 109-122. Sundborg, A., 1956. The river Klaralven: a study of fluvial processes.---Geogr. Ann. 88" 127-316. Thomsen, L., G. Graf, V. Martens & E. Steen, 1994. An instrument for sampling water from the benthic boundary layer.--Cont. Shelf Res. 14; 871-882. (accepted 11 September 1995)
Netherlands Journal of Sea Research 34 (4): 259-266 (1995)
A BOTTOM-LANDINGWATER SAMPLING SYSTEM FOR THE BENTHIC BOUNDARY LAYER A.J. BALE and C.D. BARRETT NERC Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL 1 3DH, UK
ABSTRACT A novel water sampling device which enables vertical profiles of water samples to be obtained within the benthic boundary layer in shelf sea waters is described. A maximum of ten samples spread over 2 m immediately above the seabed can be obtained on each deployment, The design of the sample bottles minimizes disturbance to particle aggregates and positive displacement sampling ensures that the samples are representative of the environment. Suspended-solids profiles sampled in the benthic boundary layer over a 15-hour period at a station in the English Channel are presented to demonstrate the utility of the system.
Key words: water sampler, benthic boundary layer, benthic lander
1. INTRODUCTION Measurements of the transport of materials across the sediment-water interface and the relationship of these exchanges to physical factors which influence mobilization and deposition are essential requirements of models which aim to describe the transport and fate of geochemical weathering products and pollutant materials in natural waters. The mobilization of dissolved and particulate materials from bed sediments, whether through physical or biological disturbance, can exert a significant impact on the nature and quality of overlying water. This can be clearly observed in estuaries where the effect of resuspension of bed sediments and pore water under the influence of tidal currents can be detected throughout the water column (Morris et aL, 1982; Ackroyd et aL, 1986). In continental shelf waters the influence of resuspension processes may not be significant in the surface water although input from the bed can often be inferred from water column profiles (Bale & Morris, 1996). For many constituents, increasing dilution and dispersion with distance from the seabed can quickly make them undetectable. Not surprisingly, the most pronounced signals are found in the region closest to the bed. However, this environment is virtually impossible to sample with any confidence using conventional oceanographic sampling apparatus lowered
from a ship because the motion of the vessel does not allow sufficient vertical resolution to be achieved and sediment can be disturbed if the sampler touches the seabed in an uncontrolled manner. In order to obtain samples which allow the flux of material to and from the sediments to be determined quantitatively, a sampler was required which would meet specific requirements: 1) The height of the samples need to be accurately registered relative to the sediment surface and thus the sampler should be vertical, within limits. That is, it would be necessary to know that the sampler was not oriented at a large angle from vertical or, in the worst case, lying on its side. 2) The system should not sample material disturbed from the bed by the apparatus itself. 3) The sampling system should have no dead volume that would allow contamination of samples by surface waters. 4) The sample containers and their mode of filling should avoid contamination of the sample and should collect representative suspensions and, for studies of particle characteristics, with minimal disruption to particle aggregates. This means that the sampler should not cause extra turbulence or excessive deceleration or acceleration of water velocities. The criteria for achieving this last requirement are outlined in Sternberg et aL (1986) after Sundborg (1956), who stated: 1) The mean intake velocity should be approximately equal to the flow velocity. 2) The intake should be pointed into the flow and pro-
260
A.J. BALE & C.D. BARRETT
Plate 1. Shows the bottom-landing sampler suspended in the A-frame of a researchvessel just prior to deployment. Note the off-centre lifting points which keep the mercury tilt switches within the control cylinder open-circuit until the frame lands on the seabed and also orientates the sampling bottles into the current during deployment. trude upstream from any zone of disturbance caused by the mounting brackets. 3) The sampler should fill smoothly without sudden inrush or gulping. 4) The sampler should not be contaminated by water or sediment accumulated in the intake prior to sampling.
