The Open Sea: The World of Plankton. Alister Hardy

or brought in by drainage from the land. If we compare measurements of light made at different depths below the surface in the waters near Plymouth we find the penetration in inshore waters at Cawsand Bay to be only half what it is at a point some 10 miles S.W. of the Eddystone Lighthouse (Poole and Atkins, 1926). Taking the light entering the sea, i.e. just below the surface, as our standard, we find in Cawsand Bay that half of it has been absorbed at 2 metres depth (i.e. 1 fathom), some 75% at 4½ metres and 90% at about 8 metres depth; whereas at 10 miles out the same percentage reductions in light intensity are found at depths of about 4¼, 9½ and 17 metres respectively. At points in the open sub-tropical or tropical Atlantic, where the phytoplankton is very sparse, as in the Sargasso Sea, the corresponding depths might be increased four or five times. A very simple bit of apparatus known as the Secchi disc, which can easily be home-made, will enable you to compare the transparency of the sea at different points; you may well be surprised at some of the results you will get with it. Take a white painted metal disc, say two feet in diameter, and drill three equidistant holes near its margin; now take three cords each about 6 feet long, tie them to a weight, say a 7–1b lead, and then tie a knot in each at 3 feet from the lead; next pass the cords through the holes in the disc and bring them together as supporting bridles to be tied to a loop or eye at the end of the line which will suspend the whole device in the water as shown in Fig. 16. If a metal disc cannot easily be obtained, a white dinner plate, with wire clips behind it to take the cords will serve the purpose quite well. To compare the transparency of the sea all you have to do is to stop your boat and lower the disc over the side at different places and find how deep it must go before you can no longer see it. The weight not only carries the disc down but, if the cords are properly adjusted, ensures that it is always kept horizontal. The line can be knotted at metre intervals to facilitate measuring the depth. It is well to raise and lower it about the disappearance point several times in order to make quite sure just at what depth it goes out of sight; at some places it may vanish in only 5 metres, at others it may be seen for as much as 12 metres or more.

      FIG. 16

      The Secchi disc for measuring the transparency of the sea.

      In passing I may say that the Secchi disc is also valuable in helping us to compare the varying colour of the water. Here, of course, I am not thinking of the often striking and delightful changes of hue which we may see as light of different quality is reflected from a changing sky: when for example dark grey clouds give place to an open space of blue or when cumulus clouds dapple the sea with purple shadows. Reference was made in Chapter 2 to the contrast between the green water of the Channel and the deep blue of the Atlantic; such differences are due to the nature of the contents of the sea itself and are examples of what I mean by the varying colour of the water. A great variety of shades and hues may be found at different times; these are mainly due to the presence of different kinds of very small plankton organisms in exceptional numbers. The light reflected back through the water from the white background of the disc enables us to judge and compare these differences more easily than by just looking into the depths. Dense concentrations of diatoms such as Rhizosolenia and Biddulphia, or of the colonial flagellate Phaeocystis, may give a brown appearance to the water over large areas; the North Sea fishermen often call such patches of water “Dutchman’s baccy juice”. Some dinoflagellates may make the water almost red, coccolithophores may give it the white milky appearance referred to in the last chapter, and other small flagellates may occasionally make it a vivid green.

      But let us return to the sunlight and these little plants of the sea; in order to flourish and grow they must produce more oxygen in the process of photosynthesis (see here) than they use up in respiration. Plants, of course, breathe as well as animals. Some very significant experiments were performed in the Clyde sea-area by Drs. Marshall and Orr (1928) of the marine biological station at Millport on the Island of Cumbrae. They grew cultures of diatoms in glass bottles in the sea at different depths; these they suspended on strings from a long thin rod between two buoys at the surface and so kept them free of shadow. All their bottles were in pairs; one of each pair was exposed to the light and the other covered with a black cloth. In each bottle the oxygen-content of the water was measured at the beginning of the experiment and again at the end of twenty-four hours. An increase in the oxygen in the uncovered bottles showed the amount produced by photosynthesis less that used up in respiration; a fall in oxygen-content in the ‘blacked-out’ bottles measured respiration alone. By adding this oxygen-loss to the oxygen measured in the uncovered bottles the total oxygen-production as a record of photosynthesis could be estimated. The experiments were repeated as the spring passed into summer, and were also made on days which were overcast and on others which were sunny. As the sun went higher in the sky and the light became more intense the depth at which diatoms could produce more oxygen than they used in respiration increased from a depth of less than 10 metres on an overcast day in March to nearly 30 metres on a sunny day at midsummer. By far the greatest photosynthetic activity—on which their growth depends—took place, however, in the top 5 metres. In the waters round Great Britain we may now say that practically all the plant-production that matters takes place in the top 10 or 15 metres. This is one important clue in the puzzle of the seasons; we must now turn to temperature.

      FIG. 17

      The Nansen-Pettersen water sampling bottle: shown open and closed.

The water round our coasts varies in temperature from about 8°C in winter to sometimes as much as 17°C in the Channel in a warm summer. It is, of course, because the sea loses and gains heat so much more slowly than the land that we in Britain have so equitable a climate compared to that of an area in the middle of a continent. Two methods are used in taking the temperature of the sea. Down to moderate depths, say to 50 metres, the insulated Nansen-Petterson water-bottle, which is shown in Fig. 17 is used; it is of metal and is sent down suspended on a wire to obtain samples of water both for chemical analysis and for temperature determination. It goes down with the bottom and top open so that water can circulate through it; then at the required depth a small ‘messenger’ weight is sent sliding down the wire to hit a trigger which releases springs to close it. Projecting through the top, in a protective casing, is the stem of a thermometer whose bulb is in the centre of the sampling bottle; its scale and mercury thread are visible through a slit in the upper casing so that it can be read as soon as the bottle is brought back to the surface. There are actually three walls to the cylindrical bottle, one inside the other, with a little space between; when the top and bottom are firmly closed there are thus two water jackets outside the bottle proper and these act as insulating chambers preventing loss or gain of heat in the water sample while it is coming up and the thermometer is being read. As soon as the temperature has been noted the water is run out from a cock at the bottom to be stored for later analysis and the bottle is opened ready to be sent down to another level. From much greater depths the bottle would take so long being drawn up that the insulation just described would not be adequate to prevent a change of temperature in the process. To get over this, special so-called reversing thermometers and bottles have been devised. The mercury tube of the thermometer, just above the bulb, has a loop and a kink in it, so that when it is swung rapidly upside down the thread of mercury breaks; as soon as this happens all the mercury that before was above the kink now runs to the opposite, and now lower, end of the tube. When it is brought up the height of this inverted column of mercury is seen against a scale which can only be read when the thermometer is upside down; it tells us the temperature that the thermometer was recording at the moment it was turned over. The bottle and thermometers (there are usually two to give check readings) are mounted in a frame which rotates when a trigger is hit by a messenger weight; the bottle, which before was open, is closed as it swings over.¹

      After this digression on thermometers, let us return to consider the temperatures of our seas