Life in Lakes and Rivers. T. Macan T.
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.