Alister Hardy - The Open Sea - The World of Plankton

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. 17is 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 with the passing of the seasons. Water, above 4°C, expands when warmed and contracts when cooled; so its density is altered: a given volume of cold water weighing more than the same volume of warm water. In winter the atmosphere is colder than the sea so that the surface waters are cooled and therefore sink beneath the warmer and less dense layers which were below; this is repeated again and again until after a time there is an almost uniform low temperature from top to bottom. The winter gales help in the process of mixing up the layers too. The sea, of course, is rarely so cold in winter or so warm in summer as is the atmosphere; as we have already noted, it gains and loses heat much more slowly. As spring passes into summer the air warms up and the radiant heat of the sun gets stronger, so we find the upper layers of the sea becoming warmer too; as they heat up they become increasingly lighter than the layers below and thus tend more and more to remain separated on the top because less and less are they likely to be mixed with the heavier waters beneath. This division between the upper and lower waters is called a discontinuity layer (or thermocline in still more technical language) and is usually set up at a depth of round about 15 metres. Let us take an actual example from the summer temperatures in the English Channel in July as found by the hydrologists of the Plymouth Laboratory. At depths from just below the surface down to 15 metres the temperature only varied from 16.5° to 15.82°C; but at 17½ metres it had dropped to 12.09°C and then, as it was sampled deeper and deeper, it remained practically constant to read 12.03°C at 60 metres. The upper layer was effectively cut off from the lower by this sudden drop in temperature of nearly 4°. A strong summer gale may destroy this discontinuity layer, but if it is not too late in the season it will soon form again. It is in the autumn that the air cools again and so the surface water loses heat; also the equinoctial gales stir up the sea and the more uniform temperatures of winter again become established from top to bottom. It will be noted that this warm summer upper layer corresponds very closely to the region (sometimes called the photic zone) in which the little plants get sufficient light to carry out effective photosynthesis. Two bits of the puzzle seem as if they would fit together; we require, however, yet another piece to go with them before we can see the explanation of the seasonal changes in the plankton. This last link concerns certain salts in the sea, and to them we must now turn.

First we must consider the general saltness of the sea; this, of course, is mainly due to the abundant sodium chloride which accounts for almost 77.8% of the total salt content. However there are many other salt constituents, of which the next more important, in order of descending quantity, are magnesium chloride (10.9%, magnesium sulphate (4.7%), calcium sulphate (3.6%, potassium sulphate (2.5%), calcium carbonate (0.3%) and magnesium bromide (0.2%). These proportions are actually those in which these different salts would be recovered from the sea on evaporation; their molecules as dissolved in the sea, however, would largely—some nine out of ten—be split up into their respective parts or ions: sodium and chlorine or magnesium and sulphate ions as the case may be. It is better to think of the salt constituents of sea water, as they mostly are in the sea itself, in terms of separate ions. We can tabulate the percentage proportions as follows, based upon a mean of 77 samples collected from different localities by the Challenger Expedition: