Читать книгу The Open Sea: The World of Plankton - Alister Hardy - Страница 8

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!

There is a somewhat similar system of a cold and less saline surface current flowing away from the north polar basin due to melting ice and the fall of snow and rain, but it is not so far-reaching as that from the south because this precipitation is less and in addition the Arctic Ocean is almost entirely enclosed by submarine ridges; this cold stream dips below the warmer water at the northern boundary of the Gulf Stream. It is partly to replace this cold water stream that the extension of the Gulf Stream—the North Atlantic Drift—is carried so far to the northward of our islands and up the northern coasts of Scandinavia. Here we see how these Archimedian forces may contribute to the North Atlantic system.

The third factor affecting ocean-currents is a much more subtle one, due directly to the actual spin of our planet. This deflecting force of the earth’s rotation is sometimes called Corioli’s force, after the French physicist, though in fact it was carefully worked out by his countryman Laplace 60 years before; it applies to the atmosphere as well as the sea. It is not a cause of the initial motion of the water but a cause of its deflection. A body of water moving in any direction is deflected to the right in the northern hemisphere and to the left in the southern hemisphere. The effect is greater towards the poles and reduced towards the equator; on the actual equator itself there is no such effect at all. It applies not only to water but to any moving object; we can perhaps understand it best by considering the effect upon a swinging pendulum. Let us suppose we could hang a fairly heavy weight, say of some 20 lbs., on a long string from a 100 foot tall gallows-like structure at the north pole; now if we set the weight swinging to-and-fro in the same direction as a straight line drawn in the snow beneath it, we should soon observe that its line of swing would deviate from the line in the snow. Its swing would be deflected in a regular fashion in a clockwise direction; even in ten minutes its path would be deflected 2½. Unperceived by us the earth and the gallows would be rotating in an anti-clockwise direction, but the heavy pendulum weight is swinging free and its path is not affected although the string at the top will twist. If we watched it for a full twenty-four hours we should see the path of the pendulum complete a deflection of 360° and once more for a moment swing directly above the line in the snow. If we repeated this experiment at the equator—drawing our line in the sand—we should see no such effect; if the pendulum was set swinging say north and south it would continue to swing thus as it would also continue to swing in any other direction in which we might choose to start it; here the earth makes no turning motion in relation to the swing; for the line in the sand and the line of swing are carried on together by the earth’s motion round its axis. At the south pole we should of course get a similar effect to that at the north pole except that the pendulum would appear to be deflected in an anti-clockwise direction. Swinging such a pendulum at places in different degrees of latitude will give a different amount of deflection. At a place situated in latitude 30° north the pendulum will swing through 180° in the 24 hours, for, speaking mathematically, the effect depends on the sine of the angle of latitude; in London at 51.5° latitude it will swing through 281°. The effect was shown very clearly by Foucault, the French physicist and inventor of the gyroscope, by swinging a hundred-foot pendulum at the Great Exhibition of 1851. This demonstration which is to be seen in the Science Museum in London and in a number of provincial museums is not difficult to set up in any building with a high roof or in any house that has a fairly wide staircase well above the entrance hall; it is an impressive sight to see in the matter of a few minutes the apparent change of motion of the pendulum, which really indicates the rotation of the hall itself or, indeed, the earth.

FIG. 8

The varying saltness (31.0 0/00 to 35.4 0/00) of the surface waters and the main circulation typical of the North Sea and English Channel in winter. Drawn from a chart kindly provided by Commander J. R. Lumby, of the Fisheries Laboratory, Lowestoft.

Just as the pendulum is deflected in relation to the objects in the hall, so any body of water in motion tends to be deflected to the right in the northern hemisphere and to the left in the south in relation to the surrounding land masses and the ocean floor; account has to be taken of it in every practical treatment of tides, wind drifts and ocean currents. Whenever a water-mass meets an obstruction, either a mass of land or an opposing water-mass, it will, other things being equal, turn to the right in the north rather than to the left, and vice versa in the south. There is another important effect. We have seen how through differences in temperature and saltness the water varies in density; the lighter water will naturally be on top. In a current system the water of a particular density—say the lightest water at the top—is not lying in a layer of uniform depth; owing to the earth’s rotation the lighter water is pushed more to the right-hand side of the current stream than the heavier water, so that imaginary surfaces separating waters of different density are not horizontal but tilted. It is from a consideration of the deflection of waters of different densities that the speed and direction of ocean currents can be mathematically worked out as mentioned earlier in the chapter.

With this slight introduction, intended merely to give an idea of the kind offerees at work to produce the circulatory ocean-systems, we may now briefly review the main streams of water in the seas around our coasts. Fig. 8, is based on the account by Comd. J. R. Lumby (1932), hydrologist at the Fisheries Laboratory, Lowestoft, with a small revision which he has kindly made in the drawing for this figure. The water in the North Sea and English Channel is slightly less salt than the Atlantic Ocean water; it is typically coastal water diluted by freshwater drainage from the land. The Baltic has a much lower salinity still. A stream of Atlantic water flows into the North Sea from the north, mainly passing round to the east of the Shetland Islands to flow due south and not usually entering between the Orkneys and the Shetlands as was originally thought; a less powerful stream flows up the English Channel and enters it from the south. The northern influx is generally thought to flow on a broad front down the middle of the North Sea forming, as it goes, swirls off the coast of Scotland especially in the Moray Firth and in the region of the Firth of Forth. Dr. J. B. Tait of the Scottish Fishery Department has in recent years, however, put forward the view (1952) that the main streams are much narrower than formerly supposed—more like rivers flowing in the sea. Which is the correct view is at present by no means certain; some evidence from plankton distribution appears to support one view and some the other. Just before reaching the Dogger Bank the main stream, whether broad or narrow, appears generally to divide into three branches: one running south-westerly, another south-easterly and a third turning east to enter the Skagerrak. The south-westerly and south-easterly branches form large swirls in the southern North Sea as they meet the stream of water entering from the Channel. Another smaller swirl is formed outside the Skagerrak as the stream entering on the southern side meets the stream flowing out of the Baltic on the northern side.¹ The stream entering the North Sea from the Channel flows north-eastwards past the Dutch and Danish coast and some of it joins the stream going into the Baltic. Most of the North Sea is shallow, but there is a deep hollow running up the western coast of Norway to the north; it is along this Norwegian trough that the water leaves the North Sea—the less saline water from the Baltic on the top and the bulk of the North Sea water proper in the deep channel below.

The extent of the inflow of Atlantic water varies from year to year; such variations affect the distribution of the plankton and are likely to influence the distribution of the herring shoals which depend upon the plankton for food. In some years of exceptional influx numbers of plankton animals usually only found in the more open ocean make their appearance in the northern North Sea. There is some evidence to support the view that it is the pressure of this water from the north (produced by the main wind systems) which, apart from the occasional effects of local winds already referred to, controls the inflow of Channel water into the southern North Sea. If the pressure from the north is high it seems that the Channel flow is reduced; if it is weak then a larger influx from the Channel seems to take place. It is in the study of this inflow into the northern North Sea that the charting of the relative movements of certain Atlantic plankton animals in different years can be most helpful. We shall see in a later chapter (see here) how, by the use of plankton-recording machines towed at monthly intervals by commercial steamships on regular routes, we can compare the areas of invasion of these more oceanic forms in different months and years. We shall find that not only does the extent of the Atlantic inflow vary from year to year, but the time of the advance of typical invading organisms will vary: in some years it may be a month earlier or later than in other years. There is an interesting suggestion now being investigated that the time of the appearance of the shoals of herring at different points down the east coast of Scotland and England, and consequently the time of the different fisheries, may be earlier or later in different years depending on whether this Atlantic inflow is earlier or later.² Whether this indication—it is no more at the moment—will be proved correct or not, there can be no doubt that the fluctuations that are found to occur in the water movements round our islands must have a profound effect upon the fish and other life inhabiting our seas.