Although Sholkovitz (1970) identified similar requirements and solved the sampling problem for deep ocean basin studies with a free landing and recovery vehicle, his apparatus was not suitable for the dynamic shelf environment where sampling often
A BOTTOM-LANDINGWATER SAMPLER
needs to be undertaken rapidly in order to follow changes which occur with time, e.g. over tidal cycles. Sholkovitz's bottom lander obtained a profile of water samples close to the seabed but his sample bottles relied on passive flushing and diffusion to obtain representative samples and required a 3-h equilibration time on the seabed; this sampler could take a minimum of 5 hours to deploy and recover. Since that work, numerous workers have contrived sampling systems which attempted to meet the Sundborg (1956) criteria with various degrees of success. These systems have included pumping via swivelled nozzles into flexible plasma bags (Sternberg et aL, 1986, 1991), pressurized chambers with inlet 'snorkels' set at specific depth intervals (Eversberg, 1990) and similar bottles pre-filled with fresh water which was exchanged with sample by use of an onboard pump (Thomsen et al., 1994). While many of these approaches, and others, had some advantages, none of the systems appeared to fully meet the requirements outlined above and virtually all of the systems used pumps or restricted orifices that could break up aggregate particles. Furthermore, none of the existing systems were capable of providing vertical sampling resolution that was compatible with electronic and optical measuring systems for determining physical parameters in the benthic boundary layer. In order to overcome these shortcomings, we have constructed a tethered, bottom-landing apparatus for use in shelf sea waters which, when lowered to the seabed, will collect a profile of ten, 4-dm 3 samples using a syringe action to fill the bottles relatively quickly. Sampling is initiated by a programmable control system which employs self-contained sensors and timers and is completely independent of the vessel once deployment has commenced. 2. DESCRIPTION 2.1. THE LANDER FRAME The complete system is shown in Plate 1 and consists of a stainless steel frame in the form of a truncated, 2.4-m-high pyramid with a 2-m square base and a 1-m square top. Two vertical masts located within the centre of the frame carry an array of ten horizontally mounted sampling bottles. The central axes of the bottom and top bottles are 10 and 212.5 cm, respectively, above the sediment surface. When the bottles are evenly spaced the vertical resolution is 22.5 cm. A control cylinder which 'senses' the frame attitude and initiates the bottle closing mechanism after a preset delay is mounted on the top of the frame. Each of the four frame legs is fitted with a 30 x 30 cm square plate to prevent the frame sinking into the bed. When the bottles are oriented into the flowing current, the feet are 1 m off to each side of the bottle opening and only 0.6 m upstream. Thus disturbances of the seabed by the feet do not influence the
261
samples taken by the lower bottles. If required, up to three 5 kg weights can be added to each foot in order to ballast the frame, e.g. in strong tidal currents. The dimensions of the frame and its form were determined by the need to provide the most stable possible platform which would protect the bottles and the control unit from damage during deployment whilst still being of a practical size to transport and handle. 2.2. ACTIVE SAMPLING BOTTLES The bottles which have been designed for this application incorporate a number of features which are intended to maximize the integrity of the sample. Each bottle consists of a 650 mm length of 100 mm diameter cast acrylic tube. For particular applications, other material could be substituted, e.g. metal for trace organic sampling or polytetrafluoroethylene (PTFE or 'Teflon(R)') lining for trace metal sampling. An end cap machined from polyvinyl chloride (PVC) is fitted to each end of the cylinder using O-ring seals (Plate 2). The PVC piston within the sample bottle cylinder has two loosely fitting but expandable PTFE piston rings which seal the piston whilst providing a low friction sliding surface. The profile of the piston matches the internal profile of the inlet end cap and also protrudes slightly through the opening with the piston in the cocked position. This means that the sample container has no dead volume when the bottles are being deployed through the water column. The piston is actuated by three 3 mm diameter, elastic shock cords. In the cocked position, the piston is retained against the pull of the shock cords by a spring clip which fits a groove machined around the circumference of the part of the piston which protrudes through the inlet. A sealing system which is taken from the end closure of a 1.7 dm3 National Institute of Oceanography water sampling bottle is located on the inlet end cap. This has a hemi-spherical butyl rubber cap which is mounted on a short arm. When cocked the arm is retained by a latch but, when released, the arm swings through 70 ° under the influence of a coil spring to seal the syringe inlet. The inlet is 60 mm diameter and is profiled so that the cap seals efficiently but also so that sample can be drawn through with minimal shear disruption to particle aggregates. This is helped by the large diameter of the opening and the relatively slow progress of the piston. The filling time is marginally, over 3 seconds in still water so that drawing 4 dm ° through the 60-mm-diameter openin9 is equivalent to a flow velocity of 0.47 m-sec" which corresponds to typical NW European shelf sea tidal currents. However, the fill rate increases slightly in fast flowing water due to the increased pressure on the piston. Thus particles tend to experience slightly greater than ambient velocities when sampling at low current speeds and approximately similar to ambient conditions at higher current
262
A.J. BALE & C.D. BARRETT
Plate 2. Close-up views of the active water bottle. Upper picture: shows the large sample inlet with the sealing mechanism in the open position. Lower picture: the base end cap with the pulley wheels which carry the elastic shock cords that operate the piston and the lever which releases the sealing arm when the piston reaches the end of its travel. The piston can be seen through the transparent walls of the bottle cylinder.
