Benthos

There are several branches of science which seek information about the sea bottom; for example, oceanography, geology and palaeontology as well as marine biology. Each makes use of apparatus designed primarily to collect information needed in that particular field of study, but there is so much overlap between these various aspects of marine science that data relevant to one may well be of interest to another, and there is consequently a variety of instruments for studying the sea

Acoustic receiver and depth telemeter

Release gear v Fixed top bar

__Sliding bar attached to top of net

Release gear v Fixed top bar

__Sliding bar attached to top of net

Samplers Net Benthos

Figure 3.17 Midwater trawl for mesopelagic organisms, used on RRS Discovery. The net can be opened and closed by sonic signals sent from the ship, operating the release mechanism. The depth of the net is indicated by the pulse frequency of sonic signals generated by the depth telemeter. (a) Net closed for lowering; (b) net open; (c) net closed for hauling.

Figure 3.17 Midwater trawl for mesopelagic organisms, used on RRS Discovery. The net can be opened and closed by sonic signals sent from the ship, operating the release mechanism. The depth of the net is indicated by the pulse frequency of sonic signals generated by the depth telemeter. (a) Net closed for lowering; (b) net open; (c) net closed for hauling.

bottom which produce information relating to marine biology. We shall refer to only a few. Those selected for mention here are of two general types; instruments which are intended mainly for collecting samples of sediment, and those for collecting benthic organisms. The distinction is not a firm one because sediment samplers are likely to include small organisms in the material brought up, and apparatus designed to catch bottom-dwelling creatures may also retain some of the deposit.

Sediment samplers

Various small spring-loaded, snapper grabs have been devised which take a shallow bite out of the sea floor (Figure 3.18). Some investigations seek

Hemispherical jaws

,Weight

Spring Herni

Hemispherical jaws

Figure 3.18 A spring-loaded snapper grab.

information about the deeper layers of deposit, and instruments known as corers are used for this purpose.

A corer is a long tube which can be driven down into the sea floor, and then withdrawn enclosing a core of sediment. The coring tube contains a separate liner to facilitate removal of the core. Considerable force is required to drive a corer far into the deposit, and several methods are used for this purpose.

The corer may be heavily weighted and allowed to descend at speed, penetrating the sediment under its own momentum. An explosive charge can be detonated to drive the tube downwards. A vibrocorer is driven into the sea-bed by a vibrating motor at the top of the tube, operated by electricity or compressed air.

The Kullenberg piston corer makes use of hydrostatic pressure to assist deep penetration. This corer consists of a weighted coring tube with brass liners, inside which fits a sliding piston attached to the lowering cable. The corer is lowered with the piston at the lower end of the tube, and the apparatus is slung from a release mechanism held in the closed position by counter-weights suspended below the nose of the coring tube. When these counter-weights touch bottom, the release mechanism opens to let the coring tube fall under its own weight. At this moment, the reduced strain on the lowering cable is indicated on the vessel by a dynamometer, and the cable winch is stopped immediately so that the piston attached to the cable is held stationary as the coring tube plunges downwards. This creates a tremendous suction inside the tube which helps to overcome the resistance of the substrate to penetration. Undisturbed cores over 20 m long have been obtained from very deep water with this device.

A type of piston corer has been used in conjunction with the drilling tube of the drilling ship Glomar Challenger. This ship makes drill borings in the deep ocean floor as part of a research project, the International Programme of Ocean

Drilling (formerly the Deep Sea Drilling Project), studying the structure of the earth's crust beneath the sea. The drilling bits used for boring hard rock disrupt the soft uppermost sediments, but by first dropping piston corers down the drill pipe it has been possible to obtain cores of undisturbed sediment up to about 200 m long. These cores contain the remains of planktonic organisms deposited on the sea-bed over a period of several hundred thousand years, and something may be learnt of oceanic conditions in the past by studying the variations in composition at different levels of the core.

For taking short cores up to about 1 m long from deep water, a 'free fall corer' may be used. This is a weighted coring tube which has no cable for lowering and hauling, but is simply thrown over the side of the vessel to sink freely. When the corer has sunk into the bottom an automatic release frees the tube, and a pair of glass floats carries the tube and enclosed sample to the surface. Retrieval is aided by a flashing light on the instrument.

Sediment traps

In the deep sea, the amount of particulate organic material reaching the deeper layers and the sea-bed affects the composition of animal communities. The amount and nature of the biogenic particles that rain down throughout the year can be measured using sediment traps. The traps can be moored at various depths above the sea-bed and left in place for a year or more. Modern traps work automatically using rotary collectors (Figure 3.19}. Each collector opens for a set number of days, commonly seven, then closes (Lampitt, 1996). These techniques and time-lapse photographic techniques have been widely used in the recent Joint Global Ocean Flux Study (JGOFS) (Ducklow and Harris, 1993).

Collecting organisms from the bottom

Methods of sampling the benthic population vary with the types of organisms under study, and the type of bottom. Demersal fish and many other creatures that live on, rather than within, the sea bottom can be captured by the trawls and seines used by commercial fisheries, described later (see page 315). For research purposes, the nets are usually of smaller mesh than is permitted for commercial fishing. Animals living within the sediment must be dug out using grabs and corers. Rock-living species can usually only be sampled using divers or photographed using remote cameras or submersibles. Practical details of all these methods are given in Holme and McIntyre (1984).