A more definitely established connection between water changes and fisheries has been demonstrated at the western entrance to the English Channel. The water of the greater part of the Channel is like that of the southern North Sea—coastal water which is less saline and less rich in plankton than the Atlantic water that flows into it. This more coastal water can readily be distinguished from the more oceanic water by the presence of certain of these indicator plankton species—particularly two species of Sagitta, the slender transparent arrow worm shown in Plate IX; Sagitta setosa being found in the coastal water and Sagitta elegans in the more oceanic water.³ The boundary between the two waters formerly used to lie somewhere in the region of Plymouth where sometimes the plankton would have elegans predominating in it and sometimes setosa) during the investigation up to 1929 it was more usually elegans, indicating Atlantic water richer in phosphates and other nutrient salts. The importance of these salts in the economy of the sea will be discussed in Chapter 4. Since 1929 the boundary between elegans and setosa water has lain much further to the west so that the water off Plymouth has been of the coastal type and much poorer in plankton. Since this date there has also been a marked reduction in the number of young fish of many kinds present in the plankton as well as a change in the herring fishing; since that time the herring which used to visit the Plymouth area around Christmas have not turned up in their usual numbers so that this winter fishery, once quite a prosperous one, now no longer takes place. An excellent account of this trend was given by the late Dr. Stanley Kemp in his presidential address to the Zoology Section of the British Association in 1938. More recently some other interesting differences between the elegans and setosa water have been discovered; these will be referred to later when the various chemical constituents of the water are being considered (see here).

FIG. 9

Map showing the distribution of three kinds of water round Great Britain each characterised by a different species of the arrow-worm Sagitta: serratodentata in open ocean water, setosa in coastal water and elegans in oceanic water mixing with the coastal water. The conditions are those which might be expected in the autumn of a year with a strong Atlantic influx into the North Sea from the north. From Russell (1939), but modified in the north in the light of more recent surveys and with some other details omitted.

Mr. F. S. Russell, the present Director of the Plymouth Laboratory, who carried out these studies on Sagitta (1935, 1936) and young fish (1940), made cruises to trace the boundaries between the different kinds of plankton. He has told me how very abruptly one type of water may give place to another. On one occasion he has said it was even possible to place the ship across the very margin between them, so that elegans water could be sampled from the bows and setosa water from the stern! Fig. 9, shows the general distribution of the elegans and setosa water round the British Isles as it might be expected in the autumn of a year in which there is a strong influx of Atlantic water into the North Sea from the north; it is taken from another of Mr. Russell’s papers (1939). These different waters may also be sometimes discernible by a difference in their colour, a contrast of shades of blue and green making a line across the sea. In 1923, when on the staff of the Fisheries Laboratory at Lowestoft, I acted as observer in some attempts to locate shoals of herring and mackerel from the air. In flying from Plymouth to the western mackerel grounds we passed over a sharp line separating the green water of the Channel from the deep blue of the Atlantic; it ran on a slightly irregular course from the Lizard to the south-west as far as we could see to the distant horizon. Then while circling over the mackerel area we saw another equally definite boundary running from Land’s End towards the Scilly Isles separating the deep blue water from a more brown-green area lying to the north. At that time I could not interpret that striking pattern of colour contrasts; now on looking at Mr. Russell’s maps I have little doubt that the blue area I saw was oceanic elegans water lying between the setosa water of the English Channel and that of the Irish Sea. Fig. 10 shows a comparison between my sketch of these colour boundaries, which was published in the official report (Hardy, 1924a) and Mr. Russell’s maps of the distribution of the setosa and elegans water in the same area but in different years (Russell, 1935, 1936). If these marked colour-changes can be correctly interpreted we may in the future find aircraft being used to make rapid surveys of the surface conditions in relation to the fisheries. The actual experiments in spotting shoals offish were not successful in these waters; in the southern North Sea the water was too opaque with the large amount of sediment constantly stirred up by tidal currents running over sand and mud banks; at the western entrance to the Channel the ocean surface was too much broken up by waves into light and shade to allow of any observations below it.

FIG. 10

Well defined areas of blue and green water (A) seen from the air during mackerel spotting tests off Cornwall in 1923 drawn from the chart by Hardy (1924) and compared (B and G) with the distribution of western and Channel water as indicated by the arrow-worms Sagitta, elegans and setosa, charted by Russell (1935 and 1936).

In the shallower waters—especially in the southern North Sea—we must not forget the influence of the tidal streams just mentioned; they may have a most profound effect in modifying the action of the main currents, especially when they vary so enormously in their force between spring and neap tides. At spring tides in certain places a mass of water may be moved for some thirty or forty miles in each direction.

The movements of water in the Irish Sea are also dominated by tidal currents; these flow into it from both ends and follow the general direction of the coast lines. Professor K. F. Bowden, who has given us such an excellent account of these tidal streams (1953), writes, “Knowledge of the non-tidal drift, however, is much less certain and is based on indirect evidence. It was recognised at an early date that the distribution of salinity indicated a north-going drift and in 1907 Knudsen estimated that the rate of flow was such that the water in the Irish Sea would be completely renewed in a year.” After saying that “this implies a flow through the Dublin-Holyhead channel at an average rate of just over a kilometre a day,” he later stresses that, although there seems little doubt about this average northward movement, “its magnitude, its variations and the degree of its dependence on the wind are still uncertain.”

We have now dealt with the main water movements round our islands; later in the book we shall see instances of more local effects and how upwellings and the mixing of waters may be important in producing a richer plankton. I will end the chapter by referring to some surprising and significant plankton records being made by Dr. J. H. Fraser (1952e, 1955) of the Scottish Fishery Laboratory at Aberdeen. In some years, over a wide area to the north of Scotland, he finds plankton animals which we should more usually associate with the latitudes of the Mediterranean; they indeed indicate a very unexpected movement of water. It now appears that some of them may in fact actually have come from the Mediterranean Sea itself.

FIG. 11.

A chart showing the northward flow of the ‘Lusitanian’ plankton; kindly provided by its discoverer Dr. J. H. Fraser of the Scottish Fishery Laboratory, Aberdeen.

It has long been known that a surface stream of Atlantic water flows eastwards through the Straits of Gibraltar and that this influx is balanced by an outpouring (at a lower level) of Mediterranean water of very high salinity; this spreads out from the Gulf of Gibraltar underneath the North Atlantic water and some of it is carried north up the edge of the European continental shelf. This movement is well summarised in Sverdrup, Johnson and Fleming’s important book The Oceans (1942, pp. 646, 685–6) which, for the serious student, gives such an excellent account of the main results of modern oceanography. Dr. L. H. N. Cooper of the Plymouth Laboratory has recently (1952) made a study of the distribution of this water to the west of the British Isles as it continues northward below the Atlantic water at a depth of some 600 to 1,200 metres. How far north it goes seems to vary greatly in different years; in some it appears to go no further than the west of Ireland, but in other years it flows onwards to upwell and spread over the continental shelf. Its course has been followed by Dr. Fraser by finding its typical but exotic fauna in his plankton nets; to the west he finds it deep down—but let me quote from his recent paper.

“It apparently follows the edge of the Hebridean Continental shelf, mixing on its western edge with open oceanic water, and upwells somewhat on the east side to overflow and mix with coastal water on the shelf. In some years this current may not reach Scotland or is too weak to be recognised, but on occasions it is sufficiently strong to continue into the South side of the Faroe Channel, though it only rarely penetrates beyond the north of Shetland. Frequently, however, it mixes with the coastal water on the shelf and the resulting mixture floods the area to the west of Orkney and often passes through the Fair Isle-Orkney Passage into the Moray Firth area.”

How lucky we are to have such a remarkable current carrying its rich and southern life far below the surface and then spreading it out, as it were, on our northern doorstep for our examination. Fraser calls this a planktonic ‘Lusitanian fauna’ and lists no fewer than 43 species characteristic of it; he has kindly prepared for me a chart of its typical distribution which is reproduced in Fig. 11. Through his recent publications I have been able to add to my account some very interesting animals which I shall be describing in Chapter 7 and Chapter 8 and which hitherto I should never have dared to include as inhabitants of British waters. He defines his Lusitanian fauna as that which “originating in the outflow from the Mediterranean, has become modified by admixture with fauna from the area between the Azores and Bay of Biscay.” This work is an outstanding example of the importance of natural history in helping us to have a better understanding of the physics of the sea. I will give a final quotation from his work:

“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.

⁴ see here.