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APPARATUS FOR STUDYING LAKES

Anyone provided with a stout net, some bottles, and a white dish or sheet can do an immense amount of work in fresh water. He can wade as far as is necessary into many ponds and streams and collect in the shallow water of lakes. He can even collect the plankton from the open water of a lake, if a suitable point of vantage is to be found. However, more serious work on the open water and any kind of work on the fauna of the mud or of the submerged weeds requires more elaborate apparatus. The first necessity is a boat. If work is to be done in deep water a winch is desirable. A very useful type of light winch which can be put on to any row-boat is made by the firm of Friedinger of Lucerne. It has the advantage that the wire is paid out over a pulley block of special circumference to which is attached a cyclometer, so that the depth at which the instrument hangs below the surface is shown accurately to the operator in the boat. With such a winch the different instruments for measuring temperature or light intensity, and for collecting water samples or plankton, can be lowered easily to any depth. It is often necessary to operate an instrument at a considerable depth in the water before hauling it back to the surface, and this may be achieved by despatching a so-called messenger down the wire. The messenger is usually a lump of metal with a hole drilled through it; on reaching the instrument at the bottom of the wire, it strikes some projection which is arranged to release a catch in order to perform the necessary operation.

An example is provided by the reversing thermometer. This thermometer is mounted on a pivot about its middle, and the pivot has a spring which turns the thermometer upside down when the catch at the top of the frame is released by the messenger. This reversal breaks the mercury column, and so, when the thermometer comes to the top, it shows the temperature at the depth at which the messenger struck it; warmer water through which it may pass leaves it unaffected. The ordinary clinical thermometer works on somewhat the same principle.

The thermometer has to be specially built to resist the high pressure which obtains under water and so it is a comparatively large instrument, which will not immediately take up the temperature of the surrounding water. Accordingly it has to be left for a few minutes at each depth from which a reading is desired, and, since, further, it must be hauled to the surface to be read, the taking of a series of observations is a long process. It is still used on expeditions and long excursions, but for regular work it has been obsolete for some years. The popular device at present contains a substance whose electrical resistance changes considerably with a relatively small change of temperature. It requires, therefore, a battery and a galvanometer but, when these are available and transport presents no problems, the apparatus, known as a thermistor, is convenient. Much of Dr Mortimer’s work, described in the preceding chapter, was carried out from a boat, but latterly he had a series of thermistors slung at intervals between the bottom and a buoy moored in the deepest part of the lake. Each was connected to a recorder in the Ferry House, and what amounted to a continuous record was obtained. Dr Mortimer had nothing to do except convert the readings to °C. and work out what was happening. One of the authors was once explaining to a group of visitors what the recorder indicated and had just got to the point where emphasis is laid on the fact that the bottom of the lake is always cold when, by unfortunate coincidence, Mortimer, out on the lake, started to haul his line of thermistors up to the surface.

Apparatus of a somewhat similar kind is used for measuring the amount of light penetrating below the surface, which, we have seen, is so important in determining the depth of plant activity. A photoelectric cell, contained in a pressure casing, is connected to the surface by wires, and a window facing upwards is inserted into the pressure casing so that rays of light penetrating from the surface can strike the cell. They cause a small electric current, varying in amount according to the intensity of the light, and this can be measured in much the same way as with the temperature apparatus by a galvanometer in the boat.

When measuring sub-aqueous light, it is necessary to lower the instrument from a long support projecting sideways from the boat, because otherwise the boat would shade the instrument hanging beneath it. Not only the general intensity of light below the surface, but also the kind of light, is of great importance. This can be determined with the same instrument by covering the window with filters of various colours.

There is a much simpler but useful instrument for giving a rough idea of the clarity of water, known as Secchi’s disc after the scientist who first used it. This consists of a white plate of 20 cm. (8 in.) diameter, which is lowered below the surface to the point at which it becomes invisible to the naked eye. This is, of course, a crude way of measuring how far light can penetrate, but Secchi’s disc is very easy to carry about and use, and is accurate enough to provide comparisons between different types of water.

