The Open Sea: The World of Plankton

CHAPTER 2 THE MOVEMENT OF THE WATERS

ONE OF THE MOST important features in the world of a marine animal is the movement of the sea itself. Apart from the wave-action near the surface and the to-and-fro tidal streams in shallow coastal areas, the whole water-mass is in continual flow as part of a greater system of oceanic circulation. Carried with the moving waters go the floating animals and plants. The main surface currents in the North Atlantic are shown in Fig. 2.

In some places the sea may be richer in plankton and in others poorer—like contrasting regions of luxuriance and barrenness on the land; unlike them, however, these areas in the sea are day by day changing their positions in relation to the coasts and the sea-bed. Not only is the basic food-supply on the move, but the delicate and helpless young of many fish and bottom-living animals are carried in one direction or another. Clearly these water movements are profound in their effect. To understand the lives of the inhabitants of our seas we must first have a knowledge of the main average pattern of the ocean currents flowing round our islands. They are by no means fixed in an unalterable course: variations are continually occurring. Sometimes these changes have striking effects; occasionally some exotic creatures from warmer seas, such as the beautiful Portuguese Man-of-war (Physalia) with its iridescent float and long trailing tentacles, or the smaller blue Velella riding the surface with a little ‘sail’, is driven ashore upon the coasts of Devon and Cornwall. (see here and Plate 5). Normally we find typical Atlantic water outside the western entrance to the English Channel, but at times there may be a marked incursion of water from the Bay of Biscay carrying with it these and other planktonic visitors from further south. It is probable that long periods of strong south-westerly winds may contribute to this movement.

FIG. 2

The surface currents of the North Atlantic Ocean. Drawn, with kind permission, from Admiralty Chart No. 5310 (1949), omitting some of the detail.

The general systems of the surface currents of the world have been worked out largely from the innumerable observations of navigators. Suppose a ship steers from point A on a particular course by compass which should carry it to a point B in, say, a day’s time; if at the end of the 24 hours the navigator finds by reference to the sun or stars that he has instead reached a point five miles to the south-east of it, then if there is no wind to account for his drift he knows that the surface water must be flowing in a south-easterly direction at a speed of five miles a day. The great pioneer in organising a systematic recording of winds and ocean currents throughout the world was Lieut, (later Admiral) M. F. Maury of the United States Navy who wrote the first text book of oceanography, The Physical Geography of the Sea in 1855, to be followed by many editions. The charts he produced of the wind and current systems of the world rendered immense service to commerce; before their production the average time of sailing between England and Australia was 124 days, but with their use it was reduced to 97, or the average passage to California (presumably from New York) was 183 days and this was reduced to 135 (from Maury, loc. cit.).

FIG. 3

Types of drift-bottle for investigating ocean currents, a, surface drifter, old type; b, bottom drifter; c, surface drifter, modern type; for further explanation see text.

Observations on the movements of ships are suitable for indicating the surface drift only in the wide open oceans. For working out the current systems in more detail in more confined areas science has made use of that romantic object, the shipwrecked sailor’s bottle with a message to his home. Thousands and thousands of these drift bottles have been cast into the sea at many different points. Each bottle displays through its glass sides, a notice in several different languages asking the finder to break it and read the instructions within; these tell him that if he will fill in and send off an enclosed postcard giving particulars of the place and date of recovery, he will receive a small reward. Many hundreds of these cards have been returned from various points along the coasts of different lands. In this way Dr. T. W. Fulton of the Scottish Fishery Board first charted the main surface movements of the North Sea at the beginning of the century. Later Dr. G. P. Bidder introduced bottom-drifting bottles, each provided with a trailing wire and weighted so as to drift along with this just touching the sea-bed; these are designed to be recovered by the many trawlers dragging their nets along the bottom of the North Sea and English Channel. Thus knowledge has been gained not only of the surface currents but also of the movements of the lower layers of water, which are often very different. Objects floating on the very surface of the sea may be driven by the wind much faster than will the true upper layer of water; surface drift bottles have now been improved by Dr. J. N. Carruthers (1928) who has provided them with little weighted ‘sea anchors’ to keep them drifting with the main body of water. Sketches of these different bottles are shown in Fig. 3 and an example of their use in studying the drift of plaice eggs is shown in Fig. 5. Quite recently oceanographers have started using water-proof plastic envelopes instead of bottles.

FIG. 4

Diagrammatic sketches showing the working of the Ekmann current-metre.