A BOTTOM-LANDING WATER SAMPLER
velocities. The base end cap fitting is machined so that, at the end of its travel, the piston fits exactly into an internal O-ring which seals the base of the syringe. A port in the base with the same cross-sectional area as the sample inlet allows water behind the piston to be vented during sampling. This balances the water pressure across the piston rings and eliminates contamination of the sample by surface water which fills the volume behind the piston when the sampler is first submerged. On reaching the end of its travel, the piston releases the latch holding the inlet seal via a mechanical linkage and closes the bottle. The bottles are secured to brackets on the lander frame using stainless steel over-centre clips and dowel pins to provide precise location. To decant the sample, the bottle is removed from the mount and opened, when vertical, by re-latching the sealing arm into the open position. 2.3. THE CONTROL SYSTEM The control system is housed within a PVC pressure cylinder machined from heavy-duty PVC pipe. This cylinder was designed for a working depth of 200 m but for deeper waters a stronger cylinder could be substituted. A 12 Volt, dry cell, rechargeable battery and electric motor with a torque output of 6 Nm at 4 rpm are housed within the cylinder. The motor is operated by a circuit linked to a number of sensors. The first sensor is a sea water switch so that the motor can only be operated when the system is in the sea. The second sensor is an array of mercury tilt switches which are arranged so that they only operate the circuit if the orientation of the frame is within 5 ° of vertical. Linked to this is a timing circuit where delay options of 30, 60, 120 or 240 seconds can be pre-selected to allow time for disturbed bed sediment to disperse. Thus, when the various sensor criteria are met, the control unit initiates a time delay after which a vertical shaft running through plain journals in each of the sample bottle mounts is rotated once by the motor and the clips are withdrawn allowing the pistons to move under the influence of the shock cords. On deck the seawater switch is open circuit and the control unit is inoperative. During deployment the frame is suspended at an angle of >40 ° from vertical using an off-centre lifting point (see Plate 1). Only when the frame settles on the seabed and becomes upright and the tilt switches detect that the frame is within 5 ° of upright is the delay timer (typically 2 min) initiated. 2.4. DEPLOYMENT To collect a profile of samples the bottles are cocked by latching the seal in the open position and pushing the piston to the limit of its travel using a metal rod
263
whereupon the piston is retained against the pull of the shock cords by fixing the spring clip in place. When the ship is positioned the apparatus is lowered to the seabed. Because of the off-centre lifting points employed to keep the tilt switches open circuit until the apparatus settles on the seabed, the bulk of the frame and sampling bottles are suspended behind the pivot point. As the ship tends to be head to tide for deployments, the frame is automatically oriented by the drag generated by the combination of current flow and vessel movement such that the bottle openings are pointed directly into the flowing currents. Small flotation units are attached to the rope at 10 and 20 m from the frame so that the rope cannot fall onto the frame and snag the bottles or disturb bottom sediment in the vicinity of the sampling frame. In order to minimize the amount of rope which must be paid out and also to prevent the frame from being pulled along or over when it is on the bed, the vessel needs to be stationary relative to the bed which may require moving ahead to offset currents. Under some wind conditions this can be difficult since the vessel will have leeway as well as motion through the water to stem the tide. For the most satisfactory results a vessel with dynamic positioning capability would be ideal. Another alternative may be to anchor if water depth is not too great although this should be avoided if at all possible because of the potential for disturbing sediment upstream of the sampling apparatus. Fortunately, the frame only needs to be on the bed for 2.5 min (using the two minute delay interval) and, except under