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