Plankton

Plankton nets

Detailed methodologies for plankton sampling are given in a variety of texts such as UNESCO (1968) and Omori and Ikeda (1984). Samples of plankton are usually collected by plankton nets. These are of many designs, but all consist essentially

Tow-rope

Nets Capture Plankton

Receiver

Figure 3.11 A simple plankton net.

Tow-rope

Canvas and tying cord

Receiver

Figure 3.11 A simple plankton net.

of a long cone of fine-mesh net. The mouth of the net is usually some 50-100 cm in diameter, and is held open by a strong hoop to which the tow-rope is attached by three bridles (Figure 3.11). The narrow end of the net is firmly tied to a small metal or plastic vessel in which much of the filtered material collects. After hauling, the net is washed into a suitable receiver to collect any material left on the mesh.

To filter efficiently, plankton nets must be towed quite slowly, not faster than about 1-1.5 knots. Fine mesh presents high resistance to the flow of water through it, and if towed too fast, the net sets up so much turbulence in the water that floating objects are deflected away from the mouth. In many designs of plankton net the aperture is reduced by a tapering canvas sleeve as in the Hensen net (Figure 3.12) which cuts down the volume of water entering the net to give more effective filtering.

In the past, silk gauze was used for plankton nets. Modern nets are mostly made from nylon gauze. These can be constructed in a variety of ways but the best material is provided by monofilament heat-set plain weave (Omori and Ikeda, 1984). This has even meshes that do not deform under the strain of towing. The gauze comes in a variety of mesh sizes or grades identified by a rather confusing array of numbering systems. For example, nets with a mesh size of 330 ¡m are commonly used to capture meso- and macroplankton. This is grade 'bolting silk GG54' or 'nylon NGG52'.

Coarse mesh is more effective than fine mesh for catching the larger planktonts because it offers less resistance and allows a faster flow of water through the net and does not clog so easily. Omori and Ikeda (1984) recommend using a mesh size of about 75 per cent of the width of the smallest organism to be sampled, when towing at normal speeds (0.7-1.0 m sec"1). For collecting organisms over a wide range of sizes several grades of net are often used together.

Towing Net Samplers

For collecting macroplankton such as euphasid shrimps, larger nets known as young fish trawls (YFTs) (Southward, 1970), are sometimes used, having a mesh of ca. 1 mm and an aperture of 1-2 m diameter. For collecting pleuston and neuston (see Section 2.1), a plankton net may be attached to a floating frame so as to skim the surface as it is towed (David, 1965).

Closing and opening-closing nets

Plankton nets can be towed behind a slowly moving vessel, or lowered from a stationary vessel and hauled vertically. When studying the vertical distribution and migration of plankton, it is necessary to have samples of plankton from particular levels. To avoid contamination of the samples by organisms entering the net while it is being lowered or raised, there must be some method of opening and closing the net at the required depth. The simplest method of closure is to encircle the mouth of the net with a noose which can be drawn tight, as in the Nansen closing net (Figure 3.13). This net is lowered vertically to the bottom of the zone to be sampled, not filtering on descent, and is then drawn up through

Nansen Net Plankton
Figure 3.13 The Nansen closing net.

the sampling depths. A messenger sliding down the tow-rope then releases the bridles from the tow-rope, causing the throttle to draw tight and close the mouth of the net, which can then be hauled to the surface without further filtering.

Nets that can be both opened and closed under water are used mostly for horizontal and oblique tows. The net is kept closed whilst being lowered to the required depth, is then opened for the tow, and closed immediately afterwards. There are many different net types of varying efficiency. They can be opened and closed by messengers, electric, sonic, time or pressure releasing mechanisms.

The Leavitt system involves two messenger-operated throttles (Figure 3.14).

Longhurst Hardy Plankton Net

Figure 3.14 An opening and closing plankton net operated by throttles. (a) The net is lowered with first throttle closed. (b) The first messenger frees the first throttle and the net opens. (c) Second messenger releases bridles and the strain is taken by second throttle, closing the net before hauling.

Figure 3.14 An opening and closing plankton net operated by throttles. (a) The net is lowered with first throttle closed. (b) The first messenger frees the first throttle and the net opens. (c) Second messenger releases bridles and the strain is taken by second throttle, closing the net before hauling.

The net is lowered with the mouth closed by a noose. The first messenger releases this noose and the mouth of the net opens. After towing, a second messenger releases the bridles and another noose closes the mouth before the net is hauled up. The Clarke-Bumpus net has messenger-operated valves which open and close the mouth. Large systems have several nets that can be used simultaneously at different depths.

Many planktonic organisms are very sensitive to temperature, and to keep a sample alive for any length of time it must be kept at an even temperature as close as possible to that of the water from which it was filtered. Vacuum flasks provide a means of doing this. For most purposes, however, preserved samples are needed. The addition to the sample of sufficient neutral formalin to produce a 4-5 per cent concentration will preserve the majority of planktonts satisfactorily.