extreme conditions, movement of the ship can generally be accommodated by paying out the rope. 3. PERFORMANCE During a series of ten deployments over a 15-h period at a station in the English Channel we were able to observe the performance of the apparatus using a self-contained underwater video camera and lighting unit attached to the frame. With this apparatus we were able to check the operation of the piston release mechanism in situ and determine that the bottles were not mis-firing before they reached the bed. On some deployments, with the video camera oriented towards the feet of the frame, we could observe the degree of bed disturbance and determine whether the programmed delay interval was sufficient to allow disturbed sediment to be flushed away. In practice, sediment disturbed by the feet quickly drifted away but, more importantly, passed down each side of the frame away from the bottle openings. We were also able to observe that the frame was stable on the bed in current velocities of 1 m.sec "1 and monitor its behaviour during recovery. We found that a rope length of five times the water depth generally allowed us to achieve stable frame deployments although recovery of the frame after a lot of rope had been paid out tended to result in the frame being tipped over
264
A.J. BALE & C.D. BARRETT
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and pulled along on its side, sledge-like, before it was lifted out of the water. On a relatively smooth seabed this was not a problem as the bottles remained attached and protected. However, it might become problematic if the bottom was very irregular. A thorough echo sounder and, if possible, video camera survey of the intended sampling site would be an essential prerequisite for interfacial sampler deployments on unfamiliar ground. Examples of profiles obtained from one series of measurements during this fieldwork at different tidal states (Fig. 1) clearly show the resolution obtainable with this sampler. At slack water the suspended sediment concentration (SSC) ranged from 0.6 to 1.0 mg-dm "3 and values were uniform through the profile even though the potential for contamination of sample by material disturbed from the bed by the lander feet was greatest at this time. The consistency of the values from sample to sample gave a large measure of confidence in the analytical measurements of SSC during this work. At maximum current velocity there was a clear increase in SSC throughout the profile with concentrations reaching 5 to 6 mg-dm "3 in the lower 1 m. When the SSC values obtained from all ten profiles taken over a 15-h period were contoured using a proprietary contouring package (Fig. 2) the results clearly showed that SSC changes were closely related to changes in current velocity at the seabed
which implies that sampling artefacts are not a problem with this instrument. Current velocity measured 1.5 m above the bed with an electromagnetic current meter mounted on a heavily weighted seabed frame is given in the box below the contoured data in Fi£1.2. The recorded velocities ranged from 0 m.sec" at slack water to 0.8 m'sec -1 at mid4ide. Only one data set is presented here to illustrate the utility of this sampling apparatus as the synthesis of the data sets collected during this study will be reported elsewhere. Although there are always risks in drawing conclusions from one set of measurements, the coherent nature of the data collected with this apparatus can allow some general observations to be made. Periods of maximum near bed SSC appear to be related to the accelerating period of the tidal cycle and maximum SSC seems to occur when velocities reach approximately 0.4 m.sec -1 to 0.5 m.sec -1. SSC appears to decrease slightly at periods of maximum velocity and this may suggest that all the easily mobilized superficial sediment has been eroded by the time this velocity has been reached and/or that local SSC is reduced due to more efficient mixing of eroded material higher into the water column with increased current velocity. SSC declines uniformly as energy levels decrease to give uniformly tow values throughout the profile at slack tide when current velocities are minimal. This suggests that the majority of material in this type of environment settles very rapidly, which is broadly consistent with previous observations in a marine environment in which fine, slowly settling material is quickly aggregated and/or otherwise incorporated into organic-rich aggregates due to biological activity and its by-products (Jago et aL, 1993). 