For most kinds of chemical work on water, and also for studying microscopic life, it is necessary to obtain samples of water from different depths. Here again the simple expedient is adopted of despatching a messenger down the wire to close a water-bottle at the desired depth. A variety of different kinds of water-samplers are used for this purpose. A simple example is a metal cylinder open at both ends so that when it is lowered it will pass through a column of water without disturbing it much. It is halted at the required depth and a messenger is sent down the wire. This releases lids which close over the top and bottom of the cylinder and are kept tightly in place by strong springs. The apparatus is now watertight, and can be hauled to the surface with a sample of water from the depth at which it was closed.

This self-closing metal water-bottle is an excellent instrument for many purposes, but for the study of bacteria, of which very many kinds inhabit fresh water, it is no use. The spores of bacteria are everywhere – in the air, in the water, on one’s fingers – and accordingly a water-sampler for bacteriological investigations has to be arranged so that every part of the instrument which comes in contact with the actual sample of water collected can be sterilized by heat and kept in a sterile condition until the sample enters it. The principle was therefore adopted of using glass sampling vessels of a simple and standard pattern, held in a metal framework fitted with the necessary gadgets to operate an opening and closing device. The bottle is sealed with a bung pierced by two tubes. One is long and runs down to the bottom of the flask, and the other ends flush with the inside of the bung and is bent into an S-shape outside. A U-shaped piece of glass rod fits into a length of rubber tubing attached to each tube. When the bottle is at the required depth, a messenger is sent down to release a strong spring which pulls the glass rod out of the two tubes. Water runs down into the flask through the long tube, driving air out of the other tube until the flask is completely filled. A bubble of air remains in the bent tube so that no mixture can take place between the sample in the flask and the surrounding water during haulage of the whole apparatus to the surface. In practice, a number of flasks, each with its stopper and tubes, are sterilized in the laboratory and then a series of samples for bacteriological examination can be taken at different depths or at different places during the same outing.

Water may be obtained from any depth by lowering a tube and sucking. In the early days of the Freshwater Biological Association, when lack of money placed a premium on ingenuity, Mortimer used a bicycle pump with the washer reversed to obtain samples. If two bottles are connected in series, with the larger nearer the pump, the smaller and the tube will have been sufficiently washed by the time the larger is full. Water can also be raised by a stream of air bubbles emitted from a small tube inside, and extending almost to the lower end of, a larger one.

Plankton is commonly caught by means of a conical net made of material woven in such a way that the holes retain their size. A mesh of 60 meshes to the inch is generally used for animals, one of 180 meshes to the inch for algae. The efficiency of a net falls as the catch blocks the pores and for quantitative work the amount of water that has passed through the mouth must be measured by means of a propeller attached to a recorder. Many methods of catching plankton have been tried, particularly at the station at Pallanza, and the quest continues. One difficulty is that some of the animals swim away from an object they see coming through the water, or away from the pull of a current caused by suction into a pipe.

The easiest medium to sample is the mud on the bottom of a lake, though each sample must be subjected to a tedious process of sieving before the animals can be isolated. Often a simple tube will secure enough animals. If they are scarce, a larger sample may be obtained with a Birge-Ekman grab, which is a metal box open at the bottom and provided with two hinged lids at the top. Two jaws to close the bottom are held along the sides against the pull of strong springs. Going down, the apparatus passes through the water with little disturbance. This is important, for if there is obstruction the apparatus will not pass through the water, but push it aside, and it will also push aside the top layers of the mud if these are fine and fluid. The lids fall when the box sinks into the mud and comes to rest. A messenger trips the bridle that holds the jaws up and the springs then pull them together to close the bottom of the box.

Stones and vegetation are less easy to sample quantitatively. Several workers have found that the number of animals caught in a given time or in a given number of sweeps of a net indicates, sometimes with unexpected accuracy, relative numbers in different places. Numbers per unit area can be calculated if samples with a quantitative sampler are taken in the same part of the lake at the same time. The Danish workers have used a square box open top and bottom to sample stony substrata near lake margins. It is placed over the bottom, stones are removed, and the water inside is baled out and poured through a net. This method cannot be used in running water because the box deflects the current downwards and causes it to scour the area that is to be sampled.

In Windermere H. P. Moon used a square frame on which he could pile stones to represent an area of natural substratum before lowering it onto the bed of the lake and leaving it until it had been colonized. The frame is one-third or one-half of a square metre in area and underneath it is covered with fine gauze to prevent the loss of animals while the frame is being hauled up. Stout wire-netting beneath the gauze adds additional support for the stones.