For the more exact measurement of the speed and direction of current flow at different depths special instruments have been invented to be used from anchored ships. One of the most successful is that of Dr. Ekmann. It is such a beautiful and ingenious device that it is worth examining in some detail to see how it works. It is suspended in the sea on a wire, as shown in Fig. 4, and, being provided with a vane like a weather-cock, always points head on into the current. It also has a little ‘propeller’ which will be turned by the water flowing past it; this however is held by a catch until the measurement of flow is about to begin. On the underside of the apparatus, and held horizontally, is a circular box, like a large and rather deep pill-box. The lower half of this is divided into a number of compartments by partitions radiating from the centre like the spokes of a wheel; swinging above these compartments, and pivotting upon a spike in the centre, is a stout compass needle having a slightly sloping groove on its upper surface running from its mid-point to the end pointing to the north. In addition the compass box has a small hole in the centre of the lid and immediately above it is another little box containing a supply of bronze shot. To take a current measurement the instrument is lowered to the desired depth—perhaps 10, 50 or 100 metres; then a small brass weight, called a messenger, is threaded on to the suspending wire and sent sliding down to release the catch and set the propeller free to turn. After one, two or more hours as may be desired, the recording is ended by the dispatch of a second ‘messenger’, which moves the catch further over to stop the propeller once more, and the machine is hauled up to the surface. The number of turns the propeller has made in the interval of time is recorded on a series of little dials and so enables the speed of the current to be estimated; also after every so many revolutions a little valve opens and allows a shot to drop into the box below and be guided by the grooved compass-needle into one of its compartments. The compass-box, being fixed to the instrument, is always orientated in relation to the direction of the current; thus as the instrument swings in response to changes in current flow, each shot is dropped into whatever compartment happens to lie below the north-pointing end of its compass needle. By counting the shot found in each of the compartments at the end of a recording we know to what extent the current has varied during the time of observation and also the average direction of its flow. The direction is, of course, given by the angle between the mid-line of the box and that of the compartment which most often pointed north, as revealed by the shot in it. Such observations are made at several depths to show how the speed and direction of the water flow varies at different levels.

The upper layers of sea are nearly always travelling faster than the lower ones. The surface layer is affected by the wind; apart from this, however, the lower layers will be retarded by the frictional resistance of the sea-bed, just as in a river the water in the middle travels faster than that near the bank. We shall later see (see here) that these differences in speed may have a marked effect on the distribution of plankton animals which make extensive day and night migrations between the upper and lower layers. In deep ocean basins we may even find currents at different levels flowing in opposite directions.

FIG. 5

Showing where drift-bottles liberated at A and B were recovered. A map reproduced from one in the museum of the Fisheries Laboratory at Lowestoft showing the results of one of many experiments made by the late Mr. J. O. Borley when investigating the drift of plaice eggs and larvae from the spawning areas to the coastal nursery grounds. The numerals show the number of bottles picked up at each point.

There are several different kinds of current-meter but most of them are similar in general principle to that described. Dr. Carruthers (1926) has devised a much more robust machine which is used on a number of lightships anchored round our coasts to record automatically the main drift of water for periods of a month at a time. It records the to-and-fro tidal stream as well as the residual current-drift. These instruments have given much valuable information on the variations in the flow of water through the English Channel into the southern North Sea (Carruthers, 1930). It is in the Flemish Bight that so many plaice congregate in winter to lay their floating eggs at the point where the Channel water enters; these eggs, and later the hatched-out fry, are carried by the current and so bring the new generation to settle down as small flat-fish on the nursery feeding grounds in the shallow waters off the Dutch and Danish coasts. Herring fry in vast numbers are also carried by the same current into the North Sea from one of the largest spawning grounds off Cape Gris Nez. The spawning migrations of many fish have been evolved in relation to the prevailing current systems; selection has naturally acted to preserve the offspring of those parents who migrate to lay their eggs at the point up-stream most favourable for their survival; eggs spawned elsewhere are less likely to be successful because they are carried to less suitable nursery grounds. Fig. 5 shows the results of two of many experiments made with drift-bottles during the English fishery investigations into the life-history of the plaice. Those liberated at a point in the main spawning area in the Southern Bight have all been picked up on the Dutch coast, whereas those from the lesser spawning area off the Yorkshire coast are carried into the Wash, which is another smaller ‘nursery’ ground; the number against each point on the coast indicates the number of bottles found there. In some years a marked variation from a normal current-flow may well have a considerable effect on the relative success of a particular brood of fry; if the flow is weaker than usual they may not reach the best ground, if it is too strong they may be carried beyond it. The speed of flow of Channel water into the North Sea may be affected by the wind at a critical time apart from any variation in the fundamental current system; a prolonged westerly wind may accelerate the flow, and an easterly one may have the reverse effect. These are just some of the many factors we must take into account in puzzling out the possible causes for this or that unusual event in the fisheries; it is so often that such factors have an effect which is most marked in the fishery several years later, i.e. when the young fish of that brood will have grown to maturity—or failed to.