4. CONCLUSIONS The experience gained during the development of this system and the results of suspended solids analyses from a series of deployments in the English Channel have convinced us that we have constructed a general purpose water and particle sampler for the benthic boundary layer which meets most of the specifications outlined in the introduction to this paper. The lander concept is not new but the design of the bottles employed in conjunction with this lander provides a significant improvement over existing sampling and control systems. The materials used in the construction of this prototype were chosen in relation to sampling suspended particles but could be changed to suit particular applications whilst retaining the same basic mechanical operation. At sea we were able to cycle the sampler at an hourly frequency but this included processing the samples by filtration on to tared GF/F filter papers. If sample processing was quicker or samples were processed independently of the sampling operation or stored for subsequent processing the sampler could be cycled at a
A BOTTOM-LANDING WATER SAMPLER
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It is recognized that the relation of sediment erosion and deposition processes to current velocities measured at a non-specific distance from the site of the observations and at only one height above the
266
A.J. BALE & C.D. BARRETT
bed is not ideal. Subsequent developments aim to include the simultaneous measurement of a current velocity profile coincident with the collection of near-bed water samples. This will be achieved with a number of electromagnetic current meters mounted on the lander frame and operated and logged by the lander control system. We also aim to measure and record the direction of the current flow relative to the axis of the bottles. Acknowledgements.--The authors are grateful for the opportunity to participate in the CPB Manche II survey, which formed part of the field programme of the French Programme Nationale d'Oceanographie Cotiere (PNOC) and to the officers and company of the research vessel 'Le Noroit" for their constructive help during the cruise. Tony Bale would like to thank Gerard Thouzeau and co-workers for practical assistance during the survey and for allowing us to use the current meter data obtained during the survey. Development and construction of the interracial sampler was partly funded by the UK Department of the Environment under contract PECD 7/8/215. 5. REFERENCES Ackroyd, D.R., A.J. Bale, R.J.M. Howland, S. Knox, G.E. Millward & A.W. Morris, 1986. Distributions and behaviour of dissolved Cu, Zn and Mn in the Tamar Estuary.--Est, coast. Shelf Sci. 23" 621-640. Bale, A.J. & A.W. Morris, 1996. Organic carbon in suspended particulate material in the North Sea: effect of
mixing resuspended and background particles.---Cont. Shelf Res. (in press). Eversberg, U., 1990. A new device for sampling water from the benthic boundary layer.--Helgol&nder Meeresunters. 44" 329-334. Jago, C.F., A.J. Bale, M.O. Green, M.J. Howarth, S.E. Jones, I.N. McCave, G.E. Mil!ward, A.W. Morris, A.AI Rowden & J.J. Williams, 1993. Resuspension processes and seston dynamics, southern North Sea.--Phil. Trans. R. Soc. Lond. A. 343: 475-491. Morris, A.W., A.J. Bale & R.J.M Howland, 1982. The dynamics of estuarine manganese cycling.--Est. coast. Shelf Sci. 14" 175-192. Shotkovitz, E.R., 1970. A free vehicle bottom-water sampler.--Limnol. Oceanogr. 1,5: 641-644. Sternberg, R.W., R.V. Johnson II, D.A. Cacchione & D.E. Drake, 1986. An instrument for monitoring and sampling suspended sediment in the benthic boundary layer.--Mar. Geol. 71: 187-199. Sternberg, R.W., G.C. Kineke & R. Johnson, 1991. An instrument for profiling suspended sediment, fluid, and flow conditions in shallow marine environments.--Cont. Shelf Res. 11: 109-122. Sundborg, A., 1956. The river Klaralven: a study of fluvial processes.---Geogr. Ann. 88" 127-316. Thomsen, L., G. Graf, V. Martens & E. Steen, 1994. An instrument for sampling water from the benthic boundary layer.--Cont. Shelf Res. 14; 871-882. (accepted 11 September 1995)