Quantitative plankton studies

The aim of quantitative plankton studies is usually to estimate numbers or weights of organisms beneath unit area of sea surface or in unit volume of water. There are many difficulties in these studies. For instance, plankton is often very patchy in distribution, and it is difficult to obtain any clear picture of the amount and variety of plankton unless samples are taken at numerous stations spread over the area of investigation. Modern navigational aids enhance the precision of the sampling grid. There is also the difficulty of knowing how much of the plankton is actually retained in a plankton net, for some may be displaced from the path of the net by turbulence, small organisms may escape through the meshes and the larger active forms may avoid capture by swimming. A further difficulty is to know the volume of water filtered. If a net could filter all the water in its path, the volume passing through it would be irr2d, r being the radius of net aperture and d the distance of the tow, but in practice this formula can only give an approximate measurement. A net does not filter all the water in its path and the filtering rate reduces as material collects on the mesh and the resistance of the net increases. There is a further difficulty in knowing precisely the distance a net has moved during towing.

To measure the filtered volume more accurately, a flowmeter can be added to a plankton net. A flowmeter has a multi-bladed propeller which is rotated by the flow of water, and a simple counter records the number of revolutions. This can be placed in the aperture of the net to measure the volume of water entering, for example Currie-Foxton and Clarke-Bumpus nets; or the net may be surrounded by an open-ended cylinder and the flowmeter placed behind the net to measure the volume filtering through, for example the Gulf III Sampler (Figure 3.15).

Another method of collecting plankton is the plankton pump. This draws water up a hose and pumps it through nets or filters to trap the plankton. Simple pumps sampling shallow depths, can be operated from small boats. Large ships can operate submersible electric pumps capable of raising many litres of water a minute from depths down to about 100 m.

Tow line

Stabilizing

Tow line

Stabilizing

Gulf Iii Phytoplankton Net

Flowmeter

Figure 3.15 A high-speed plankton sampler of the Gulf III type.

Depressor weight

Opening rear section

Flowmeter

Figure 3.15 A high-speed plankton sampler of the Gulf III type.

With this method it is possible to measure quite accurately the volume of water filtered and the depth and to sample the actual water from which the plankton is filtered. After filtering, the water can be centrifuged or otherwise sampled for the smallest organisms that escape through nets. Despite these advantages the method has some drawbacks. Large creatures are prone to damage as they pass through the pump, the stronger-swimming planktonts may escape being sucked into the hose and there are difficulties in the use of pumps to obtain samples from deep levels.

Nanoplankton and ultraplankton

The smallest planktonts such as bacteria and microflagellates escape through the meshes of ordinary nets. Materials of finer mesh are now becoming available which retain much smaller organisms than hitherto, but if too fine, the water will not pass easily through them. So the sampling of nanoplankton and ultraplankton is usually done by collecting samples of seawater in sterile bottles or by pumping (see Section 3.1.2) and then concentrating the organisms by allowing them to settle, by centrifuging or by fine filtration.

Plankton counting

Plankton counters can be constructed for towing at sea and counting directly the number of small organisms present in the water which flows through the instrument. The apparatus is essentially a tube containing electrodes connected to circuitry which records the change of impedance when objects pass between the electrodes. This enables an estimate to be made of both number and size of organisms.

For most purposes quantitative investigations are done on plankton samples which have been filtered from a known volume of water, the method depending on the type of study. Sometimes an estimate is wanted of the gross quantity of plankton of all types. A rough volume estimate can be made very simply by allowing the sample to settle in a measuring cylinder and reading the volume directly from the scale. Measurements of displacement volume are probably rather more accurate, and estimates can also be made by weighing, either as a rough wet weight, or, better, by drying to constant weight.

The most detailed investigations are by direct counting. Large organisms are usually few in number and can be individually picked out and counted. Smaller organisms may be so numerous that the sample must be sub-sampled to reduce them to a number that it is practicable to count. The sub-sample can be spread out in a flat glass dish and examined, with a microscope if necessary, against a squared background. The count is then made, a square at a time. For very small organisms present in large numbers, the haemocytometer used by physiologists for counting blood cells can be used for plankton counting, or a Coulter particle counter can be adapted for this purpose. Other methods applicable to phytoplankton are mentioned on page 175.

Underway and continuous plankton samplers

We have previously pointed out that ordinary plankton nets must be towed slowly to be effective. They can therefore be used only from vessels operating for scientific purposes. Plankton samplers that can be used at higher speeds have some advantages; for example, they interfere little with the normal cruising of the ship and can therefore be towed behind commercial vessels proceeding on their normal routes, extending the scope of plankton studies. Also, they are probably more effective than slow-moving nets in capturing the more actively swimming creatures.