Without the use of drift bottles or current-meters some indication of the direction of current-flow may also be got by mapping the contours of varying salinity (measurements of salt-content) obtained by the analysis of water-samples collected at a number of different points in the area; for instance, a tongue of very salt water projecting into a less saline area might indicate a flow of ocean water into a more coastal region where the salt water has been diluted by drainage from the land. Indeed modern oceanography has developed an elaborate mathematical system for estimating the direction and relative speeds of water movements from a knowledge of the varying densities of the water at a number of different points. We shall later see how at times certain planktonic animals and plants characteristic of one particular type of water may be used as indicators of the incursion of such water into other areas: in fact one such example, that of the tropical Physalia and Velella reaching our coasts, has already been mentioned, and we shall discuss others at the end of the chapter. We are apt to think of the use of plankton animals as current indicators as rather a modern idea; I was interested to find that Alexander Agassiz in 1883 was emphasising the importance in this respect of the animals just mentioned. “This group of Hydrozoa,” he wrote “is eminently characteristic of the Gulf Stream, and wherever its influence extends these Velellae and Physaliae have been found. In fact these surface animals are excellent guides to the course of the current of the Gulf Stream—natural current bottles, as it were.”

In the Department of Natural History in the University of Aberdeen there is a cabinet containing a remarkable collection of South American and West Indian seeds picked up on the shores of the Outer Hebrides. They were gathered from 1908 to 1919 by William L. MacGillivray who was a nephew of a former Regius Professor; most of them he found on the West Sand of Eoligarry, Barra, where he lived, but some came from Lewis and the Island of Fudag. There are Brazil nuts, the seeds of a leguminous liana Diodea, the Virgin Mary nut, palm seed of different kinds, the pecan nut, the Calabar bean, nutmeg, the seeds of the Central American soapberry tree and many others. Altogether seventeen tropical species are represented. Just as these seeds are drifted to our shores, so also are the baby eels carried round by the ocean circulation from where they were spawned—from a small area situated between Bermuda and the Leeward Islands—and eventually scattered by the Gulf Stream to enter the rivers along the whole seaboard of Europe. This was the amazing discovery made by the famous Danish oceanographer Professor Johannes Schmidt. On many special voyages he plotted the distribution of the tiny eel fry all over the Atlantic until at last he could show that there is only one limited area where the very smallest and newly hatched young are to be found—a breeding ground some 3,000 miles from the rivers in which they grow to maturity. In Fig. 6, I reproduce his map showing the spread of the fry of different sizes. Their drift round the ocean to Europe takes from 2 to 2½ years, and during this phase they are little flat and quite transparent creatures having the shape of a willow leaf. They used to be thought to be a separate species of fish, called Leptocephalus brevirostris, until the Italian naturalists Grassi and Calandruccio kept some in an aquarium and were surprised to find they turned into the common elvers, as the young freshwater eels are called when they ascend the rivers from the sea. The story is now very well known and I only recall it here because it is, to use Agassiz’s simile, nature’s greatest drift-bottle experiment and demonstrates so clearly the constancy of this vast current system; year after year, for many millions of years, the eel-fry must have been transported in this way with never a break in the sequence. How the adult eels navigate back to breed in this one place is one of the most profound mysteries of the sea; a discussion of this, however, belongs to a chapter on fish and must await the subsequent volume.

FIG. 6

The distribution of the common European Eel (Anguilla vulgaris) during its various stages of development. The contoured areas represent those in which the larvae of various sizes, 10, 15, 25 and 45 mm. are found; the line ul represents the limit of occurrence of unmetamorphosed larvae; the black bands along the coasts indicate the countries where the adult is found in fresh water (after Schmidt).

The causes of the great circulations of water—as distinct from mere tidal streams—are of three kinds; oceanographers, however, are still not fully agreed as to which is the most important: indeed all three play their part together. Primarily there are the effects of the prevailing winds over wide stretches of ocean, particularly the northeast and south-east trade winds blowing obliquely towards the equator from north and south respectively. These certainly take a great part in driving the equatorial water towards Central America, so that from the Gulf of Florida emerges the powerful warm Gulf Stream to flow across the North Atlantic and give to our islands and the north of Europe so temperate a climate compared with that of the corresponding latitudes of North America. The latter are cooled by the Arctic Stream of the Labrador Current. The Gulf Stream, or the North Atlantic Drift as it is more correctly called on this side of the ocean, has a profound effect upon our waters.