The Surber sampler is used by some workers to sample the stony substratum of streams and rivers. It consists of two frames, generally about one tenth of a square metre in area. These fold into one plane for transport and open at right angles for use. The horizontal one is placed on the bottom and the vertical one supports a net. Stones are then removed from the bottom inside the horizontal frame, and brushed in the mouth of the net to dislodge animals clinging to them, after which the remaining small stones, gravel and debris are stirred with a stick until it is believed that all living material has been swept into the net. We have not found this a satisfactory instrument because the current is often so swift that when one stone is picked up the stones above it shift to fill the gap. If the current is slow many good swimmers probably swim out of the net, if they are ever carried into it. More satisfactory, though not by any means free of error, is a shovel of some kind which can be pushed into the substratum for a known distance. Designs have ranged from a shovel with high sides with a net at the back, to a cutting edge connected to the handle by two strips which also support the frame of the net. A strong coarse net arrests the stones and a long tapering fine one any animals that have let go. If the stones are tipped into a solution of high specific gravity, calcium chloride or magnesium sulphate are suitable, the animals float to the top.

Weeds in rivers trail downstream and may be severed with shears and caught in a large bag. Another method is to hold a box with sharp edges a known distance above a lid and then bring the two together enclosing and severing the weed in a known volume. Weeds in still water rise vertically, and a device that cuts each leaf or stem as it meets it is preferable to one that pushes them downwards and does not cut them until they are pressed against the bottom. One such instrument consists of two tubes, about 8 cm. across, fitting one within the other. A boss on the inner passes through a slit in the outer and holds it in position, allowing a small amount of rotation to and fro. As the tubes are lowered into a weed-bed, the outer tube is rotated and the vegetation is severed between the sharp teeth which have been cut in the lower end of both tubes. They pass across each other like the teeth of a haycutter.

Incidentally parallel samples with this instrument and a net have shown the latter to be unexpectedly selective. It collects an unduly high proportion of species that tend to flee and an unduly low proportion of those which, like leeches, tend to cling to the substratum.

Larvae of Chironomids and many Trichoptera cannot yet be named, and in order to find out what species are present it is necessary to trap the emerging adults. In still water a box open at the bottom may be floated in a frame. The top should be of some transparent plastic material to keep the rain out, but at least one side should be of gauze to prevent condensation. Dr. J. H. Mundie has devised various modifications for use in both still and running water. In a lake he used conical traps into the top of which a screw-top jar could be screwed. Entrance to it is through a cone which prevents the animals falling back into the water. The whole apparatus can be submerged, an advantage in a lake to which the general public has access. For use in streams he built a heavy trap that could be anchored to the bottom. Triangular in both plan and elevation it offered minimum resistance to the current, which tended to press it downwards. Three legs kept it raised off the bottom and the catch entered a screw-top jar as in the other model.

Much decomposition takes place in the top few centimetres of the mud, and substances diffuse from it into the water. A study of these processes, important to the general economy of the lake, requires a sample disturbed as little as possible. The Birge-Ekman grab does not bring up such a sample and for this purpose the Jenkin surface-mud-sampler was invented. It consists of a large glass tube about 6 cm. in diameter, held by a band about its middle on to a metal frame, standing on four spreading legs. The sampler sinks into the soft mud when lowered to the bottom, but without disturbing it, and then, a messenger sent down the wire having released a catch, two pairs of arms travel forward to place a cap on either end of the glass tube. The speed at which these caps move into position has to be very slow in order not to disturb the mud and water in the tube, and this is effected by means of a pressure chamber of the same kind as that used for preventing doors from slamming. When closed, the glass tube contains a sample of the top few inches of deposit together with the water above, and the caps at either end are held tightly in place by springs. At this point the whole apparatus is hauled to the surface gently to avoid disturbance. The glass tube is detached from the frame, and the sample of bottom deposit, complete with the water immediately above it, just as it was at the bottom of the lake, can be carried into a laboratory for chemical and other tests. The person who devised this most useful apparatus was a retired engineer, Mr B. M. Jenkin, and it is worthy of note that the first experimental model, which he made largely out of meccano, operated so well that it was still in frequent use at the laboratories of the Freshwater Biological Association at Windermere ten years later.