For high-speed sampling the net area must be very large in relation to the aperture to reduce the high back pressure developed when a fine-mesh net is towed rapidly through water. A simple sampler of this type is the Hardy Plankton Indicator consisting of a torpedo-shaped cylinder with stabilizing fins. Water enters a small opening at the front, filters through a disc or cone of bolting cloth and leaves through a rear aperture. A rather larger apparatus, the Gulf III Sampler (Southward, 1962) illustrated in Figure 3.15, incorporates a flowmeter. The Jet Net is similar in appearance and designed to reduce the speed of water flow through the mesh, thus reducing damage to organisms.

The Hardy Continuous Plankton Recorder (Figure 3.16) is a high-speed sampler which provides a continuous record of the plankton collected over a long-distance haul. The recorders are towed at a depth of 10 metres on a standard length of wire rope and are deployed at monthly intervals as far as possible. Modifications can be made so that the angle of the depressor plate alters during towing, causing the apparatus to take an undulating course and sample over a range of depths. Electronic sensors for depth, salinity and temperature are also often fitted.

Water enters through a small aperture (1.25 cm) in the nose-cone. Once inside, the water is slowed to about a thirtieth of its original speed by the enlargement of the tunnel. It is then filtered through a slowly moving band of filtering gauze. Organisms trapped on the cloth are covered by a second band of gauze to form a 'sandwich' which is wound onto a take-up spool in a formalin reservoir where the plankton is preserved undisturbed.

At the end of a voyage, the instrument is sent to the headquarters of the CPR survey (Sir Alister Hardy Foundation for Ocean Science) in Plymouth, UK. The spool of gauze is unwound and each 10 cm length, representing 10 nautical miles of tow, is subjected to a standard routine analysis. The plankton is identified and counted and the data fed into a computer. By comparing the sample number with the ship's log, a general picture of the distribution of plankton can be built up.

Tow line

Take-up spool

Scientific Diagram Wire Gauze

Depressor plate Moving strip of gauze (B1)

filtering out plankton

Figure 3.16 Diagrammatic section of a Hardy Continuous Plankton Recorder.

Tow line

Take-up spool

Propeller turned by flow of water, driving internal rollers

Depressor plate Moving strip of gauze (B1)

filtering out plankton

Figure 3.16 Diagrammatic section of a Hardy Continuous Plankton Recorder.

These instruments have been in regular use for over 60 years, towed by commercial vessels on many routes around the British Isles. Between 1931 and 1990, recorders have been towed for 3 733 746 nautical miles by merchant ships and ocean weather ships operating in the North Atlantic and North Sea (Colebrook et al., 1991). The survey is on-going and continues to provide a means of mapping plankton populations and pin-pointing changes. This is of particular interest in terms of possible changes resulting from global warming. The results of these investigations are published at intervals in the Bulletin of Marine Ecology. The design of the recorder has not been changed in any way that would affect the sampling characteristics.

In the Longhurst-Hardy Plankton Recorder the principle of collecting the catch on a long strip of mesh is applied to a unit attached to the apex of a conventional plankton net in place of the usual receiver. Instead of continuously winding on, the filtering strip moves intermittently at 30 s intervals. It therefore carries a series of catches each representing filtering for a 30 s period. This mechanism is electrically driven, powered by batteries carried within the apparatus. It also contains a flowmeter and sensors for salinity and depth, these data being recorded on a miniature chart-recorder contained in the unit.

Diver collection and observation

Gelatinous plankton such as scyphozoans and thaliaceans are best captured by divers using wide-mouthed containers. Divers have also contributed much useful information on the behaviour and mode of life of these animals (Hamner, 1974; Hamner, 1975; Hamner et al., 1975). Plankton for use in the laboratory study of living animals can also be collected using special buckets instead of the normal cod-end collectors on plankton nets. The buckets are generally much larger than normal and have mesh windows in the top part.

Was this article helpful?

+1 0
Enneagram Essentials

Enneagram Essentials

Tap into your inner power today. Discover The Untold Secrets Used By Experts To Tap Into The Power Of Your Inner Personality Help You Unleash Your Full Potential. Finally You Can Fully Equip Yourself With These “Must Have” Personality Finding Tools For Creating Your Ideal Lifestyle.

Get My Free Ebook


Responses

  • segan yemane
    Who developed plankton tow nets?
    8 years ago
  • bisirat
    When was the phytoplankton net?
    8 years ago
  • Pupa
    What is hardy continuous plankton recorder?
    8 years ago
  • russom
    Why must a plankton net be towed slowly?
    8 years ago
  • lorenzo bruce
    What is an ideal plankton net?
    2 years ago
  • sarah
    Why is silk gauze used for plankton nets?
    2 years ago

Post a comment