Life in Lakes and Rivers. T. Macan T.

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       CHAPTER 3

      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 СКАЧАТЬ