Mr Jenkin was set another and much more complicated problem, namely to devise an instrument capable of extracting cores from the bottom deposits in lakes, if possible to a depth of twenty or thirty feet below the mud surface. A good deal of trial showed that an ordinary open tube or pipe was useless for this purpose because it compresses and disturbs the layers of deposit too much. After some thought, Mr Jenkin hit upon the idea of a sampler which could be thrust into the deposit first and then made to carve out a core by means of a curved cutting blade working on a long pivot. The business end of the instrument, which cuts out the core, is about four feet long, and consists of a tube cut in half lengthways and covered with a metal plate except for a slit down one side. A second half-tube lies within the first, attached on an axis in such a way that it can be rotated out through the slit. When the apparatus has been driven to the required depth, the inner half-tube is rotated, its sharp leading edge passes out of the slit, and, travelling through 180°, comes up against the far side of the plate. Between the inner half-tube and the plate there is now a sample of mud isolated from its surroundings with the minimum of disturbance. The rotation of the inner half-tube is effected by a system of cogs and a driving-wheel worked by a wire from the surface. The cutter can be attached to a series of tubes so that it can be driven to the desired depth in the mud before it is operated. The force required to press the whole instrument into the bottom is provided by a series of heavy lead weights, of which the number is adjusted according to the depth at which the particular sample is required. Thus a complete core, say twenty feet in length, is obtained in a series of overlapping cores each four feet in length.

The method of using this instrument is briefly as follows. First a pontoon with a derrick is firmly anchored over the spot from which the core is to be taken. Next a flat weight on a thin wire is lowered to the bottom to serve as a guide and as an exact measure of the depth to which the main instrument is subsequently lowered. Then the coring machine itself is lowered from the derrick on a stout wire with a pair of arms clutching the aforementioned guiding wire. The machine is allowed to sink into the deposit to the required depth, when a sample is required from near the surface; for a deep core the machine is allowed to sink as far as it will go and driven the rest of the way. Then a messenger is despatched down the guiding wire in order to release the arms, and the guiding wire is hauled up, an action which also operates the machinery for cutting out the core. It remains for the whole machine to be hauled to the surface and laid flat before the half revolution of the cutting blade is reversed and the core is exposed ready for transfer to the laboratory. It will be appreciated that the successful handling of this apparatus is no mean task; in fact it requires a team of three or four operators well trained in the particular functions which each has to perform at the right moment. With its aid, however, a large number of cores, some of them covering twenty-one vertical feet of deposit, have been collected from many parts of Windermere, and these have provided valuable information about the history of lakes since the Ice Age.

Fig. 5 Diagrammatic cross-section of the Mackereth core-sampler (not to scale). The letters are referred to in the text. (Limnol. Oceanogr. 1958)

Mr Jenkins’ apparatus proved excellent for use on Windermere, where the necessary pontoons could be borrowed, but not elsewhere, and accordingly Mr F. J. H. Mackereth devised a portable model (Fig. 5). The problem was to ensure stability while the core was being obtained. This he solved by basing the corer on a large cylinder resembling a dustbin (G). This sinks some way into the mud when the apparatus has been lowered, and is then forced farther in by means of a pump (P) which removes the water from its upper portion. A secure base has now been secured for the rest of the operation. The core is obtained in a long tube (B) housed inside a second tube (A), which is attached to the centre of the top of the anchoring cylinder. The second problem was how to drive the corer into the mud from a small boat that could not easily be kept exactly above the apparatus. Compressed air passing down a flexible tube (O) was the solution. It involved a piston fitting inside the outer tube and closing the top of the inner one (C). Some means of evacuating the inner tube as it moved downwards was essential, for otherwise a solid cylinder rather than a tube was being forced into the mud. This is achieved by a fine central tube (D) which holds in position a piston (F) at the mouth of the inner tube when this is retracted, passes through the upper piston and out through the top of the outer tube (L) to which it is attached. When compressed air admitted to the top of the outer tube forces the inner tube down into the mud, the air in the inner tube escapes through the fine central tube and the corer passes into the mud, causing no more compression than is due to the friction of the walls. When the inner tube is nearly fully extended, the compressed air escapes into a side tube (I), which leads it into the anchoring cylinder. This is forced out of the mud and brings the whole apparatus to the surface. Compressed air passed into the fine inner tube brings the inner tube back into the outer and, at the same time, ejects the core. This apparatus has been carried to tarns in the mountains by a helicopter and successfully used there.