Although marine physicists are beginning to believe that the stress of the wind on the sea surface, together with the effect of the earth’s rotation, can give rise to slow movements of water in the deep layers of the ocean as well as near the surface, they have to consider a second kind of cause: the action of what are often termed Archimedian forces. These are the forces due to internal changes in the water-mass causing alterations in its density. Such changes may be due to the expansion or contraction of the water on being warmed or cooled; they may also be due to an increase in the salt-content caused by excessive evaporation of water at the surface, as in the tropics, or to a decrease in saltness caused by large additions of fresh water from melting ice or excessive rainfall. Whether these causes actually produce the deep water movements, or do no more than make the water take the path of least resistance, they have far-reaching effects, particularly in giving rise to vertical as well as horizontal differences in the great ocean basins. One of the most surprising discoveries of worldwide oceanography was made between the two world wars—indeed through the work of our own Discovery Expeditions and the German Meteor Expedition, in the Antartic and Atlantic Oceans; it is the fact that the heavy snow or rainfall and the melting ice in the Antarctic seas has an immense influence that extends across the equator into the northern hemisphere. It is so striking an example of these Archimedian forces that I cannot resist using it as an illustration.

FIG. 7

A section through the Atlantic Ocean, from latitude 55°S to 15°N along the meridian 30°W, showing the water of varying saltness (34–00/00 to 37–0 0/00) and the directions of the main water movements at different depths. It shows how the great Antarctic icecap extends its influence into the northern hemisphere. Redrawn in diagrammatic form from Deacon (1933).

The great ice-cap at the south pole dominates the oceans of the world. The Antarctic continent rising in a plateau to elevations of some 8,000 feet is covered with a sheet of ice many hundreds of feet thick; this is continually being added to by the frequent heavy falls of snow, and is constantly and slowly moving as a vast glacier to the coast and beyond into the ocean where it juts out as the floating ice barrier. This is shown on the left of Fig. 7. At its edge this barrier from time to time breaks up into the massive tabular icebergs so characteristic of the south polar seas. This is freshwater ice, which on melting, helps to form a cold but light surface layer; in spite of being colder it is lighter than the normal sea-water because its salt content is reduced by the addition of the fresh-water. All round the pole this cold surface layer flows away to the north. Below this is water that is heavier because it is just cooled and not diluted with fresh-water; this sinks and forms a cold current, also flowing north but over the ocean floor. To take the place of these two streams of water flowing away from the pole, a mass of warmer water flows southwards and wells up against the ice, to be itself diluted, cooled and turned north again. The surface current thus formed continues till it meets warmer water which, although more saline, is lighter because it is so much warmer; the cold current now dips below this warmer water but still travels northwards and can be traced to a point some 30° of latitude north of the equator; it then sinks and joins in the return flow going south again to complete the circulation.

Here we have a striking case of waters at different levels travelling in opposite directions: a layer going south flows in between two layers coming north. We shall see in a later chapter how the behaviour of some plankton animals is adapted in a most remarkable manner to take advantage of this fact.

Dr. G. E. R. Deacon, who has done so much to increase our knowledge of this remarkable system (1933 and 1937), while on one of the Discovery expeditions took water-samples all the way along the path of this northward-flowing current after it had dipped below the surface; when he had analysed them he found something very extraordinary. We have said that this water was of low salt-content because of the melted ice; it also has a high oxygen-content because of a great production of planktonic plants in the polar surface waters (due to the rich nutrient salts and to the long hours of daylight in high latitudes). Now as this water travels north the salt-content increases by diffusion from the surrounding layers and the oxygen-content is lowered by the respiratory requirements of animals. As he went along the path of the current Dr. Deacon obtained a clear indication that the salt and oxygen values did not increase and decrease respectively in a perfectly steady manner as one might have expected, but in a series of waves. As far could be judged from the graph of the increase in saltness there were seven undulations in the curve from south to north; likewise in a graph of oxygen-decrease there were also seven undulations. Now the crests of the undulations of one curve corresponded in position with the troughs of the undulations of the other curve; in other words as we passed along the stream of water, regions of higher oxygen-content and lower salinity alternated with regions of lower oxygen-content and higher salinity. This water had come originally from the Antarctic surface layer in which during the summer more ice melts and also more plants are produced than in the winter. More ice melting means a lowering of salinity and more plants mean a greater production of oxygen. Clearly these regions of lower salinity and higher oxygen-content alternating with regions of higher salinity and lower oxygen-content represent the water which left the antarctic surface layers in past summers and winters respectively. Dr. Deacon tells me that he now fears that there are not sufficient observations along the path of the current to make their number quite certain; but since the indicated rates of water movement agree very well with most other estimates, he feels that they give us a fairly reliable time scale for this great circulating system. This water takes at least seven years on its journey from the antarctic to the northern hemisphere!