“The whole of this oceanic system to the north and west of Scotland overlies a south tending mass of artic or boreal water. The main flow of this water mass is to the west of Faroe from whence it thrusts southwards in deep water (below about 1,000m.), but part also penetrates the Faroe Channel where it is checked by the Wyville Thomson Ridge.4 Although this water affects the inflowing system where it mixes at its interface it is not of such importance as are the more massive cold water currents on the other side of the Atlantic.
“Each of the above water masses has a typical plankton fauna (see Russell 1939, and earlier works), which varies within certain limits, in the abundance and in the proportions of its constituent species from year to year. As these organisms are transported further from their natural habitat they gradually die as their limit of tolerance is reached, and they are replaced by other species through mixing either with other oceanic streams or with coastal water. The fauna of an incoming water mass thus gradually changes along its length; for example, few of the oceanic species noted off Scotland normally reach north-western Norway (Wiborg 1954). The degree of survival of the original fauna gives a measure of the purity of the inflow, and the relative life of the species less tolerant to various factors may give an indication of the type of dilution or change involved.”
1 A valuable review of the changes in the southern North Sea, clue to variations in the influence of the low salinity water from the Baltic and that of the higher salinity water from the Channel, has been made by Lucas and Rae (1946).
2 This will be referred to again in chapter 15, see here.
3 The difference between the two is shown in Fig. 42.
CHAPTER 3 PLANTS OF THE PLANKTON
HAVING DISCUSSED the movement of the waters it might perhaps seem more logical to pass on at once to consider other physical characters of the sea and something of its chemistry before proceeding to deal with any of the life within it. On the other hand, since the plants of the open sea are so intimately dependent upon their physical and chemical background it will be more interesting if we know what kind of plants we are dealing with before we actually discuss the conditions which are most favourable for their growth.
The vegetation of the open sea must be floating freely in the water in order to be sufficiently near the surface to get enough light; the great difference between it and that of the coasts or land, is that it consists entirely of plants of microscopic size. They are, as we have already seen, part of the plankton: the phytoplankton. Each is composed of just one unit of life—a single cell—instead of being made up, as are the larger plants, of a vast number of such units. Instead of having various kinds of cells specialised to perform different functions in organs such as roots, stems, leaves and reproductive bodies, all these activities of life are carried out by just one highly organised unit. It does at first sight seem strange that there should not be even a few larger plants adapted for such a floating existence. There is the famous Gulf-weed Sargassum which, buoyed up by the little floats upon its fronds, is found in masses drifting round that great eddy of the tropical Atlantic—the Sargasso Sea; this however is not a true open-ocean plant for it has been broken away by wave action from the coasts of Central America and the West Indies. It continues to grow for a long time but never produces reproductive organs as it does when attached to its native rocks. In our own waters we may sometimes meet with patches of bladder-wrack, Fucus vesiculosus, torn from the sea-shore by storms and floating in the same way. They are but mutilated stragglers, out of place and lost in the open sea.
The microscopic plants must have some great advantage over larger plants in this floating drifting life. The smaller an object is the larger is its surface in relation to its volume. If we increase the size of an object—keeping its shape in the same form—the volume increases by the cube of linear measurement but the surface does so only by the square. This elementary fact is so important in the present discussion that it may be well to emphasise it by a simple concrete example. If we have eight small cubes of soap of the same size and press them together to form one big cube, the volume of this new cube will then be eight times that of one of the smaller ones, whereas its surface will be only four times as large. We have of course lost all the surfaces that were pressed and fused together. Inversely, the more we cut up our soap into smaller and smaller cubes, the more surface will each new cube have in proportion to its volume. A cube the size of one of our little plants will have a surface-volume ratio many hundreds of times as large as that of a cube with a side no more than an inch or two. A large surface-volume ratio is a great advantage to our little plants in at least two important respects. Firstly, the larger the surface in relation to mass the greater will be the frictional resistance to the water which will retard its sinking and so enable it to remain more easily in the upper sunlit layers. Secondly, since absorption must take place through the surface, the larger its surface in proportion to its volume the more readily will it be able to take up for its needs enough of the necessary mineral salts which may be present in the water in only very small amounts. This indeed may be the cardinal factor which has prohibited the development of larger plants in the plankton; but for this they might well have evolved bladder-like floats to support their larger mass, as some animals have done. Each tiny plant, as a single cell, can also take better advantage of the scattered sunlight than can a number of such cells massed together.
It is at first difficult to believe that these finely scattered and microscopic plants can really form a vegetation which has sufficient bulk to support all the teeming animal life of the sea: the dense populations of planktonic crustaceans, the vast shoals offish and all the invertebrate animals on the sea-bed. Yet we know this must be so. Some estimates of the actual quantities of plants present in a cubic metre of sea-water will be given later in the chapter; here, in passing, we will only note that, given suitable conditions, the amount of plant life produced under a given area of sea may well exceed that produced for the same area in a tropical forest. Just as our coal supplies are giving us the energy stored up in the great primaeval forests of some hundred million years ago—so is the energy in the petrol, which drives our motor and flying age, derived from that originally trapped from the sunlight by the tiny planktonic plants in the seas of long ago. According to current geological theory, the great supplies of mineral oil have been formed, in the course of ages, from the remains of marine organisms buried in sedementation under specially favourable conditions which are not yet fully understood. It is most likely that the planktonic crustaceans, whose modern representatives are so rich in oil, would in the past be the main contributors to the supplies of petrol we are burning up today; those crustaceans, of course, derived their energy either directly or indirectly from such tiny plants as we are now considering.
A microscope of sufficient power to enable us to see a great deal of this world of planktonic plants and animals need not be an elaborate one, nor need it cost much more than a good pair of field-glasses. We shall want some glass slides and coverslips, small dishes (such as watch-glasses), pipettes,