Trawls and dredges (qualitative)

Trawl nets are designed to skim over the bottom and as they cover a large area, they have a good chance of collecting widely dispersed species. They are, however, often rather selective. A net much used for biological work is the Agassiz trawl

Agassiz Trawl
Figure 3.19 A time series sediment trap. Falling material enters the cone and falls down into a collecting cup filled with formalin. After a set number of days, the next collecting cup moves round into position.

(Figure 3.20), which has the advantage of very easy handling because it does not matter which side up it reaches the bottom. The mouth of this net is held open by a metal frame, and it can be fitted with fine-mesh net to retain small creatures. It is simply dragged along the bottom.

To capture animals that live beneath the surface, the sampling device must be capable of digging into the deposit. The naturalist's dredge is a simple device which can be operated from a small boat. It consists of a bag of strong sacking or wire mesh held open by a heavy, rectangular metal frame. This can bite a few inches into a soft sediment as it is hauled along, but tends to fill mainly with material lying on the bottom. The leading edges of the frame can be angled and sharpened to increase the tendency to dig rather than to ride along the surface, but it does not catch the deeper-burrowing creatures.

An example of an instrument that takes a considerably deeper bite is the Forster anchor dredge (Forster, 1953) (Figure 3.21). This requires a sizeable vessel for its operation. The net is attached to a strong rectangular metal frame with a long, forward-projecting upper arm and a lower, downward-sloping digging-plate. The dredge is lowered to the bottom and remains stationary as the ship moves slowly

Anchor Dredge Benthic
Figure 3.20 The Agassiz trawl.
Agassiz Trawl
Figure 3.21 The Forster anchor dredge. (Based on Forster, 1953.)

astern paying out a long length of cable, three to five times the depth of the water. The winch brakes are then applied to the cable, and the strain exerted as the ship is stopped causes the dredge to tilt and bite deeply into the substrate. So instead of sliding along the bottom the dredge digs in like an anchor to a depth of about 25 cm. Finally, the cable is winched in until the dredge eventually breaks out, the contents being retained within the net. For ease of use in deep water this type of dredge can be made with digging-plates on both sides of the arm, so that it will bite whichever way up it lands on the bottom.

Epibenthic sledges

Epibenthic sledges are widely used for sampling smaller seabed animals, especially in the deep sea. The simplest sledge consists of a mesh bag or bags mounted in a metal frame on runners. The collecting bag is protected inside a steel mesh cage. The angle and height of the cutting plates on the mouth of the frame can be adjusted and the sledge can operate either way up.

More sophisticated sledges (Gage and Tyler, 1991) have various instruments mounted on the top and so must operate the right way up. Cameras photograph the bottom area ahead of the sledge. Acoustic devices indicate the position of the sledge relative to the bottom, and an odometer wheel coupled to a potentiometer measures the distance the sledge has travelled over the sea-bed.

Plankton living near the bottom (hypoplankton) can be collected in a plankton net attached to a sledge (Figure 3.22) and dragged over the sea floor.

Towed Submersible Sledge

towing

Runners

Doors to close mouth of net

Figure 3.22 The Bossanyi hypoplankton net.

Receiver Springs to close doors

I /during raising and

' Plankton net Alowering towing

Runners

Doors to close mouth of net

Figure 3.22 The Bossanyi hypoplankton net.

Quantitative bottom sampling

Quantitative studies of benthic populations require samplers which take a standard bite of known area and depth. A large number of remote samplers have been designed for use in a variety of depths and conditions. For small organisms (micro- and meiobenthos), most of which live close to the surface, short coring tubes can provide satisfactory samples from soft deposits. Capturing larger creatures presents more difficulty because some can escape the sampling gear by crawling away or moving deeper down their burrows. The main types of grabs and corers in current use have been reviewed by Eleftheriou and Holme (1984) and aspects of their use are described in Hartley and Dicks (1987). Only those most commonly used are described here.

On soft sediments the Petersen grab (Figure 3.23) has been much used. It consists of a pair of heavy metal jaws which are locked wide apart while lowering to the sea-bottom. The grab sinks into the deposit under its own weight, and as the cable goes slack the lock holding the jaws apart is automatically released. On hauling, before the grab lifts off the bottom, the tightening cable first draws the jaws together enclosing a bite of the substrate of approximately 0.1 m2 surface area. The grab bites fairly well into soft mud, but on sand or gravel it digs only to a depth of some 3-4 cm and many creatures escape. Stones or pieces of shell may wedge between the jaws, preventing complete closure, and much of the catch may then be lost during hauling.

The Petersen grab is less used nowadays than several other samplers which work on similar principles. Three grabs in common use because of their simplicity and ease of handling are the Smith-Mclntyre, Day and Van Veen grabs.

The much used Smith-Mclntyre grab has jaws which are spring-loaded to drive them into the sediment. Hauling on the cable then closes the buckets before lifting the grab off the bottom. The Day grab is based on the Smith-McIntyre design but

Smith Mcintyre Grab Sampler
Figure 3.23 (a) The Petersen grab. (b) The Van Veen grab.
Van Veen Dredge
Figure 3.24 The Day grab. (a) End view open for lowering; (b) Side view, one bucket open

the other closed. On reaching the sea-bed, the two pressure plates are pushed upwards releasing the transverse beam so that the hooks holding the buckets open are released. The buckets are closed by tension on the two cables. (From: Day (unpublished manuscript) in Holme and McIntyre, 1984.)

is not spring-loaded, and is therefore possibly somewhat safer in operation. It has a strong pyramid-shaped frame and can be almost guaranteed to land, and stay the right way up (Figure 3.24). The simple design of the Van Veen grab makes it especially suitable for small boat use (Figure 3.23b). It has arms attached to the jaws to give greater leverage for forcing the jaws together. Its lightness means it works best in soft sediments and calm weather.

An example of a sampler which can be used on rather coarser deposits is the Holme scoop sampler, used in studies of the biomass of the English Channel. This digs by means of semi-circular scoops. Two models have been designed, one having a single scoop and the other a pair of counter-rotating scoops (Figure

Cable to ship

Weighted frame

Cables to rotate scoops

Rotating semicircular scoops

Figure 3.25 Diagrammatic representation of the Holme double scoop sampler. (Based on Holme, 1953.)

Rotating semicircular scoops

Figure 3.25 Diagrammatic representation of the Holme double scoop sampler. (Based on Holme, 1953.)

3.25). The apparatus is lowered with the scoops in a fully open position. On reaching the bottom, a release mechanism operates so that, when hauling commences, the strain on the cable is first applied to the scoops, turning them through 180° so that they dig into the substrate. Each scoop samples a rectangular area of approximately 0.05 m2 although a later model has scoops twice the width. In favourable conditions each bite is semi-circular in vertical section with a maximum depth of 15 cm.

In deep-sea investigations, the USNEL box corer (Hessler and Jumars, 1974), is now the standard gear for quantitative sampling. It consists of a square, open-ended steel core box fixed to a weighted column, mounted on a support frame. After the corer has sunk into the sediment, a spade swings down to close the bottom and the top is closed by flaps (Figure 3.26). When operated carefully the sample retains an undisturbed sediment surface.

One of the problems with both grabs and box corers is the 'bow wave' generated as the apparatus nears the sea-bed. This can 'blow away' small surface-dwelling animals such as amphipods. The USNEL box corer has been designed to minimize this effect. Its operation is described in detail in Gage and Tyler (1991).

Another type of bottom sampler for quantitative work is the Knudsen Suction sampler. This is a short corer of wide bore with a suction pump in the upper part

Cable to ship

Weighted frame

Cables to rotate scoops

Usnel Box Corer

Figure 3.26 USNEL box corer. (a) Gear in ready position before reaching sea-bed; (b) on sea-bed, heavy corer enters sediment; (c) the spade swings down to seal the box as the slack wire is winched in; (d) gear and sample being hauled to surface. (From Gage and Tyler (1991), by courtesy of Cambridge University Press.)

Figure 3.26 USNEL box corer. (a) Gear in ready position before reaching sea-bed; (b) on sea-bed, heavy corer enters sediment; (c) the spade swings down to seal the box as the slack wire is winched in; (d) gear and sample being hauled to surface. (From Gage and Tyler (1991), by courtesy of Cambridge University Press.)

of the instrument. After reaching the bottom, tension on the hauling cable first turns the pump, thereby generating a suction inside the tube to assist penetration. When the corer breaks out of the substrate it automatically turns upside down to avoid loss of contents.

Rough quantitative comparisons can also be made using the anchor dredge (see page 75), which takes a fairly uniform bite.

Diver-operated samplers

In shallow water (less than about 25 m), diver-operated suction samplers provide accurate quantitative samples of soft-bottom benthos. All the designs utilize the lift generated by compressed air to suck up the sediment. They are most effective in sand and least in mud. Their use is described in detail in Hiscock (1987).

Early designs are based on the Barnett-Hardy suction corer. The diver positions the corer and presses it into the sediment (Figure 3.27). A compressed air line from the surface, or more usually a diving cylinder, then generates an air-lift in a suction pipe connected to the coring tube, whereby water is sucked out of the corer to

Marine Archaeology Diver
Figure 3.27 The Barnett-Hardy diver-operated suction corer.

force it down into the mud. When it has penetrated fully, the top of the coring tube is opened and the air-lift sucks the contents of the tube up into a sieve which separates the sample into a collecting bag. There are many variations on this theme, all utilizing compressed air to create a lift. Similar devices are also extensively used in underwater archaeology to remove sediment from around artifacts.

In soft sediments, small quantitative samples can be collected by divers using something as unsophisticated as plastic drain pipe. The volume sampled can be adjusted by using pipes of different diameter and sampling to a known depth. Use of a 'milk crate' to hold the tubes and rubber bungs to retain the samples allows for replicate samples and for sampling along a transect line.

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